The team from Oxford, Germany and Japan are said to have observed conclusive signatures of superconductivity after hitting a non-superconductor with a strong burst of laser light. (D. Fausti et al., "Light-Induced Superconductivity in a Stripe-Ordered Cuprate", Science v.331, p.189 (2011)).
Abstract: One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional “striped” spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La1.675Eu0.2Sr0.125CuO4, into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.
‘We have used light to turn a normal insulator into a superconductor,’ said Prof Andrea Cavalleri of the Department of Physics at Oxford University and the Max Planck Department for Structural Dynamics, Hamburg. ‘That’s already exciting in terms of what it tells us about this class of materials. But the question now is can we take a material to a much higher temperature and make it a superconductor?’
The material the researchers used is closely related to high-temperature copper oxide superconductors, but the arrangement of electrons and atoms normally act to frustrate any electronic current.
In the journal Science, they describe how a strong infrared laser pulse was used to perturb the positions of some of the atoms in the material. The compound, held at a temperature just 20 degrees above absolute zero, almost instantaneously became a superconductor for a fraction of a second, before relaxing back to its normal state.
Superconductivity describes the phenomenon where an electric current is able to travel through a material without any resistance.
High-temperature superconductors can be found among a class of materials made up of layers of copper oxide, and typically superconduct up to a temperature of around –170°C. They are complex materials where the right interplay of the atoms and electrons is thought to ‘line up’ the electrons in a state where they collectively move through the material with no resistance.
‘We have shown that the non-superconducting state and the superconducting one are not that different in these materials, in that it takes only a millionth of a millionth of a second to make the electrons ‘synch up’ and superconduct,’ said Professor Cavalleri. ‘This must mean that they were essentially already synched in the non-superconductor, but something was preventing them from sliding around with zero resistance. The precisely tuned laser light removes the frustration, unlocking the superconductivity.’
The advance immediately offers a new way to probe with great control how superconductivity arises in this class of materials.
The researchers are hopeful it could also offer a new route to obtaining superconductivity at higher temperatures. If superconductors that work at room temperature could be achieved, it would open up many more technological applications.
‘There is a school of thought that it should be possible to achieve superconductivity at much higher temperatures, but that some competing type of order in the material gets in the way,’ said Prof Cavalleri. ‘We should be able to explore this idea and see if we can disrupt the competing order to reveal superconductivity at higher temperatures. It’s certainly worth trying.’
Monday, January 17, 2011
Light turns insulator into a superconductor
Sunday, January 16, 2011
NRL begins field tests of laser acoustic propagation
An NRL research team led by physicist, Dr. Ted Jones, Plasma Physics Division, performed the first successful long distance acoustic propagation and shock generation demonstration of their novel underwater photo-ionization laser acoustic source. These tests, performed at the Lake Glendora Test Facility of Naval Surface Warfare Center-Crane, expanded on their earlier laboratory research on pulsed laser propagation through the atmosphere.
Using a pulsed Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) 532 nanometer wavelength laser housed in a floating platform, pulses were directed by steering mirrors down through a focusing lens and into the water surface. Each laser pulse produced an acoustic pulse with a sound pressure level of approximately 190 decibels (dBs), which was detected and measured by boat-mounted hydrophones at distances up to 140 meters, roughly the length and a half of a football field. Prior laboratory acoustic propagation distances were limited to about three meters.
"The goal of this laser acoustic source development is to enable efficient remote acoustic generation from compact airborne and ship-borne lasers, without the need for any source hardware in the water," said Jones. "This new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications."
The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. A properly tailored laser pulse has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, a highly useful and efficient tool for undersea communications from aircraft.
Using a pulsed Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) 532 nanometer wavelength laser housed in a floating platform, pulses were directed by steering mirrors down through a focusing lens and into the water surface. Each laser pulse produced an acoustic pulse with a sound pressure level of approximately 190 decibels (dBs), which was detected and measured by boat-mounted hydrophones at distances up to 140 meters, roughly the length and a half of a football field. Prior laboratory acoustic propagation distances were limited to about three meters.
"The goal of this laser acoustic source development is to enable efficient remote acoustic generation from compact airborne and ship-borne lasers, without the need for any source hardware in the water," said Jones. "This new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications."
The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. A properly tailored laser pulse has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, a highly useful and efficient tool for undersea communications from aircraft.
Monday, January 10, 2011
Few femtosecond, few kiloampere electron bunch produced by a laser–plasma accelerator
Particle accelerators driven by the interaction of ultraintense and ultrashort laser pulses with a plasma can generate accelerating electric fields of several hundred gigavolts per metre and deliver high-quality electron beams with low energy spread, low emittance and up to 1 GeV peak energy. Moreover, it is expected they may soon be able to produce bursts of electrons shorter than those produced by conventional particle accelerators, down to femtosecond durations and less. Here we present wide-band spectral measurements of coherent transition radiation which we use for temporal characterization. Our analysis shows that the electron beam, produced using controlled optical injection, contains a temporal feature that can be identified as a 15 pC, 1.4–1.8 fs electron bunch (root mean square) leading to a peak current of 3–4 kA depending on the bunch shape. We anticipate that these results will have a strong impact on emerging applications such as short-pulse and short-wavelength radiation sources, and will benefit the realization of laboratory-scale free-electron lasers.
An ultrashort and ultraintense laser pulse (red) is focused on a gas jet in which a plasma wave is excited. Electrons are injected into the plasma wave during the collision with the injection pulse (green), which arrives at a relative angle.
(via Nature Physics doi:10.1038/nphys1872)
An ultrashort and ultraintense laser pulse (red) is focused on a gas jet in which a plasma wave is excited. Electrons are injected into the plasma wave during the collision with the injection pulse (green), which arrives at a relative angle.(via Nature Physics doi:10.1038/nphys1872)
Thursday, December 23, 2010
Dutch researchers build affordable alternative to mega-laser X-FEL
Stanford University in the USA has an X-FEL (X-ray Free Electron Laser) with a pricetag of hundreds of millions. It provides images of 'molecules in action', using a kilometer-long electron accelerator. Dutch researchers at Eindhoven University of Technology (TU/e) have developed an alternative that can do many of the same things. However this alternative fits on a tabletop, and costs around half a million euro. That's why the researchers have jokingly called it 'the poor man's X-FEL'.
It's one of the few remaining 'holy grails' of science: a system that allows you to observe the extremely high-speed molecular processes at an atomic scale. You could call it an ultra-fast video microscope. Instead of visible light this kind of system uses X-rays or electrons, because it requires radiation with a wavelength of less than a nanometer. The X-rays or electrons have to be emitted in ultra short pulses, so that the exposure time is extremely short. However these pulses are not easy to generate. An X FEL uses X-ray pulses for this purpose, generated by accelerating electrons in an accelerator of a kilometer, or longer. These electrons are then converted into X-rays. An installation of this kind is very costly, uses large amounts of energy and needs a whole team to operate it. A European X-FEL, which will cost a billion euro, is currently under construction in Hamburg (Germany).
TU/e doctoral candidate ir. Thijs van Oudheusden has developed a machine that in many respects can compete with this billion-euro facility, based on ideas from his co supervisor dr.ir. Jom Luiten. The essence of their 'poor man's X-FEL' is that it uses electrons instead of X-rays. "Why convert electrons into X-rays if you can use the electrons themselves?", asks Van Oudheusden. "As well as that you only need to give the electrons a low energy, so you can accelerate them in just a centimeter. That's why the whole system fits on a tabletop."
The physical barrier that Van Oudheusden had to overcome is that the electrons in electron bunches repel each other. This causes the electron bunches to expand, making them longer than the desired 100 femtoseconds (1 femtosecond is 10-15 second), which in turn would make the 'video microscope' too slow. Jom Luiten thought of a solution to prevent the undesired expansion. The key was to create bunches of exactly the right shape, so they can be controlled and focused by means of electrical fields into bunches of the desired type and length. All with a number of electrons (1 million) that is sufficient to create a diffraction pattern in just a single shot.
Supervisor prof.dr. Marnix van der Wiel believes that half to three-quarters of the kind of research that can be done on an X-FEL can also be done with the 'poor man's X_FEL'. But this doesn't immediately mean that the latter is automatically a lot cheaper in relation to the scientific output that can be generated with it. "The X-FEL at Stanford works non-stop, all year round, and is used by thousands of research groups over several decades. So if you're allocated time on the system you have to take all your equipment to the USA, where you have to stick to a very strict schedule. Our finding is a good alternative for people who want to have the freedom to do research in their own labs. As far as the costs are concerned, it depends on the user if our system will turn out to be cheaper on a per publication basis."
TU/e spin-off AccTec BV intends to build the machine developed by Van Oudheusden and Luiten and to sell it to scientific users. AccTec expects the total price to be below half a million euro.
Thijs van Oudheusden gained his PhD on 13 December with his doctoral thesis entitled 'Electron source for sub-relativistic single-shot femtosecond diffraction'.
It's one of the few remaining 'holy grails' of science: a system that allows you to observe the extremely high-speed molecular processes at an atomic scale. You could call it an ultra-fast video microscope. Instead of visible light this kind of system uses X-rays or electrons, because it requires radiation with a wavelength of less than a nanometer. The X-rays or electrons have to be emitted in ultra short pulses, so that the exposure time is extremely short. However these pulses are not easy to generate. An X FEL uses X-ray pulses for this purpose, generated by accelerating electrons in an accelerator of a kilometer, or longer. These electrons are then converted into X-rays. An installation of this kind is very costly, uses large amounts of energy and needs a whole team to operate it. A European X-FEL, which will cost a billion euro, is currently under construction in Hamburg (Germany).
TU/e doctoral candidate ir. Thijs van Oudheusden has developed a machine that in many respects can compete with this billion-euro facility, based on ideas from his co supervisor dr.ir. Jom Luiten. The essence of their 'poor man's X-FEL' is that it uses electrons instead of X-rays. "Why convert electrons into X-rays if you can use the electrons themselves?", asks Van Oudheusden. "As well as that you only need to give the electrons a low energy, so you can accelerate them in just a centimeter. That's why the whole system fits on a tabletop."
The physical barrier that Van Oudheusden had to overcome is that the electrons in electron bunches repel each other. This causes the electron bunches to expand, making them longer than the desired 100 femtoseconds (1 femtosecond is 10-15 second), which in turn would make the 'video microscope' too slow. Jom Luiten thought of a solution to prevent the undesired expansion. The key was to create bunches of exactly the right shape, so they can be controlled and focused by means of electrical fields into bunches of the desired type and length. All with a number of electrons (1 million) that is sufficient to create a diffraction pattern in just a single shot.
Supervisor prof.dr. Marnix van der Wiel believes that half to three-quarters of the kind of research that can be done on an X-FEL can also be done with the 'poor man's X_FEL'. But this doesn't immediately mean that the latter is automatically a lot cheaper in relation to the scientific output that can be generated with it. "The X-FEL at Stanford works non-stop, all year round, and is used by thousands of research groups over several decades. So if you're allocated time on the system you have to take all your equipment to the USA, where you have to stick to a very strict schedule. Our finding is a good alternative for people who want to have the freedom to do research in their own labs. As far as the costs are concerned, it depends on the user if our system will turn out to be cheaper on a per publication basis."
TU/e spin-off AccTec BV intends to build the machine developed by Van Oudheusden and Luiten and to sell it to scientific users. AccTec expects the total price to be below half a million euro.
Thijs van Oudheusden gained his PhD on 13 December with his doctoral thesis entitled 'Electron source for sub-relativistic single-shot femtosecond diffraction'.
Friday, April 16, 2010
FEMTOLASERS: An Ultrafast Success Story
When lasers were discovered in 1960, it was unclear just how this new kind of light could be used. Similar uncertainty faced Ferenc Krausz and his fellow scientists in 1994 when, as a young postdoc at Vienna University of Technology in Austria, he set up a company to market femtolasers, FEMTOLASERS Produktion.
For the first time it became possible to generate light pulses with a duration of around 10 femtoseconds.
The success story of FEMTOLASERS began ....
At the end of the 1980s, a revolution was underway in the field of laser technology. In the early part of the decade, scientists had discovered titanium ions as a crystalline gain medium in lasers. Then along came a new technology to generate short pulses, "Kerr-lens mode locking," courtesy of University of St Andrews (UK) physicist Wilson Sibbet. This young era of new inventions was set to take over from femtosecond laser technology, which worked with liquid dyes as a gain medium.
By the early 1990s, the field of ultrashort laser physics was gripped by a pioneering spirit. Ferenc Krausz, his PhD recently completed at Vienna University of Technology (TU Vienna), was enthusiastic about the new possibilities promised by solid-state laser technology.
The first working group Krausz led at TU Vienna in 1994 was dedicated to the study of new types of solid-state and short-pulse lasers. This team was driven by a desire to significantly shorten the duration of light pulses, using Ti:Sa lasers. The 100-femtosecond pulse barrier had already been cracked. But the broadband laser material titanium-doped sapphire offered an opportunity to get much lower than that.
The greatest challenge to be faced was how to counteract the dispersion that became more and more noticeable as pulse duration shortened. Dispersion occurs in all materials. It delays the colour components in broadband light to varying degrees and thus leads to a lengthening of the pulse. The shorter the pulse, the wider its bandwidth, an effect which causes the pulse to disperse even faster.
Krausz' working group solved the problem by inventing "chirped mirrors" and using them to control the dispersion of femtosecond Ti:Sa oscillators, in cooperation with Robert Szipöcs and Kárpát Ferencz of Budapest.
Chirped mirrors (CM) are made from alternating nanometer-scale layers of two different transparent optical materials with different indices of refraction. They can reflect up to 99.9% of the incident light over a wide bandwidth. In addition, they let light of different wavelengths penetrate to differing depths before it is reflected.
As a result, it now became possible to control dispersion across wide bandwidths and thereby support the formation of pulses of hitherto unachieved shortness.
The journal Optics Letters (Vol. 19, No. 3) published two separate articles on the subjects of the invention of chirped mirrors (U.S. patent 5,734,503) and the concurrent development of a Ti:Sa oscillator which could generate pulses lasting just 11 femtoseconds.
Femtosecond Pioneers
Spurred on by this success, Krausz, together with Andreas Stingl and Christian Spielmann, both scientists in his team at TU Vienna in 1994, decided to set up a company, FEMTOLASERS Produktion in Vienna.
In the first two years, the team produced a small number of Ti:Sa laser systems with chirped mirrors as a kit. These were the very first chirped mirrors ever made.
In 1997, the first commercial laser with a pulse duration of less than 12 femtoseconds was ready for market. Demand for these laser systems was rising steadily, and FEMTOLASERS was the only company serving the market.
Ultrafast Atoms
Although in the beginning it was not yet clear just what the unusually short pulses could be used for, it quickly became apparent that they could open up spectacular new perspectives in biology and physics in particular.
Ever shorter pulses permitted the observation of ever faster processes, for example, in nature. Here, time-resolved femtosecond spectroscopy has done much to further the understanding of elementary processes that occur in chemical reactions. With the aid of this technique, it is possible to track-in real time-the ultrafast movement of atoms in chemical reactions.
The laser pulses are used, for example, to capture momentary snapshots of molecular arrangements. These individual snapshots can then be put together, like an animated film, to depict the time sequence of an event. In 1999, Ahmed Zewail was awarded the Nobel Prize in Chemistry for this development.
Competition to FEMTOLASERS was not long in coming, and soon the only way to survive in a tough market environment was to keep improving the technology of short pulse lasers. FEMTOLASERS developed new amplifiers which injected a million times higher energy into the light pulses.
This development opened the way to the generation of coherent radiation; in addition to pulses in the visible spectrum it was now also possible to generate femtosecond pulses in the ultraviolet and x-ray ranges.
To date, FEMTOLASERS has sold well over 500 systems and is one of the market leaders in this area.
Further development in femtosecond pulse technology since 2001 has opened the door to the attosecond time dimension, 1000 times shorter than femtoseconds. At that time, the team at TU Vienna was the first group worldwide to succeed in generating individual attosecond pulses with a length of 650 as.
Attosecond flashes arise when electrons in an inert gas are excited by femtosecond laser pulses of a few wave cycles. Today femtosecond pulse technology is capable of generating light flashes which carry over 70% of their energy in a single oscillation cycle.
Thanks to these advances, the team succeeded in 2008 in generating light pulses shorter than 100 attoseconds.
By then, the work was being continued at the Max Planck Institute of Quantum Optics (MPQ) in Garching (Germany) by a working group led by Krausz. The group had moved there from Vienna in 2004. Krausz is director of the MPQ, and he also holds a chair in Experimental Physics at the Ludwig-Maximilians-Universität (LMU) of Munich.
Attosecond flashes below 100 as lift the veil on previously invisible electron movements. In particular it becomes possible to observe interactions between individual electrons in real time, because within and between atoms, these particles generally move on an attosecond timescale.
Attosecond Flashes
For attosecond technology to emerge, it was necessary both to shorten the femtosecond laser pulses to around the ultimate limit set by the oscillation period and to precisely control the wave form of the oscillating light field. These wave-form-controlled few-cycle light pulses, which Krausz and his group demonstrated for the first time in cooperation with Nobel Prize winner Theodor Hänsch and his team in 2003 (and which FEMTOLASERS was the first to market a little later), are not just essential for being able to consistently generate and measure individual attosecond flashes. They also open up for the first time the way to controlling the movement of atoms in atomic systems.
Imaging Potential
The potential significance of this is enormous. It may become possible, for example, to control biological processes at a molecular level, or even accelerate microelectronics to its ultimate limits, which are defined by light frequencies.
In many other ways, too, short pulse laser technology will open up brand new insights into the microcosm in the future. For example, it will improve imaging techniques such as multiphoton microscopy by generating non-linear optical effects. In non-linear microscopy, a femtosecond laser excites biomolecules. The light emitted by them is used for imaging and enables a 3D representation.
Work is also proceeding on optimizing coherence tomography. Medical applications are mainly in ophthalmology and early diagnosis of skin cancer.
Terahertz imaging also has great potential and could benefit from the revolution in femtosecond laser technology. In the terahertz frequency range (100 GHz to 10 THz) materials like paper and many plastics become transparent and can be probed. Water and metals, however, have strong absorption lines at these frequencies. This offers potential applications in the examination of solid-state materials, plastics, and biomedical textiles and also in environmental monitoring, quality control, package testing, and security checks.
Future of Ultrafast Lasers
In addition to short pulse technology, Krausz and his team devote special attention to the further development of chirped mirrors. For this purpose in 2009, the team formed a new company in Garching, UltraFast Innovations (www.ultrafast-innovations.com), a spinoff from the excellence cluster, Munich-Centre for Advanced Photonics (www.munich-photonics.de). MPQ and LMU each have a 50% share in the company, which designs and manufactures custom optics.
The optical components offered by UltraFast Innovations are suitable for almost all areas of laser technology. The scientists test the new developments in their own research work, an aspect customers very much appreciate.
We can only guess at the enormously diverse range of potential applications that will emerge from new developments in ultrashort laser pulses and improvements in optical components in laser technology. Almost every new technological advance brings with it new applications, expanding ever further the reach of our understanding.
SPIE Professional April 2010: Open access article
For the first time it became possible to generate light pulses with a duration of around 10 femtoseconds.
The success story of FEMTOLASERS began ....
At the end of the 1980s, a revolution was underway in the field of laser technology. In the early part of the decade, scientists had discovered titanium ions as a crystalline gain medium in lasers. Then along came a new technology to generate short pulses, "Kerr-lens mode locking," courtesy of University of St Andrews (UK) physicist Wilson Sibbet. This young era of new inventions was set to take over from femtosecond laser technology, which worked with liquid dyes as a gain medium.
By the early 1990s, the field of ultrashort laser physics was gripped by a pioneering spirit. Ferenc Krausz, his PhD recently completed at Vienna University of Technology (TU Vienna), was enthusiastic about the new possibilities promised by solid-state laser technology.
The first working group Krausz led at TU Vienna in 1994 was dedicated to the study of new types of solid-state and short-pulse lasers. This team was driven by a desire to significantly shorten the duration of light pulses, using Ti:Sa lasers. The 100-femtosecond pulse barrier had already been cracked. But the broadband laser material titanium-doped sapphire offered an opportunity to get much lower than that.
The greatest challenge to be faced was how to counteract the dispersion that became more and more noticeable as pulse duration shortened. Dispersion occurs in all materials. It delays the colour components in broadband light to varying degrees and thus leads to a lengthening of the pulse. The shorter the pulse, the wider its bandwidth, an effect which causes the pulse to disperse even faster.
Krausz' working group solved the problem by inventing "chirped mirrors" and using them to control the dispersion of femtosecond Ti:Sa oscillators, in cooperation with Robert Szipöcs and Kárpát Ferencz of Budapest.
Chirped mirrors (CM) are made from alternating nanometer-scale layers of two different transparent optical materials with different indices of refraction. They can reflect up to 99.9% of the incident light over a wide bandwidth. In addition, they let light of different wavelengths penetrate to differing depths before it is reflected.
As a result, it now became possible to control dispersion across wide bandwidths and thereby support the formation of pulses of hitherto unachieved shortness.
The journal Optics Letters (Vol. 19, No. 3) published two separate articles on the subjects of the invention of chirped mirrors (U.S. patent 5,734,503) and the concurrent development of a Ti:Sa oscillator which could generate pulses lasting just 11 femtoseconds.
Femtosecond Pioneers
Spurred on by this success, Krausz, together with Andreas Stingl and Christian Spielmann, both scientists in his team at TU Vienna in 1994, decided to set up a company, FEMTOLASERS Produktion in Vienna.
In the first two years, the team produced a small number of Ti:Sa laser systems with chirped mirrors as a kit. These were the very first chirped mirrors ever made.
In 1997, the first commercial laser with a pulse duration of less than 12 femtoseconds was ready for market. Demand for these laser systems was rising steadily, and FEMTOLASERS was the only company serving the market.
Ultrafast Atoms
Although in the beginning it was not yet clear just what the unusually short pulses could be used for, it quickly became apparent that they could open up spectacular new perspectives in biology and physics in particular.
Ever shorter pulses permitted the observation of ever faster processes, for example, in nature. Here, time-resolved femtosecond spectroscopy has done much to further the understanding of elementary processes that occur in chemical reactions. With the aid of this technique, it is possible to track-in real time-the ultrafast movement of atoms in chemical reactions.
The laser pulses are used, for example, to capture momentary snapshots of molecular arrangements. These individual snapshots can then be put together, like an animated film, to depict the time sequence of an event. In 1999, Ahmed Zewail was awarded the Nobel Prize in Chemistry for this development.
Competition to FEMTOLASERS was not long in coming, and soon the only way to survive in a tough market environment was to keep improving the technology of short pulse lasers. FEMTOLASERS developed new amplifiers which injected a million times higher energy into the light pulses.
This development opened the way to the generation of coherent radiation; in addition to pulses in the visible spectrum it was now also possible to generate femtosecond pulses in the ultraviolet and x-ray ranges.
To date, FEMTOLASERS has sold well over 500 systems and is one of the market leaders in this area.
Further development in femtosecond pulse technology since 2001 has opened the door to the attosecond time dimension, 1000 times shorter than femtoseconds. At that time, the team at TU Vienna was the first group worldwide to succeed in generating individual attosecond pulses with a length of 650 as.
Attosecond flashes arise when electrons in an inert gas are excited by femtosecond laser pulses of a few wave cycles. Today femtosecond pulse technology is capable of generating light flashes which carry over 70% of their energy in a single oscillation cycle.
Thanks to these advances, the team succeeded in 2008 in generating light pulses shorter than 100 attoseconds.
By then, the work was being continued at the Max Planck Institute of Quantum Optics (MPQ) in Garching (Germany) by a working group led by Krausz. The group had moved there from Vienna in 2004. Krausz is director of the MPQ, and he also holds a chair in Experimental Physics at the Ludwig-Maximilians-Universität (LMU) of Munich.
Attosecond flashes below 100 as lift the veil on previously invisible electron movements. In particular it becomes possible to observe interactions between individual electrons in real time, because within and between atoms, these particles generally move on an attosecond timescale.
Attosecond Flashes
For attosecond technology to emerge, it was necessary both to shorten the femtosecond laser pulses to around the ultimate limit set by the oscillation period and to precisely control the wave form of the oscillating light field. These wave-form-controlled few-cycle light pulses, which Krausz and his group demonstrated for the first time in cooperation with Nobel Prize winner Theodor Hänsch and his team in 2003 (and which FEMTOLASERS was the first to market a little later), are not just essential for being able to consistently generate and measure individual attosecond flashes. They also open up for the first time the way to controlling the movement of atoms in atomic systems.
Imaging Potential
The potential significance of this is enormous. It may become possible, for example, to control biological processes at a molecular level, or even accelerate microelectronics to its ultimate limits, which are defined by light frequencies.
In many other ways, too, short pulse laser technology will open up brand new insights into the microcosm in the future. For example, it will improve imaging techniques such as multiphoton microscopy by generating non-linear optical effects. In non-linear microscopy, a femtosecond laser excites biomolecules. The light emitted by them is used for imaging and enables a 3D representation.
Work is also proceeding on optimizing coherence tomography. Medical applications are mainly in ophthalmology and early diagnosis of skin cancer.
Terahertz imaging also has great potential and could benefit from the revolution in femtosecond laser technology. In the terahertz frequency range (100 GHz to 10 THz) materials like paper and many plastics become transparent and can be probed. Water and metals, however, have strong absorption lines at these frequencies. This offers potential applications in the examination of solid-state materials, plastics, and biomedical textiles and also in environmental monitoring, quality control, package testing, and security checks.
Future of Ultrafast Lasers
In addition to short pulse technology, Krausz and his team devote special attention to the further development of chirped mirrors. For this purpose in 2009, the team formed a new company in Garching, UltraFast Innovations (www.ultrafast-innovations.com), a spinoff from the excellence cluster, Munich-Centre for Advanced Photonics (www.munich-photonics.de). MPQ and LMU each have a 50% share in the company, which designs and manufactures custom optics.
The optical components offered by UltraFast Innovations are suitable for almost all areas of laser technology. The scientists test the new developments in their own research work, an aspect customers very much appreciate.
We can only guess at the enormously diverse range of potential applications that will emerge from new developments in ultrashort laser pulses and improvements in optical components in laser technology. Almost every new technological advance brings with it new applications, expanding ever further the reach of our understanding.
SPIE Professional April 2010: Open access article
Monday, March 22, 2010
Landmarks: Lasing with Electrons
Traditional lasers have been essential in science and technology, but each one is limited in the wavelengths at which it can operate. Two papers published in Physical Review Letters in 1976 and 1977 described a wholly new kind of laser that could in principle operate over a wide range of wavelengths. Today, these so-called free electron lasers provide high intensity from microwaves all the way up to x rays, with applications ranging across biology, chemistry, and physics.
Conventional lasers rely on the process of stimulated emission, described by Albert Einstein. Electromagnetic radiation of the correct wavelength triggers atoms or molecules in an excited state to emit more radiation of the same wavelength, and the emitted radiation is in phase (or "coherent") with the triggering radiation [see 2005 Focus story, Invention of the Maser and Laser].
In 1971, John M. J. Madey of Stanford University in California showed theoretically that stimulated emission can also occur with bremsstrahlung, the radiation that a charged particle emits when forced to follow a curved path [1]. He considered a beam of electrons traveling close to the speed of light through a magnetic field oriented perpendicular to its path. He assumed a field that varies periodically in a way that forces the electrons to wiggle from side to side (or up and down), and as they wiggle, they emit radiation, concentrated in the forward direction.
Madey showed that if radiation of the right frequency travels along the same axis as the electron beam, it can stimulate additional bremsstrahlung radiation with the same frequency and phase. Unlike a conventional laser, however, where the operating frequency is a fixed property of the atoms or molecules at hand, the frequency of stimulated emission from "free electrons" depends only on the energy of the electrons and the magnetic field periodicity. So the wavelength can in principle be adjusted over a very wide range, just by varying the electron beam energy.
It was five years before Madey and several Stanford colleagues demonstrated the effect and reported it in Physical Review Letters. To provide a periodic magnetic field with the necessary strength and structure, the team built a superconducting electromagnet just over 5 meters long, with a period of 3.2 centimeters. Through the center of this magnet they directed a pulsed beam of electrons with energy of about 24 MeV from the Stanford Linear Accelerator Center (SLAC), along with laser light with a 10.6-micron wavelength. They observed a 7 percent amplification of the laser light when the electron beam energy had the right value to allow stimulated emission at the laser frequency.
The following year, Madey and colleagues published their account of the first free electron laser (FEL). It used the same apparatus as the earlier experiment, with a beam energy of 43.5 MeV, but now enclosed between two mirrors and without an external light source. Radiation generated spontaneously by the electron beam reflected back and forth between the mirrors, stimulating further emission to form a strong beam at an infrared wavelength of 3.4 microns.
Although Madey couched his theory in quantum mechanical terms, the working of FELs is now almost always described classically, says Joe Frisch of SLAC's Linac Coherent Light Source (LCLS), an FEL operating at x-ray wavelengths. In this picture, there is an interaction between the sideways motion of electrons and the radiation's sideways electric field that ultimately causes the electrons to form into "microbunches" spaced just a wavelength apart. Each bunch is at the same point in the wave, so their wiggling creates additional waves that are guaranteed to be in synch with the radiation that is already present.
At the LCLS, says Frisch, the electron-wiggling magnets (the undulators) have a 3-centimeter period, and electrons passing through them form into microbunches over about the first 100 periods. During the rest of their trip, the bunches generate coherent x rays with enough intensity that mirrors aren't necessary--a good thing, as no such mirrors exist for x rays. LCLS was the first FEL to generate x rays of sub-nanometer wavelength, with an energy of about 10 keV. Brief, intense pulses of x rays can image individual atoms, making it possible to follow the progress of chemical reactions. FELs are also invaluable in generating high power at infrared frequencies and below, with uses ranging from studies of molecular structure to medical diagnostics.
References:
[1] John M. J. Madey, "Stimulated Emission of Bremsstrahlung in a Periodic Magnetic Field," J. Appl. Phys. 42, 1906 (1971).
Conventional lasers rely on the process of stimulated emission, described by Albert Einstein. Electromagnetic radiation of the correct wavelength triggers atoms or molecules in an excited state to emit more radiation of the same wavelength, and the emitted radiation is in phase (or "coherent") with the triggering radiation [see 2005 Focus story, Invention of the Maser and Laser].
In 1971, John M. J. Madey of Stanford University in California showed theoretically that stimulated emission can also occur with bremsstrahlung, the radiation that a charged particle emits when forced to follow a curved path [1]. He considered a beam of electrons traveling close to the speed of light through a magnetic field oriented perpendicular to its path. He assumed a field that varies periodically in a way that forces the electrons to wiggle from side to side (or up and down), and as they wiggle, they emit radiation, concentrated in the forward direction.
Madey showed that if radiation of the right frequency travels along the same axis as the electron beam, it can stimulate additional bremsstrahlung radiation with the same frequency and phase. Unlike a conventional laser, however, where the operating frequency is a fixed property of the atoms or molecules at hand, the frequency of stimulated emission from "free electrons" depends only on the energy of the electrons and the magnetic field periodicity. So the wavelength can in principle be adjusted over a very wide range, just by varying the electron beam energy.
It was five years before Madey and several Stanford colleagues demonstrated the effect and reported it in Physical Review Letters. To provide a periodic magnetic field with the necessary strength and structure, the team built a superconducting electromagnet just over 5 meters long, with a period of 3.2 centimeters. Through the center of this magnet they directed a pulsed beam of electrons with energy of about 24 MeV from the Stanford Linear Accelerator Center (SLAC), along with laser light with a 10.6-micron wavelength. They observed a 7 percent amplification of the laser light when the electron beam energy had the right value to allow stimulated emission at the laser frequency.
The following year, Madey and colleagues published their account of the first free electron laser (FEL). It used the same apparatus as the earlier experiment, with a beam energy of 43.5 MeV, but now enclosed between two mirrors and without an external light source. Radiation generated spontaneously by the electron beam reflected back and forth between the mirrors, stimulating further emission to form a strong beam at an infrared wavelength of 3.4 microns.
Although Madey couched his theory in quantum mechanical terms, the working of FELs is now almost always described classically, says Joe Frisch of SLAC's Linac Coherent Light Source (LCLS), an FEL operating at x-ray wavelengths. In this picture, there is an interaction between the sideways motion of electrons and the radiation's sideways electric field that ultimately causes the electrons to form into "microbunches" spaced just a wavelength apart. Each bunch is at the same point in the wave, so their wiggling creates additional waves that are guaranteed to be in synch with the radiation that is already present.
At the LCLS, says Frisch, the electron-wiggling magnets (the undulators) have a 3-centimeter period, and electrons passing through them form into microbunches over about the first 100 periods. During the rest of their trip, the bunches generate coherent x rays with enough intensity that mirrors aren't necessary--a good thing, as no such mirrors exist for x rays. LCLS was the first FEL to generate x rays of sub-nanometer wavelength, with an energy of about 10 keV. Brief, intense pulses of x rays can image individual atoms, making it possible to follow the progress of chemical reactions. FELs are also invaluable in generating high power at infrared frequencies and below, with uses ranging from studies of molecular structure to medical diagnostics.
References:
[1] John M. J. Madey, "Stimulated Emission of Bremsstrahlung in a Periodic Magnetic Field," J. Appl. Phys. 42, 1906 (1971).
Saturday, March 20, 2010
Graphene makes ultrafast laser
Researchers at the University of Cambridge in the UK and CNRS in Grenoble, France, have fabricated an ultrafast "mode-locked" graphene laser. The result – which comes as quite a surprise, given the absence of a band gap in graphene – paves the way to photonic devices based on the material.Since its discovery in 2004, graphene has continued to amaze scientists thanks to its unique electronic and mechanical properties that make it useful for a host of device applications. The "wonder material", as it is called, might even replace silicon as the electronic material of choice in the future. Graphene consists of a planar single sheet of carbon arranged in a honeycombed lattice and electrons travel through the material at extremely high speeds thanks to the fact that they behave like relativistic, or "Dirac", particles with no rest mass.
Now, Andrea Ferrari and colleagues say that graphene might be used in optoelectronics applications too, by demonstrating an ultrafast laser made from the material.
Ultrafast lasers are widely used in science and technology, and there is an increasing demand for compact, tunable laser sources. Today, the dominating technology in so-called mode-locked lasers – that is, lasers that produce ultrashort pulses at a very high repetition rate – is based on semiconductor saturable absorber mirrors (SESAMs). However, such devices are complicated and expensive to make, and are severely limited in their bandwidth.
Graphene mode-lockers
The new ultrafast laser exploits graphene and graphene layers as mode-lockers.
"In principle, this is quite a surprising result because graphene has no band gap, which is a key requirement for mode-locking in SESAMs," said Ferrari.
The team studied how light is absorbed in graphene and how photo-excited charge carriers behave in the material. In particular, they highlighted the key role of "Pauli blocking" in saturating the light absorption. Because of the Pauli exclusion principle, when pumping of electrons in the excited state is quicker than the rate at which they relax, the absorption saturates. This is because no more electrons can be excited until there is "space" available for them in the excited state.
Since the Dirac electrons in graphene linearly disperse, this means that it is the most wideband saturable light absorber ever, far out-passing the bandwidth provided by any other known material.
The researchers made their laser by starting with a graphene-polymer composite, obtained from a solution of graphene. Next, they placed this composite between two optical fibres in a laser cavity.
"Graphene is the ideal wideband saturable absorber, able to operate from the UV to visible and far-infrared wavelengths," Ferrari told our sister website nanotechweb.org. "Our graphene-based ultrafast laser, which harnesses the wideband optical nonlinearity of graphene, with no need for band gap engineering, extends the practical application of this novel material from nanoelectronics to optoelectronics and integrated photonics."
The team are now in the process of optimizing a fully functioning wideband tunable laser based on graphene, as well as trying similar experiments with graphene oxide.
The work was reported in ACS Nano.
Friday, January 22, 2010
Disk Laser Technology
A disk laser or active mirror is a type of solid-state laser characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium. It was introduced in the 1990s by the group of Adolf Giesen at the University of Stuttgart, Germany.
The gain medium of a thin-disk laser is a laser crystal (often Yb:YAG) in the form of a disk with a thickness of 100-200 µm, which is fixed on a water-cooled heat sink. The cooled end face has a dielectric coating that reflects both the laser radiation and the pump radiation.
The heat is extracted dominantly through the cooled end face, and because the disk thickness is considerably smaller than the laser beam diameter, the heat flow is largely in the direction of the beam, rather than in a transverse direction, as for a laser rod. As a consequence, thermal lensing is weak. The figure 1 illustrates the difference between the two types. Hence, the beam quality achievable with Disk Lasers can be much higher than that of a rod system, improving the Beam Parameter Product (BPP) up to 6 times.
The small disk thickness, as required to limit the heating, leads to incomplete pump absorption in a double pass. Therefore, one usually uses some multipass pumping scheme, which can be realized with very compact optics.
Due to improvements in the area of semiconductor pumping diodes the potential of Disk Lasers is not exhausted. While the first generation "only" extracted 1kW of laser power out of one disk, today's generation already generates 2kW out of one disk crystal. Still, the potential for this technology is not limited and expected to increase to 4kW per disk towards the end of 2008. Further, by combining several individual disk cavities, as illustrated in Figure 2,
the total available laser power of a Disk Laser is virtually unlimited. The pumping beam from diode pumping stacks is reflected multi-fold via mirrors inside the cavity to pass up to 20 times through the disk. The disk "converts" the optical pumping light into a laser beam for processing. Based on an existing 4-cavity design, a laser power of 16kW will soon be available. The beauty of this Disk Laser principle over the fiber laser principle is that there are no losses in beam quality when scaling up laser power. These improvements in beam quality and power also lead to significant advantages for the design of processing optics and allowed the development of high-power scanner optics.
It is hardly necessary to mention that indispensable features known from conventional lamp-pumped lasers have not changed: Disk Lasers offer closed-loop power control, are insensitive against back reflections returning from the workpiece, their availability (uptime) is greater than 99 per cent and due to their modular construction all components can be replaced and maintained in the field. Last, but not least, for users of Disk Laser this means that not only the performance of such devices improves, but prices for say a 4 kW Disk Laser are falling because less cavities are required to generate the same laser power.
Q switching is possible with high pulse energies but not with very short pulses because the laser gain is quite limited.
The gain medium of a thin-disk laser is a laser crystal (often Yb:YAG) in the form of a disk with a thickness of 100-200 µm, which is fixed on a water-cooled heat sink. The cooled end face has a dielectric coating that reflects both the laser radiation and the pump radiation.
The heat is extracted dominantly through the cooled end face, and because the disk thickness is considerably smaller than the laser beam diameter, the heat flow is largely in the direction of the beam, rather than in a transverse direction, as for a laser rod. As a consequence, thermal lensing is weak. The figure 1 illustrates the difference between the two types. Hence, the beam quality achievable with Disk Lasers can be much higher than that of a rod system, improving the Beam Parameter Product (BPP) up to 6 times.The small disk thickness, as required to limit the heating, leads to incomplete pump absorption in a double pass. Therefore, one usually uses some multipass pumping scheme, which can be realized with very compact optics.
Due to improvements in the area of semiconductor pumping diodes the potential of Disk Lasers is not exhausted. While the first generation "only" extracted 1kW of laser power out of one disk, today's generation already generates 2kW out of one disk crystal. Still, the potential for this technology is not limited and expected to increase to 4kW per disk towards the end of 2008. Further, by combining several individual disk cavities, as illustrated in Figure 2,
the total available laser power of a Disk Laser is virtually unlimited. The pumping beam from diode pumping stacks is reflected multi-fold via mirrors inside the cavity to pass up to 20 times through the disk. The disk "converts" the optical pumping light into a laser beam for processing. Based on an existing 4-cavity design, a laser power of 16kW will soon be available. The beauty of this Disk Laser principle over the fiber laser principle is that there are no losses in beam quality when scaling up laser power. These improvements in beam quality and power also lead to significant advantages for the design of processing optics and allowed the development of high-power scanner optics.It is hardly necessary to mention that indispensable features known from conventional lamp-pumped lasers have not changed: Disk Lasers offer closed-loop power control, are insensitive against back reflections returning from the workpiece, their availability (uptime) is greater than 99 per cent and due to their modular construction all components can be replaced and maintained in the field. Last, but not least, for users of Disk Laser this means that not only the performance of such devices improves, but prices for say a 4 kW Disk Laser are falling because less cavities are required to generate the same laser power.
Q switching is possible with high pulse energies but not with very short pulses because the laser gain is quite limited.
Sunday, January 10, 2010
Two Photon Absorption
The amount of light absorbed by a substance under normal single photon conditions is given by Beer’s Law, in which the amount of absorbed light is (in the weak absorption limit) proportional to the absorption cross-section of the molecule, s, the pathlength, l, and the concentration of absorbing species, C. In Two Photon Absorption (2PA), the absorption is proportional to the square of the light intensity. As a consequence, 2PA occurs only for very intense light, which, in common application, occurs at the focus of a laser beam. The photo at right shows fluorescence of a dye following 1PA and 2PA. The laser at the top of the cuvette is exciting the dye by 1PA causing yellow fluorescence emission. The emission can be seen clearly along the whole focussed laser path. The laser at the bottom of the cuvette is exciting the dye by 2PA, which causes the same yellow fluorescence. This time emission only occurs at the focal point of the laser because of the (intensity)2 dependence of the 2PA.
Simulated effects of excitation wavelength and numerical aperture on the dimensions of the 2PA volume. a, Normalized distributions of laser intensity-squared in the x-y and x-z plane for three different numerical apertures of water-immersion objective lenses at an excitation wavelength of 850 nm. Intensities-squared at lateral (x,y,0) and axial (x,0,z) positions were calculated using an ellipsoidal Gaussian approximation to the diffraction limited focus7,11 and expressed as the fraction of the intensity-squared at the focal point [I(0,0,0)2=1]. Color-coded contour plots depict isointensity lines in the x-z and x-y plane at the levels of 0.1, 0.3, 0.5, 0.7, and 0.9 of I(0,0,0)2. Note different scales for each panel. b, Dependence of 2PA volume on numerical aperture of the objective lens and illumination wavelength. Values were obtained by approximating the intensity-squared distribution as a three-dimensional Gaussian volume.12 For all calculations, it is assumed that the objective lens is uniformly illuminated (overfilled) and that no saturation of the fluorescence excitation process occurs. NA indicates numerical aperture.
Figure right provides a simplified illustration of the difference between single photon and two-photon activated processing. A material is polymerized along the trace of the moving laser focus, thus enabling fabrication of any desired polymeric 3D pattern by direct “recording” into the volume of photosensitive material. In a subsequent processing step the material, which was not exposed to the laser radiation, and therefore, stayed unpolymerized, is removed and the fabricated structure is revealed. The material sensitive in the UV range (λUV) can be polymerized by irradiation with the infra-red light of approximately double wavelength (λIR=2λUV), under the condition that the intensity of the radiation is high enough to initiate two-photon absorption.
Wednesday, November 25, 2009
First Pump-Probe Experiment at Linac Coherent Light Source Completed
(PhysOrg.com) -- The first experiment using the Linac Coherent Light Source to illuminate molecules via a "pump-probe" technique has been completed by an international team of more than 30 scientists from institutions including Lawrence Berkeley National Laboratory, LCLS and the joint SLAC/Stanford PULSE Institute. Ryan Coffee, physicist with the LCLS Laser Group, presented initial results in a seminar at SLAC on Wednesday, November 18.
Pump-probe experiments use one laser pulse, in this case an infrared pulse, to pump energy into a sample and then probe it with another laser pulse, in this case an LCLS X-ray pulse. Such experiments are ideal for looking at atomic and molecular interactions, which take place in tiny fractions of a second. The LCLS probe pulses were as short as a few quadrillionths of a second and a billion times brighter than any X-ray source produced in a laboratory.
Coffee and his colleagues looked at the quantum behavior of electrons in nitrogen molecules, N2. The results represent a step toward a fundamental understanding of how nature converts light into chemical energy and might one day help revolutionize solar power, Coffee said.
Nitrogen atoms distribute their electrons between a lower and a higher energy shell. Using X-rays, the team picked off two electrons from the lower level, allowing a higher shell electron to descend and fill the vacancy. The energy released during this downward plunge ejected another electron from the atom, a phenomenon known as the Auger effect.
The team wanted to study how the nitrogen molecules' orientation affected this reaction. To do this, they used the infrared laser to line up the nitrogen molecules so that they were all facing the same direction.
"In a sense, we tried to make the gas act a little bit like a crystal," Coffee said.
After hitting the nitrogen with X-rays, the researchers detected electrons flying off and measured how the molecules' alignment with respect to the X-rays influenced the Auger effect. They observed numerous features that had strong dependence on the molecules’ direction. The results are currently being prepared for publication.
Future work will focus on how atomic bonds change as molecules either break apart or rearrange. Coffee thinks such work will lead to a deeper understanding of how nature converts light into energy. Ultimately, he hopes the results will lead to technology that will help humans generate power from the sun.
"I'm going for the solar power revolution, though I don't know where it will come from," he said. His gut feeling is that the important atoms to look at are carbon, nitrogen and oxygen.
"That's where energy in nature comes from," he said.
Coffee added that the team owes a debt of gratitude to the LCLS Controls, Accelerator, and Laser Groups, who made the experiment's success possible.
Pump-probe experiments use one laser pulse, in this case an infrared pulse, to pump energy into a sample and then probe it with another laser pulse, in this case an LCLS X-ray pulse. Such experiments are ideal for looking at atomic and molecular interactions, which take place in tiny fractions of a second. The LCLS probe pulses were as short as a few quadrillionths of a second and a billion times brighter than any X-ray source produced in a laboratory.
Coffee and his colleagues looked at the quantum behavior of electrons in nitrogen molecules, N2. The results represent a step toward a fundamental understanding of how nature converts light into chemical energy and might one day help revolutionize solar power, Coffee said.Nitrogen atoms distribute their electrons between a lower and a higher energy shell. Using X-rays, the team picked off two electrons from the lower level, allowing a higher shell electron to descend and fill the vacancy. The energy released during this downward plunge ejected another electron from the atom, a phenomenon known as the Auger effect.
The team wanted to study how the nitrogen molecules' orientation affected this reaction. To do this, they used the infrared laser to line up the nitrogen molecules so that they were all facing the same direction.
"In a sense, we tried to make the gas act a little bit like a crystal," Coffee said.
After hitting the nitrogen with X-rays, the researchers detected electrons flying off and measured how the molecules' alignment with respect to the X-rays influenced the Auger effect. They observed numerous features that had strong dependence on the molecules’ direction. The results are currently being prepared for publication.
Future work will focus on how atomic bonds change as molecules either break apart or rearrange. Coffee thinks such work will lead to a deeper understanding of how nature converts light into energy. Ultimately, he hopes the results will lead to technology that will help humans generate power from the sun.
"I'm going for the solar power revolution, though I don't know where it will come from," he said. His gut feeling is that the important atoms to look at are carbon, nitrogen and oxygen.
"That's where energy in nature comes from," he said.
Coffee added that the team owes a debt of gratitude to the LCLS Controls, Accelerator, and Laser Groups, who made the experiment's success possible.
Monday, November 09, 2009
Laser creates record-breaking protons
An international group of physicists working at the Los Alamos Laboratory in the US has used a laser to generate 67.5 MeV protons – the highest-energy protons yet produced in this way. Their work points the way to new laser-based devices for proton therapy, which would be far smaller and cheaper than existing particle-accelerator sources.
When a high-energy proton beam travels through the human body it deposits most of its energy within a small volume, the size and location of which can be calculated to great precision. As a result, protons offer a distinct advantage over other forms of radiation used to destroy tumour cells because they cause less damage to surrounding healthy tissue. Unfortunately, the accelerators needed to generate the protons can cover thousands of square metres and cost some $100m. This has limited the number of proton-therapy facilities available and patients often have to travel considerable distances to be treated in this way.
Some physicists believe that a laser-based proton generator could be made for about one tenth of the cost of a conventional accelerator and be small enough to be contained within a classroom-sized laboratory. The idea is that ultra-powerful laser pulses knock electrons out of the atoms within a tiny target, causing the electrons to accumulate on the target's rear surface. This sets up an electric field across the target, accelerating the resultant ions and forcing them to leave the material as a very high-energy beam.
Energy is a problem
In practice, however, some of the world's most powerful petawatt (10^15 W) lasers have only been able to generate protons with a maximum energy of about 58 megaelectronvolts (MeV). While tumours of the eye can be treated using protons of 60–70 MeV, deeper tumours require energies of about 300 MeV.
The latest breakthrough was carried out by Kirk Flippo of Los Alamos, Sandrine Gaillard of the Forschungszentrum Dresden–Rossendorf research centre (FZD) in Germany and colleagues, who used Los Alamos' Trident laser to generate 67.5 MeV protons. The work relies on a novel target design – an anvil-shaped piece of copper comprising a cone around 100 µm long with a 100 µm flat disc across perched on its tip. Flippo's team directed the laser beam to the inside of the cone, liberating electrons that were guided to the tip and which set up an electric field that accelerated protons away from the disc. The researchers claim that this arrangement is far more efficient than the thin films used in previous experiments – they used 80 J laser pulses, whereas the previous record of 58 MeV involved 450 J laser pulses.
Team-member Michael Bussmann of the FZD says that this significant step forward in maximum proton energy was also made possible by increasing the intensity of the main part of each pulse relative to the "pre-pulse", which precedes the main pulse and can damage the target.
Not enough protons
However, it might take a decade or more before laser-generated protons can be used to combat cancer. Another major challenge is that Trident and the other more intense lasers simply require too much energy to be able to function at the roughly 10 Hz pulse rate needed to produce enough protons for cancer therapy.
According to Bussmann, reaching the sought-after high production rates will be a matter of getting the target right. One possibility will be some kind of refinement of the anvil shape, he says. Others, however, believe that the answer lies in reducing the size of the target, allowing electrons to be heated and ejected from the target much more quickly and therefore with a more uniform energy distribution, in other words leading to fewer low-energy electrons. "We already have enough energy in our lasers, the question is how can we use it more efficiently," says Bussmann. "Nobody has the final idea right now," he said, "but we are in a position to test all these different theories and see which works best."
Looking beyond cancer therapy, Flippo believes that such proton sources could also be used to create medical isotopes and employed to generate neutrons for research in condensed-matter physics and other areas of science. They might also be used to search for nuclear materials inside cargo, given that the characteristics of a proton beam are altered in a well defined way by radioactive substances.
The research was presented at the annual meeting of the Division of Plasma Physics of the American Physical Society, held in Atlanta on 2–6 November.
When a high-energy proton beam travels through the human body it deposits most of its energy within a small volume, the size and location of which can be calculated to great precision. As a result, protons offer a distinct advantage over other forms of radiation used to destroy tumour cells because they cause less damage to surrounding healthy tissue. Unfortunately, the accelerators needed to generate the protons can cover thousands of square metres and cost some $100m. This has limited the number of proton-therapy facilities available and patients often have to travel considerable distances to be treated in this way.Some physicists believe that a laser-based proton generator could be made for about one tenth of the cost of a conventional accelerator and be small enough to be contained within a classroom-sized laboratory. The idea is that ultra-powerful laser pulses knock electrons out of the atoms within a tiny target, causing the electrons to accumulate on the target's rear surface. This sets up an electric field across the target, accelerating the resultant ions and forcing them to leave the material as a very high-energy beam.
Energy is a problem
In practice, however, some of the world's most powerful petawatt (10^15 W) lasers have only been able to generate protons with a maximum energy of about 58 megaelectronvolts (MeV). While tumours of the eye can be treated using protons of 60–70 MeV, deeper tumours require energies of about 300 MeV.
The latest breakthrough was carried out by Kirk Flippo of Los Alamos, Sandrine Gaillard of the Forschungszentrum Dresden–Rossendorf research centre (FZD) in Germany and colleagues, who used Los Alamos' Trident laser to generate 67.5 MeV protons. The work relies on a novel target design – an anvil-shaped piece of copper comprising a cone around 100 µm long with a 100 µm flat disc across perched on its tip. Flippo's team directed the laser beam to the inside of the cone, liberating electrons that were guided to the tip and which set up an electric field that accelerated protons away from the disc. The researchers claim that this arrangement is far more efficient than the thin films used in previous experiments – they used 80 J laser pulses, whereas the previous record of 58 MeV involved 450 J laser pulses.
Team-member Michael Bussmann of the FZD says that this significant step forward in maximum proton energy was also made possible by increasing the intensity of the main part of each pulse relative to the "pre-pulse", which precedes the main pulse and can damage the target.
Not enough protons
However, it might take a decade or more before laser-generated protons can be used to combat cancer. Another major challenge is that Trident and the other more intense lasers simply require too much energy to be able to function at the roughly 10 Hz pulse rate needed to produce enough protons for cancer therapy.
According to Bussmann, reaching the sought-after high production rates will be a matter of getting the target right. One possibility will be some kind of refinement of the anvil shape, he says. Others, however, believe that the answer lies in reducing the size of the target, allowing electrons to be heated and ejected from the target much more quickly and therefore with a more uniform energy distribution, in other words leading to fewer low-energy electrons. "We already have enough energy in our lasers, the question is how can we use it more efficiently," says Bussmann. "Nobody has the final idea right now," he said, "but we are in a position to test all these different theories and see which works best."
Looking beyond cancer therapy, Flippo believes that such proton sources could also be used to create medical isotopes and employed to generate neutrons for research in condensed-matter physics and other areas of science. They might also be used to search for nuclear materials inside cargo, given that the characteristics of a proton beam are altered in a well defined way by radioactive substances.
The research was presented at the annual meeting of the Division of Plasma Physics of the American Physical Society, held in Atlanta on 2–6 November.
Monday, November 02, 2009
Electron self-injection into an evolving plasma bubble
Just five years ago, experimentalists finally demonstrated that such laser-plasma accelerators could produce monoenergetic, collimated electron beams with quality comparable to conventional accelerators. The secret was for the laser to produce a "bubble" almost completely devoid of electrons in its immediate wake that captured electrons from the surrounding plasma and accelerated them in an exceptionally uniform way. Yet the precise mechanism by which the bubble captured these electrons and accelerated them with such uniformity has remained one of the outstanding mysteries of this field.
Now new theoretical work by scientists from the University of Texas and Commissariat à l'Énergie Atomique (CEA, France), to be reported at the 2009 APS Division of Plasma Physics Annual Meeting, has shed light on this mystery. Formation of the exceptional quality electron beam is attributed to the evolution of the bubble shape which, in turn, is directly associated with the nonlinear evolution of the driving laser pulse (nonlinear focusing and defocusing).
The basic premise of this work is that the size of the bubble—the cavity of electron density traveling over the positive ion background with nearly the speed of light—is determined by the spot size of the driving laser pulse. Plasma nonlinearities cause the laser to focus and defocus in the course of propagation. Once the laser diffracts, the bubble expands. Electrons that constitute a dense electron shell surrounding the bubble move with relativistic speeds and thus have high inertia. As a consequence, some of them become too heavy to follow the expanding shell; they fall inside the bubble, stay inside till the end of the plasma (i.e. get trapped) and finally gain multi-GeV energy.
The trapped charge is proportional to the bubble growth rate. Once the laser becomes self-guided, and the spot size oscillations saturate, the injection process clamps. Simultaneously, longitudinal non-uniformity of the accelerating gradient equalizes the trapped electron energy. This scenario of self-injection and monoenergetic bunch formation is discovered and explored in fine detail in the 3-D particle-in-cell simulations. This is fundamentally different from the previous work which concentrated on either one-dimensional models of electron trapping or on the reduced description of transverse plasma wave breaking in planar 2-D geometry.
The discussed mechanism of electron self-injection is very robust in experiments with the high-power laser (tens of terawatts to petawatt). In addition, an appropriate modification of the plasma density (e.g. using a thin dense slab as a nonlinear lens for the laser) may cause the laser to self-focus and defocus faster, which results in a single self-injection event. This kind of laser beam manipulation may lead to the generation of a 2.5 GeV mono-energetic (~1% energy spread) electron bunch containing ~1010 electrons in a future experiment with the recently commissioned Texas Petawatt (TPW) laser - the most powerful laser in the world. Electrons with 2.5 GeV of energy are traveling at 99.999998% of the speed of light. Electron beams with such unique properties are clearly beneficial for medical applications, radiation physics, material science, and homeland security.
Source: American Physical Society
Now new theoretical work by scientists from the University of Texas and Commissariat à l'Énergie Atomique (CEA, France), to be reported at the 2009 APS Division of Plasma Physics Annual Meeting, has shed light on this mystery. Formation of the exceptional quality electron beam is attributed to the evolution of the bubble shape which, in turn, is directly associated with the nonlinear evolution of the driving laser pulse (nonlinear focusing and defocusing).The basic premise of this work is that the size of the bubble—the cavity of electron density traveling over the positive ion background with nearly the speed of light—is determined by the spot size of the driving laser pulse. Plasma nonlinearities cause the laser to focus and defocus in the course of propagation. Once the laser diffracts, the bubble expands. Electrons that constitute a dense electron shell surrounding the bubble move with relativistic speeds and thus have high inertia. As a consequence, some of them become too heavy to follow the expanding shell; they fall inside the bubble, stay inside till the end of the plasma (i.e. get trapped) and finally gain multi-GeV energy.
The trapped charge is proportional to the bubble growth rate. Once the laser becomes self-guided, and the spot size oscillations saturate, the injection process clamps. Simultaneously, longitudinal non-uniformity of the accelerating gradient equalizes the trapped electron energy. This scenario of self-injection and monoenergetic bunch formation is discovered and explored in fine detail in the 3-D particle-in-cell simulations. This is fundamentally different from the previous work which concentrated on either one-dimensional models of electron trapping or on the reduced description of transverse plasma wave breaking in planar 2-D geometry.
The discussed mechanism of electron self-injection is very robust in experiments with the high-power laser (tens of terawatts to petawatt). In addition, an appropriate modification of the plasma density (e.g. using a thin dense slab as a nonlinear lens for the laser) may cause the laser to self-focus and defocus faster, which results in a single self-injection event. This kind of laser beam manipulation may lead to the generation of a 2.5 GeV mono-energetic (~1% energy spread) electron bunch containing ~1010 electrons in a future experiment with the recently commissioned Texas Petawatt (TPW) laser - the most powerful laser in the world. Electrons with 2.5 GeV of energy are traveling at 99.999998% of the speed of light. Electron beams with such unique properties are clearly beneficial for medical applications, radiation physics, material science, and homeland security.
Source: American Physical Society
Monday, October 26, 2009
Laser recreates X-rays emitted by a black hole
Physicists have used high-power lasers to recreate X-ray spectra emanating from some black holes and neutron stars. Conclusions drawn from the experiment appear to conflict with previous interpretations of astronomical data, suggesting that we may have to rethink our view of the structure surrounding black holes and neutrons stars.
Large quantities of X-rays are produced when a black hole or neutron star sucks in matter from a companion star, creating a ring of matter known as an accretion disc. As matter spirals into the black hole or neutron star, gravitational energy is converted into kinetic energy and heat. The intense radiation that is released travels outwards (in the form of photons) and ionizes material closer to the outer edge of the accretion disc – creating an X-ray emitting plasma.
Interpreting the X-ray spectrum of such a plasma is key to understanding the physics of such systems, because it is impossible for astronomers to directly measure its temperature, density and pressure. It has also proven very difficult to recreate such a "photo-ionized" plasma here on Earth because it requires an extremely hot source of radiation.
But now researchers in Japan, Korea and China are helping to address this weakness by studying the spectra of plasmas created in the lab. Such spectra are very similar to that produced by Cygnus X-3, a black hole and a companion star with highly ionized silicon ions on its surface. A similar X-ray spectrum has also been recorded from Vela X-1, a neutron-star binary system.
The researchers produced their X-ray spectra at the GEKKO-XII laser facility, which is located at Osaka University, Japan. The system combines a 10 TW laser that is capable of producing nanosecond pulses from twelve beams with a 10 PW laser that can deliver four picosecond beams.
"We used 12 nanosecond laser beams with wavelength, energy and pulse duration of 0.53 µm, 4 kJ in total and 1.2 ns [respectively]," explained Shinsuke Fujioka from Osaka University, who proposed and organized the experiment.
The beams are fired at a tiny plastic capsule, causing it to implode. "As it shrinks, a hot and dense plasma core forms inside the capsule," says Fujioka. The radiation produced then photo-ionizes a nearby sample of cold silicon gas.
Fujioka says that the shape of their X-ray spectra is quite similar to that recorded by astronomers. However, interpretations of the origin of characteristic lines emissions differ.
Astrophysicists claim that an X-ray peak at 1.84 keV stems from a forbidden transition of silicon ions. But Fujioka says that calculations performed by his team – which consider experimental measurements of the temperature and density of the plasma – suggest that the peak is associated with a different resonance transition of silicon ions. However, the researchers admit that they cannot provide a definite explanation for the origin of this peak. That is because the radiation flux produced in the laboratory lasts for tiny fractions of a second, while that produced by compact astrophysical objects is continuous.
The work is reported in Nature Physics and, writing in a companion piece, Paul Drake of the University of Michigan described the technique as having "great potential for further development," because it allows the energy of the photon source to be varied over a wide range while allowing a great deal of control over the photo-ionized material. However, Drake also cautions that more work must be done in terms of characterizing the physical properties of the resulting plasmas.
Fujioka says that the team may now turn its attention to investigations of the absorption of intense beams of X-rays. It is widely believed that the X-ray absorption rate in materials and plasmas is independent of the intensity of the beam, but they suspect that a plasma may become transparent in incredibly intense X-ray beams. If this is the case, it will modify our understanding of how plasmas behave in supernovae.
Large quantities of X-rays are produced when a black hole or neutron star sucks in matter from a companion star, creating a ring of matter known as an accretion disc. As matter spirals into the black hole or neutron star, gravitational energy is converted into kinetic energy and heat. The intense radiation that is released travels outwards (in the form of photons) and ionizes material closer to the outer edge of the accretion disc – creating an X-ray emitting plasma.Interpreting the X-ray spectrum of such a plasma is key to understanding the physics of such systems, because it is impossible for astronomers to directly measure its temperature, density and pressure. It has also proven very difficult to recreate such a "photo-ionized" plasma here on Earth because it requires an extremely hot source of radiation.
But now researchers in Japan, Korea and China are helping to address this weakness by studying the spectra of plasmas created in the lab. Such spectra are very similar to that produced by Cygnus X-3, a black hole and a companion star with highly ionized silicon ions on its surface. A similar X-ray spectrum has also been recorded from Vela X-1, a neutron-star binary system.
The researchers produced their X-ray spectra at the GEKKO-XII laser facility, which is located at Osaka University, Japan. The system combines a 10 TW laser that is capable of producing nanosecond pulses from twelve beams with a 10 PW laser that can deliver four picosecond beams.
"We used 12 nanosecond laser beams with wavelength, energy and pulse duration of 0.53 µm, 4 kJ in total and 1.2 ns [respectively]," explained Shinsuke Fujioka from Osaka University, who proposed and organized the experiment.
The beams are fired at a tiny plastic capsule, causing it to implode. "As it shrinks, a hot and dense plasma core forms inside the capsule," says Fujioka. The radiation produced then photo-ionizes a nearby sample of cold silicon gas.
Fujioka says that the shape of their X-ray spectra is quite similar to that recorded by astronomers. However, interpretations of the origin of characteristic lines emissions differ.
Astrophysicists claim that an X-ray peak at 1.84 keV stems from a forbidden transition of silicon ions. But Fujioka says that calculations performed by his team – which consider experimental measurements of the temperature and density of the plasma – suggest that the peak is associated with a different resonance transition of silicon ions. However, the researchers admit that they cannot provide a definite explanation for the origin of this peak. That is because the radiation flux produced in the laboratory lasts for tiny fractions of a second, while that produced by compact astrophysical objects is continuous.
The work is reported in Nature Physics and, writing in a companion piece, Paul Drake of the University of Michigan described the technique as having "great potential for further development," because it allows the energy of the photon source to be varied over a wide range while allowing a great deal of control over the photo-ionized material. However, Drake also cautions that more work must be done in terms of characterizing the physical properties of the resulting plasmas.
Fujioka says that the team may now turn its attention to investigations of the absorption of intense beams of X-rays. It is widely believed that the X-ray absorption rate in materials and plasmas is independent of the intensity of the beam, but they suspect that a plasma may become transparent in incredibly intense X-ray beams. If this is the case, it will modify our understanding of how plasmas behave in supernovae.
Sunday, September 13, 2009
Airborne laser ready for flight tests
IT SHOULD be the moment of truth for the Airborne Laser (ABL). In the coming months, the multibillion-dollar laser built into a customised Boeing 747 will try to shoot a ballistic missile as it rises above the clouds.Don't expect instant reports of success, though. Instead, if all goes to plan, we're likely to hear about a series of incremental improvements.
Developed by the US Department of Defense's Missile Defense Agency (MDA), the ABL aims to focus a beam of laser energy in the megawatt range for several seconds onto a missile at a "militarily significant distance" - more than 100 kilometres.
So far, the laser has only operated at near full power on the ground. On 18 August it was fired successfully from the air, but at reduced power. That, however, was no mean feat: aircraft vibrations play havoc with the precisely aligned optical components needed to generate a laser beam.
Firing at full power poses other challenges too. At powers high enough to destroy missiles, any surface contamination or tiny flaw in the laser optics can absorb so much heat that they crack or shatter.
High-power laser beams also heat the air they pass through, creating perturbations that can disperse or divert the beam. To counteract those effects, the ABL uses an adaptive system that senses atmospheric changes along its path and makes optical adjustments to compensate.
To test that system, the MDA plans a series of increasingly powerful shots at modified ballistic missiles loaded with sensors to measure the distribution of laser power on the target. Engineers will assess each shot's performance and use the results to fine-tune the adaptive optics. Once this is done, the MDA will test the laser again in varying conditions, and attempt to destroy actual missiles. The first of these tests is planned to take place late this year, with two more to follow in early 2010, according to an MDA spokeswoman.
A sister project, the Advanced Tactical Laser, which aims to use an airborne high-powered laser to hit targets on the ground, recently completed its first successful test. With future funding dependent upon the success of these tests, the pressure is on the ABL team to prove its efficacy.
Tuesday, September 08, 2009
Lasers Generate Sound in H2O
A new technology that uses flashes of laser light to remotely create underwater acoustics is being developed by scientists at the Naval Research Laboratory. The new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications, including undersea communications, navigation and acoustic imaging.
Dr. Ted Jones, a physicist in the Plasma Physics Division, is leading a team of researchers from the Plasma Physics, Acoustics, and Marine Geosciences Divisions in developing this acoustic source.
Efficient conversion of light into sound can be achieved by concentrating the light sufficiently to ionize a small amount of water, which then absorbs laser energy and superheats. The result is a small explosion of steam, which can generate a 220 decibel pulse of sound. Optical properties of water can be manipulated with very intense laser light to act like a focusing lens, allowing nonlinear self-focusing (NSF) to take place.
In addition, the slightly different colors of the laser, which travel at different speeds in water due to group velocity dispersion (GVD), can be arranged so that the pulse also compresses in time as it travels through water, further concentrating the light. By using a combination of GVD and NSF, controlled underwater compression of optical pulses can be attained.
The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. Since GVD and NSF effects are much stronger in water than air, a properly tailored laser has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, such as undersea communications from aircraft.
Also, commercially available, high-repetition-rate pulsed lasers, steered by a rapidly movable mirror, can generate arbitrary arrays of phased acoustic sources. On a compact underwater platform with an acoustic receiver, such a setup can rapidly generate oblique-angle acoustic scattering data, for imaging and identifying underwater objects. This would be a significant addition to traditional direct backscattering acoustic data.
For more information, visit: www.nrl.navy.mil
Dr. Ted Jones, a physicist in the Plasma Physics Division, is leading a team of researchers from the Plasma Physics, Acoustics, and Marine Geosciences Divisions in developing this acoustic source.
Efficient conversion of light into sound can be achieved by concentrating the light sufficiently to ionize a small amount of water, which then absorbs laser energy and superheats. The result is a small explosion of steam, which can generate a 220 decibel pulse of sound. Optical properties of water can be manipulated with very intense laser light to act like a focusing lens, allowing nonlinear self-focusing (NSF) to take place.
In addition, the slightly different colors of the laser, which travel at different speeds in water due to group velocity dispersion (GVD), can be arranged so that the pulse also compresses in time as it travels through water, further concentrating the light. By using a combination of GVD and NSF, controlled underwater compression of optical pulses can be attained.
The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. Since GVD and NSF effects are much stronger in water than air, a properly tailored laser has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, such as undersea communications from aircraft.
Also, commercially available, high-repetition-rate pulsed lasers, steered by a rapidly movable mirror, can generate arbitrary arrays of phased acoustic sources. On a compact underwater platform with an acoustic receiver, such a setup can rapidly generate oblique-angle acoustic scattering data, for imaging and identifying underwater objects. This would be a significant addition to traditional direct backscattering acoustic data.
For more information, visit: www.nrl.navy.mil
Tuesday, September 01, 2009
Laser pulses control single electrons in complex molecules
Physicists of the Max Planck Institute of Quantum Optics (MPQ) in Garching and chemists of the Ludwig-Maximilians-Universität (LMU) in Munich, Germany, succeeded for the first time to use light for controlling single, negatively charged elementary particles in a bunch of electrons. The scientists achieved a major milestone that they aimed for within the excellence cluster "Munich Center for Advanced Photonics" (MAP). They report their results in the journal Physical Review Letters.
Electrons are extremely fast moving particles. In atoms and molecules they move on attosecond timescales. An attosecond is only a billionth of a billionth of a second. With light pulses that last only a few femtoseconds down to attoseconds it is possible to achieve control over these particles and to interact with them on the timescale of their motion. These short light pulses exhibit strong electric and magnetic fields influencing the charged particles. A femtosecond lasts 1000 times longer than an attosecond. In molecules with only a single electron, such as the deuterium molecular ion, their control with such light pulses is relatively easy. This was demonstrated in 2006 by a team of physicists including Professor Marc Vrakking and Dr. Matthias Kling from AMOLF in Amsterdam and Professor Ferenc Krausz in Garching (MPQ).
Scientists led by the junior research group leader Dr. Matthias Kling (MPQ) in collaboration with Professor Marc Vrakking (AMOLF) and Professor Regina de Vivie-Riedle (LMU) have managed to control and monitor the outer electrons from the valence shell of the complex molecule carbon monoxide (CO) utilizing the electric field waveform of laser pulses. Carbon monoxide has 14 electrons. With increasing number of electrons in the molecule the control over single electrons becomes difficult as their states lie energetically very close to each other.
In their experiments the scientists used visible (740 nm) laser pulses with 4 femtoseconds duration. The control was experimentally determined via an asymmetric distribution of C+ and of O+ fragments after the breaking of the molecular bond. The measurement of C+ and O+ fragments implies a dynamic charge shift along the molecular axis in one or the other direction, controlled via the laser pulse.
The femtosecond laser pulses initially detached an electron from a CO molecule. Subsequently the electron was driven by the laser field away from and back to the ion, where it transferred its energy in a collision. The whole process took only ca. 1.7 femtoseconds. "The collision produces an electronic wave packet which induces a directional movement of electrons along the molecular axis," says Regina de Vivie-Riedle. "The excitation and subsequent interaction with the remainder of the intense laser pulse leads to a coupling of electron and nuclear motion and gives a contribution to the observed asymmetry," explains Matthias Kling.
The scientists could also image the structure and form of the outer two electron orbitals of carbon monoxide via the ionization process. The extremely short femtosecond laser pulses allowed the scientists to explore this process in the outermost orbitals. They found the ionization of the molecules to take place with a distinct angular dependence with respect to the laser polarization direction. This observation was found to be in good agreement with theoretical calculations and also gave a contribution to the observed asymmetry. The scientists could show that the strength of this asymmetry strongly depends on the duration of the laser pulses.
With their experiments and calculations, the researchers from Garching and Munich have achieved an important milestone that they aimed for within the excellence cluster "Munich Center for Advanced Photonics" (MAP). The goals were to achieve and observe the control of single electrons within a multi-electron system.
Electrons are present in all important microscopic biological and technical processes. Their extremely fast motion on the attosecond timescale, determines biological and chemical processes and also the speed of microprocessors - technology at the heart of computing. With their experiments the researchers have made a further, important step towards the control of chemical reactions with light. The results are also related to basic research on lightwave electronics aiming at computing speeds on attosecond timescales.
More information: Znakovskaya, P. von den Hoff, S. Zherebtsov, A. Wirth, O. Herrwerth, M.J.J. Vrakking, R. de Vivie-Riedle, M.F. Kling: “Attosecond control of electron dynamics in carbon monoxide”, Physical Review Letters (4. September 2009)
Electrons are extremely fast moving particles. In atoms and molecules they move on attosecond timescales. An attosecond is only a billionth of a billionth of a second. With light pulses that last only a few femtoseconds down to attoseconds it is possible to achieve control over these particles and to interact with them on the timescale of their motion. These short light pulses exhibit strong electric and magnetic fields influencing the charged particles. A femtosecond lasts 1000 times longer than an attosecond. In molecules with only a single electron, such as the deuterium molecular ion, their control with such light pulses is relatively easy. This was demonstrated in 2006 by a team of physicists including Professor Marc Vrakking and Dr. Matthias Kling from AMOLF in Amsterdam and Professor Ferenc Krausz in Garching (MPQ).
Scientists led by the junior research group leader Dr. Matthias Kling (MPQ) in collaboration with Professor Marc Vrakking (AMOLF) and Professor Regina de Vivie-Riedle (LMU) have managed to control and monitor the outer electrons from the valence shell of the complex molecule carbon monoxide (CO) utilizing the electric field waveform of laser pulses. Carbon monoxide has 14 electrons. With increasing number of electrons in the molecule the control over single electrons becomes difficult as their states lie energetically very close to each other.
In their experiments the scientists used visible (740 nm) laser pulses with 4 femtoseconds duration. The control was experimentally determined via an asymmetric distribution of C+ and of O+ fragments after the breaking of the molecular bond. The measurement of C+ and O+ fragments implies a dynamic charge shift along the molecular axis in one or the other direction, controlled via the laser pulse.
The femtosecond laser pulses initially detached an electron from a CO molecule. Subsequently the electron was driven by the laser field away from and back to the ion, where it transferred its energy in a collision. The whole process took only ca. 1.7 femtoseconds. "The collision produces an electronic wave packet which induces a directional movement of electrons along the molecular axis," says Regina de Vivie-Riedle. "The excitation and subsequent interaction with the remainder of the intense laser pulse leads to a coupling of electron and nuclear motion and gives a contribution to the observed asymmetry," explains Matthias Kling.
The scientists could also image the structure and form of the outer two electron orbitals of carbon monoxide via the ionization process. The extremely short femtosecond laser pulses allowed the scientists to explore this process in the outermost orbitals. They found the ionization of the molecules to take place with a distinct angular dependence with respect to the laser polarization direction. This observation was found to be in good agreement with theoretical calculations and also gave a contribution to the observed asymmetry. The scientists could show that the strength of this asymmetry strongly depends on the duration of the laser pulses.
With their experiments and calculations, the researchers from Garching and Munich have achieved an important milestone that they aimed for within the excellence cluster "Munich Center for Advanced Photonics" (MAP). The goals were to achieve and observe the control of single electrons within a multi-electron system.
Electrons are present in all important microscopic biological and technical processes. Their extremely fast motion on the attosecond timescale, determines biological and chemical processes and also the speed of microprocessors - technology at the heart of computing. With their experiments the researchers have made a further, important step towards the control of chemical reactions with light. The results are also related to basic research on lightwave electronics aiming at computing speeds on attosecond timescales.
More information: Znakovskaya, P. von den Hoff, S. Zherebtsov, A. Wirth, O. Herrwerth, M.J.J. Vrakking, R. de Vivie-Riedle, M.F. Kling: “Attosecond control of electron dynamics in carbon monoxide”, Physical Review Letters (4. September 2009)
Monday, August 31, 2009
New study will contribute to better understanding of nuclear ignition
UCSD scientists create computer simulations like the ones above to determine how to successfully achieve controlled, miniaturized nuclear ignition of spherical fuel pellets in laboratory environments using lasers as energy drivers.Under a recent three-year, $510,000 grant from the National Nuclear Security Administration (NNSA), Vu and his colleagues at Los Alamos National Laboratory (NM), Lodestar Research Corporation and the Laboratory for Laser Energetics at the University of Rochester in New York, are using computer simulation tools to figure out how to successfully achieve controlled, miniaturized nuclear ignition of spherical fuel pellets in laboratory environments using lasers as energy drivers.
Vu said the primary lasers for these studies are the Omega laser at the University of Rochester (NY) and the newly built National Ignition Facility at Lawrence Livermore National Laboratory (CA).
"What we would like to do is take the laser and shine the laser onto what we call a hohlraum, a cylindrically shaped black-body radiator made of high-Z materials (typically gold), in the middle of which the miniaturized fuel pellet is placed," he explained. "The material on the wall of the hohlraum absorbs the laser energy, heats up, and becomes a plasma. The plasma in turn irradiates off its newly acquired energy, and the resulting black-body radiation is what drives the miniaturized fuel pellets to nuclear ignition. It's like sunlight hitting the dashboard of a car - the energy of the sunlight is absorbed by the dashboard and is irradiated as heat, essentially electromagnetic radiation on a different wavelength spectrum from the original sunlight. It's a lot of fancy physics. But if you think about in on a fundamental level, it's pretty simple.
Source: University of California - San Diego
Sunday, August 02, 2009
Transparent aluminium is 'new state of matter'
(PhysOrg.com) -- Oxford scientists have created a transparent form of aluminium by bombarding the metal with the world’s most powerful soft X-ray laser. 'Transparent aluminium' previously only existed in science fiction, featuring in the movie Star Trek IV, but the real material is an exotic new state of matter with implications for planetary science and nuclear fusion.
In this week’s Nature Physics an international team, led by Oxford University scientists, report that a short pulse from the FLASH laser ‘knocked out’ a core electron from every aluminium atom in a sample without disrupting the metal’s crystalline structure. This turned the aluminium nearly invisible to extreme ultraviolet radiation.
''What we have created is a completely new state of matter nobody has seen before,’ said Professor Justin Wark of Oxford University’s Department of Physics, one of the authors of the paper. ‘Transparent aluminium is just the start. The physical properties of the matter we are creating are relevant to the conditions inside large planets, and we also hope that by studying it we can gain a greater understanding of what is going on during the creation of 'miniature stars' created by high-power laser implosions, which may one day allow the power of nuclear fusion to be harnessed here on Earth.’
The discovery was made possible with the development of a new source of radiation that is ten billion times brighter than any synchrotron in the world (such as the UK’s Diamond Light Source). The FLASH laser, based in Hamburg, Germany, produces extremely brief pulses of soft X-ray light, each of which is more powerful than the output of a power plant that provides electricity to a whole city.
The Oxford team, along with their international colleagues, focused all this power down into a spot with a diameter less than a twentieth of the width of a human hair. At such high intensities the aluminium turned transparent.
Whilst the invisible effect lasted for only an extremely brief period - an estimated 40 femtoseconds - it demonstrates that such an exotic state of matter can be created using very high power X-ray sources.
Professor Wark added: ‘What is particularly remarkable about our experiment is that we have turned ordinary aluminium into this exotic new material in a single step by using this very powerful laser. For a brief period the sample looks and behaves in every way like a new form of matter. In certain respects, the way it reacts is as though we had changed every aluminium atom into silicon: it’s almost as surprising as finding that you can turn lead into gold with light!’
The researchers believe that the new approach is an ideal way to create and study such exotic states of matter and will lead to further work relevant to areas as diverse as planetary science, astrophysics and nuclear fusion power.
A report of the research, 'Turning solid aluminium transparent by intense soft X-ray photoionization', is published in Nature Physics. The research was carried out by an international team led by Oxford University scientists Professor Justin Wark, Dr Bob Nagler, Dr Gianluca Gregori, William Murphy, Sam Vinko and Thomas Whitcher.
In this week’s Nature Physics an international team, led by Oxford University scientists, report that a short pulse from the FLASH laser ‘knocked out’ a core electron from every aluminium atom in a sample without disrupting the metal’s crystalline structure. This turned the aluminium nearly invisible to extreme ultraviolet radiation.
''What we have created is a completely new state of matter nobody has seen before,’ said Professor Justin Wark of Oxford University’s Department of Physics, one of the authors of the paper. ‘Transparent aluminium is just the start. The physical properties of the matter we are creating are relevant to the conditions inside large planets, and we also hope that by studying it we can gain a greater understanding of what is going on during the creation of 'miniature stars' created by high-power laser implosions, which may one day allow the power of nuclear fusion to be harnessed here on Earth.’
The discovery was made possible with the development of a new source of radiation that is ten billion times brighter than any synchrotron in the world (such as the UK’s Diamond Light Source). The FLASH laser, based in Hamburg, Germany, produces extremely brief pulses of soft X-ray light, each of which is more powerful than the output of a power plant that provides electricity to a whole city.
The Oxford team, along with their international colleagues, focused all this power down into a spot with a diameter less than a twentieth of the width of a human hair. At such high intensities the aluminium turned transparent.
Whilst the invisible effect lasted for only an extremely brief period - an estimated 40 femtoseconds - it demonstrates that such an exotic state of matter can be created using very high power X-ray sources.
Professor Wark added: ‘What is particularly remarkable about our experiment is that we have turned ordinary aluminium into this exotic new material in a single step by using this very powerful laser. For a brief period the sample looks and behaves in every way like a new form of matter. In certain respects, the way it reacts is as though we had changed every aluminium atom into silicon: it’s almost as surprising as finding that you can turn lead into gold with light!’
The researchers believe that the new approach is an ideal way to create and study such exotic states of matter and will lead to further work relevant to areas as diverse as planetary science, astrophysics and nuclear fusion power.
A report of the research, 'Turning solid aluminium transparent by intense soft X-ray photoionization', is published in Nature Physics. The research was carried out by an international team led by Oxford University scientists Professor Justin Wark, Dr Bob Nagler, Dr Gianluca Gregori, William Murphy, Sam Vinko and Thomas Whitcher.
Saturday, July 11, 2009
Military mega-lasers are too hot to handle
HIGH-ENERGY laser weapons have been hailed as the future of anti-missile defence, but they may be further from being battle-ready than military chiefs hoped.
In recent tests, several prototypes have suffered serious damage to their optics at intensities well below the expected levels of tolerance. "Optical damage has been quietly alarming upper management in most major programmes," Sean Ross of the US Air Force Research Laboratory in New Mexico told a meeting of the Directed Energy Professional Society in Newton, Massachusetts, last week. There are also big problems managing the waste heat generated by high-intensity beams.
Laser weapons require mirrors and lenses to focus powerful beams onto distant moving targets, and to compensate for atmospheric perturbations that can reduce the power they deliver. The higher the intensity of the beam, the more likely it is to damage the surface of its optical components.
Optical surfaces are designed to withstand powers up to a specific damage threshold, but tiny flaws or irregularities - which can be extremely difficult to spot - reduce this threshold by making them more vulnerable to heat. Contaminants deposited on the surface can also reduce this threshold by forcing the surface to absorb energy.
These problems have begun to stall the development of laser weapons. Earlier this year in the US, engineers halted tests of the $4.3 billion megawatt-class Airborne Laser short of full power to avoid damaging "a handful of optics in the turret", according to Mike Rinn, a Boeing vice-president who manages the programme. They realised that the optics, designed years ago, would be "frail" in the presence of any contamination, which would be virtually inevitable in flight. In the next week or so, Boeing engineers will install replacement optics and test them on the ground before running the laser at full power in flight.
Finding a way of preventing laser weapons from frying themselves is proving just as troublesome. Depending on the type of laser, generating 1 watt of laser beam produces about 4 watts of waste heat that must be dissipated. The challenge is to develop a cooling system that is both small and extremely robust.
In recent tests, several prototypes have suffered serious damage to their optics at intensities well below the expected levels of tolerance. "Optical damage has been quietly alarming upper management in most major programmes," Sean Ross of the US Air Force Research Laboratory in New Mexico told a meeting of the Directed Energy Professional Society in Newton, Massachusetts, last week. There are also big problems managing the waste heat generated by high-intensity beams.
Laser weapons require mirrors and lenses to focus powerful beams onto distant moving targets, and to compensate for atmospheric perturbations that can reduce the power they deliver. The higher the intensity of the beam, the more likely it is to damage the surface of its optical components.
Optical surfaces are designed to withstand powers up to a specific damage threshold, but tiny flaws or irregularities - which can be extremely difficult to spot - reduce this threshold by making them more vulnerable to heat. Contaminants deposited on the surface can also reduce this threshold by forcing the surface to absorb energy.
These problems have begun to stall the development of laser weapons. Earlier this year in the US, engineers halted tests of the $4.3 billion megawatt-class Airborne Laser short of full power to avoid damaging "a handful of optics in the turret", according to Mike Rinn, a Boeing vice-president who manages the programme. They realised that the optics, designed years ago, would be "frail" in the presence of any contamination, which would be virtually inevitable in flight. In the next week or so, Boeing engineers will install replacement optics and test them on the ground before running the laser at full power in flight.
Finding a way of preventing laser weapons from frying themselves is proving just as troublesome. Depending on the type of laser, generating 1 watt of laser beam produces about 4 watts of waste heat that must be dissipated. The challenge is to develop a cooling system that is both small and extremely robust.
Monday, June 29, 2009
Laser-created temporal lens could lead to movies of molecular processes
(PhysOrg.com) -- Finding a way to observe and record the behavior of matter at the molecular level has long been a holy grail among physicists. That ability could open the door to a wide range of applications in ultrafast electron microscopy used in a large array of scientific, medical and technological fields.
Now, a team at the University of Nebraska-Lincoln has figured out a possible way to do that. Working in collaboration with Nobel laureate Ahmed Zewail (chemistry, 1999) of the California Institute of Technology in Pasadena, they developed mathematical models to show that laser beams create ultra-high-speed "temporal lenses" that would be capable of making "movies" of molecular processes. The finding was published in the June 15-19 online edition of the Proceedings of the National Academy of Sciences.
The "lenses" in question are not made of glass like those found in standard tabletop microscopes. They're created by laser beams that would keep pulses of electrons from dispersing and instead focus the electron packets on a target. The timescales required, however, are hardly imaginable on a human scale -- measured in femtoseconds (quadrillionths of a second) and attoseconds (quintillionths of a second).
The physicists modeled two types of lenses. One was a temporal "thin" lens created using one laser beam that could compress electron pulses to less than 10 femtoseconds. The second was a "thick" lens created using two counterpropagating laser beams that showed the potential of compressing electron pulses to reach focuses of attosecond duration.
Now, a team at the University of Nebraska-Lincoln has figured out a possible way to do that. Working in collaboration with Nobel laureate Ahmed Zewail (chemistry, 1999) of the California Institute of Technology in Pasadena, they developed mathematical models to show that laser beams create ultra-high-speed "temporal lenses" that would be capable of making "movies" of molecular processes. The finding was published in the June 15-19 online edition of the Proceedings of the National Academy of Sciences.
The "lenses" in question are not made of glass like those found in standard tabletop microscopes. They're created by laser beams that would keep pulses of electrons from dispersing and instead focus the electron packets on a target. The timescales required, however, are hardly imaginable on a human scale -- measured in femtoseconds (quadrillionths of a second) and attoseconds (quintillionths of a second).
The physicists modeled two types of lenses. One was a temporal "thin" lens created using one laser beam that could compress electron pulses to less than 10 femtoseconds. The second was a "thick" lens created using two counterpropagating laser beams that showed the potential of compressing electron pulses to reach focuses of attosecond duration.
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