(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.
Monday, June 29, 2009
Friday, June 19, 2009
Europe's big lasers: the exawatt roadmap
Europe is thinking big – big lasers, big science, big budgets. Over the next decade, a trio of planned pan-European research facilities will give scientists access to unprecedented laser powers and intensities, opening the door to exotic science that will shed light on the origins of the universe and, it is hoped, provide the foundations for a sustainable energy future.
The overall construction cost for this new generation of "super lasers" is in excess of €2 bn, with operational budgets running to several hundred million euros per year. That's a price worth paying, says Christian Kurrer, research programme officer at the European Commission.
"International infrastructures attract the best research scientists," Kurrer told delegates attending the "Emerging European Laser Facilities: Beyond Petawatt" workshop at the recent SPIE Europe conference in Prague, Czech Republic. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."
One of those collaborations is the High Power Laser Energy Research (HiPER) facility. Headed up by the UK Science and Technology Facilities Council (STFC), a research funding body, HiPER's mission is to carry out proof-of-principle research into energy generation from laser-driven inertial-confinement fusion. The grand challenge: to initiate and study nuclear-fusion reactions via laser heating of a millimetre-sized fuel pellet (containing a mixture of deuterium and tritium) to temperatures greater than 100 million °C.
Although construction of HiPER is not slated to begin until 2014, the process of whipping existing laser technology into shape to deliver a light source with the requisite capabilities is already under way. "Current laser capability has reached its culmination in the petawatt (10^15 W) scale," observed Mike Dunne, project director of HiPER and a senior scientist at the Rutherford Appleton Laboratory (RAL), UK. "We're looking at how to take it [the technology] to the next generation."
This is the purpose of the three-year preparatory phase on HiPER, which is running alongside initial experiments at the US Department of Energy's $4 bn (€3.1 bn) National Ignition Facility (NIF) in California. (As with HiPER, the end-game for NIF, a huge facility consisting of 192 pulsed laser beams with a total energy of 1.8 MJ, is the creation of nuclear fusion in the laboratory.)
Another major European laser facility in the works is the Extreme Light Infrastructure (ELI), a project that's being led by scientists at the Laboratoire d'Optique Appliquée (LOA) at the Ecole Polytechnique, Palaiseau, France. Scheduled to fire up in 2015, ELI will enable fundamental science to be carried out at the very highest laser powers (in the exawatt regime, 10^18 W) and intensities (10^24 W/cm2).
Like HiPER, ELI will allow academic researchers to explore fundamental science at the extremes (stuff like photon–photon scattering and other nonlinear quantum vacuum effects). Other missions outlined in the ELI project include attosecond science (e.g. the study of the ultrafast motion of electrons inside atoms over timescales of the order of 10^–18 s) and generating a secondary source of electron beamlines from the light-matter interaction. HiPER, meanwhile, will also enable scientists to study laser–plasma interactions and "laboratory astrophysics" (e.g. the creation of conditions in the lab that could yield insights into supernovae evolution).
The European X-ray Free Electron Laser (European XFEL) is dedicated to generating ultrashort, hard X-ray flashes for a range of basic and applied research, including atomic-scale metrology and time-resolved studies of chemical reactions down to the 100 fs regime. Construction began on the 3.4 km long laser facility at DESY, an established particle physics and photonics research laboratory in Hamburg, Germany, at the beginning of the year.
DESY has taken the technology and knowhow from an existing pilot facility, FLASH, which is optimized for the extreme UV and soft X-ray range. "The European XFEL is based on the knowledge that has been accumulated at FLASH," said Tschentscher. "DESY will continue to operate FLASH as a user facility for the 6–60 nm regime and the European XFEL will basically build a new machine covering the wavelength range from below 0.1 nm up to 6 nm."
Upon completion, the European XFEL is intended to provide the brightest source of hard X-ray pulses at the highest repetition rate (30,000 flashes per second). In an initial version electron bunches will be separated into three beamlines delivering coherent pulses to six different experimental stations tuned to specific wavelengths.
• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.
The overall construction cost for this new generation of "super lasers" is in excess of €2 bn, with operational budgets running to several hundred million euros per year. That's a price worth paying, says Christian Kurrer, research programme officer at the European Commission.
"International infrastructures attract the best research scientists," Kurrer told delegates attending the "Emerging European Laser Facilities: Beyond Petawatt" workshop at the recent SPIE Europe conference in Prague, Czech Republic. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."
One of those collaborations is the High Power Laser Energy Research (HiPER) facility. Headed up by the UK Science and Technology Facilities Council (STFC), a research funding body, HiPER's mission is to carry out proof-of-principle research into energy generation from laser-driven inertial-confinement fusion. The grand challenge: to initiate and study nuclear-fusion reactions via laser heating of a millimetre-sized fuel pellet (containing a mixture of deuterium and tritium) to temperatures greater than 100 million °C.
Although construction of HiPER is not slated to begin until 2014, the process of whipping existing laser technology into shape to deliver a light source with the requisite capabilities is already under way. "Current laser capability has reached its culmination in the petawatt (10^15 W) scale," observed Mike Dunne, project director of HiPER and a senior scientist at the Rutherford Appleton Laboratory (RAL), UK. "We're looking at how to take it [the technology] to the next generation."
This is the purpose of the three-year preparatory phase on HiPER, which is running alongside initial experiments at the US Department of Energy's $4 bn (€3.1 bn) National Ignition Facility (NIF) in California. (As with HiPER, the end-game for NIF, a huge facility consisting of 192 pulsed laser beams with a total energy of 1.8 MJ, is the creation of nuclear fusion in the laboratory.)
Another major European laser facility in the works is the Extreme Light Infrastructure (ELI), a project that's being led by scientists at the Laboratoire d'Optique Appliquée (LOA) at the Ecole Polytechnique, Palaiseau, France. Scheduled to fire up in 2015, ELI will enable fundamental science to be carried out at the very highest laser powers (in the exawatt regime, 10^18 W) and intensities (10^24 W/cm2).Like HiPER, ELI will allow academic researchers to explore fundamental science at the extremes (stuff like photon–photon scattering and other nonlinear quantum vacuum effects). Other missions outlined in the ELI project include attosecond science (e.g. the study of the ultrafast motion of electrons inside atoms over timescales of the order of 10^–18 s) and generating a secondary source of electron beamlines from the light-matter interaction. HiPER, meanwhile, will also enable scientists to study laser–plasma interactions and "laboratory astrophysics" (e.g. the creation of conditions in the lab that could yield insights into supernovae evolution).
The European X-ray Free Electron Laser (European XFEL) is dedicated to generating ultrashort, hard X-ray flashes for a range of basic and applied research, including atomic-scale metrology and time-resolved studies of chemical reactions down to the 100 fs regime. Construction began on the 3.4 km long laser facility at DESY, an established particle physics and photonics research laboratory in Hamburg, Germany, at the beginning of the year.
DESY has taken the technology and knowhow from an existing pilot facility, FLASH, which is optimized for the extreme UV and soft X-ray range. "The European XFEL is based on the knowledge that has been accumulated at FLASH," said Tschentscher. "DESY will continue to operate FLASH as a user facility for the 6–60 nm regime and the European XFEL will basically build a new machine covering the wavelength range from below 0.1 nm up to 6 nm."
Upon completion, the European XFEL is intended to provide the brightest source of hard X-ray pulses at the highest repetition rate (30,000 flashes per second). In an initial version electron bunches will be separated into three beamlines delivering coherent pulses to six different experimental stations tuned to specific wavelengths.
• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.
Tuesday, June 16, 2009
Study gives clues to increasing X-rays' power
In a paper to be published in an upcoming edition of Physical Review Letters, UNL Physics and Astronomy Professor Anthony Starace and his colleagues give scientists important clues into how to unleash coherent, high-powered X-rays.
"This could be a contributor to a number of innovations," Starace said.
Starace's work focuses on a process called high-harmonic generation, or HHG. X-ray radiation can be created by focusing an optical laser into atoms of gaseous elements - usually low-electron types such as hydrogen, helium, or neon. HHG is the process that creates the energetic X-rays when the laser light interacts with those atoms' electrons, causing the electrons to vibrate rapidly and emit X-rays.
But the problem with HHG has been around almost as long as the onset of the method in 1988: The X-ray light produced by the atoms is very weak. In an effort to make the X-rays more powerful, scientists have attempted using higher-powered lasers on the electrons, but success has been limited.
"Using longer wavelength lasers is another way to increase the energy output of the atoms," Starace said. "The problem is, the intensity of the radiation (the atoms) produce drops very quickly."
Instead of focusing on low-electron atoms like hydrogen and helium, Starace's group applied HHG theory to heavier (and more rare) gaseous atoms having many electrons - elements such as xenon, argon and krypton. They discovered that the process would unleash high-energy X-rays with relatively high intensity by using longer wavelength lasers (with wavelengths within certain atom-specific ranges) that happen to drive collective electron oscillations of the many-electron atoms.
"If you use these rare gases and shine a laser in on them, they'll emit X-Rays with an intensity that is much, much stronger (than with the simple atoms)," Starace said. "The atomic structure matters."
Starace said that unlocking the high-powered X-rays could lead one day, for example, to more powerful and precise X-ray machines. For instance, he said, heart doctors might conduct an exam by scanning a patient and creating a 3D hologram of his or her heart, beating in real time.
Nanoscientists, who study the control of matter on an atomic or molecular scale, also may benefit from this finding, Starace said. Someday, the high-intensity X-rays may be used to make 3D images of the microscopic structures with which nanoscientists work.
"With nanotechnology, miniaturization is the order of the day," he said. "But nanoscientists obviously could make use of a method to make the structures they're building and working with more easily visible."
"This could be a contributor to a number of innovations," Starace said.
Starace's work focuses on a process called high-harmonic generation, or HHG. X-ray radiation can be created by focusing an optical laser into atoms of gaseous elements - usually low-electron types such as hydrogen, helium, or neon. HHG is the process that creates the energetic X-rays when the laser light interacts with those atoms' electrons, causing the electrons to vibrate rapidly and emit X-rays.
But the problem with HHG has been around almost as long as the onset of the method in 1988: The X-ray light produced by the atoms is very weak. In an effort to make the X-rays more powerful, scientists have attempted using higher-powered lasers on the electrons, but success has been limited.
"Using longer wavelength lasers is another way to increase the energy output of the atoms," Starace said. "The problem is, the intensity of the radiation (the atoms) produce drops very quickly."
Instead of focusing on low-electron atoms like hydrogen and helium, Starace's group applied HHG theory to heavier (and more rare) gaseous atoms having many electrons - elements such as xenon, argon and krypton. They discovered that the process would unleash high-energy X-rays with relatively high intensity by using longer wavelength lasers (with wavelengths within certain atom-specific ranges) that happen to drive collective electron oscillations of the many-electron atoms.
"If you use these rare gases and shine a laser in on them, they'll emit X-Rays with an intensity that is much, much stronger (than with the simple atoms)," Starace said. "The atomic structure matters."
Starace said that unlocking the high-powered X-rays could lead one day, for example, to more powerful and precise X-ray machines. For instance, he said, heart doctors might conduct an exam by scanning a patient and creating a 3D hologram of his or her heart, beating in real time.
Nanoscientists, who study the control of matter on an atomic or molecular scale, also may benefit from this finding, Starace said. Someday, the high-intensity X-rays may be used to make 3D images of the microscopic structures with which nanoscientists work.
"With nanotechnology, miniaturization is the order of the day," he said. "But nanoscientists obviously could make use of a method to make the structures they're building and working with more easily visible."
Friday, May 01, 2009
Big lasers, big science, big questions
Leading optical scientists agree that research and industry stakeholders need to do more if Europe is to maximize the benefits from a planned new generation of high-power laser facilities. That was one of the headline messages from the "Emerging European Laser Facilities: Beyond Petawatt" workshop at the SPIE Europe conference in Prague, Czech Republic, last week.
Marking 50 years since the invention of the laser, the workshop was intended to open debate among senior figures from planned pan-European petawatt laser facilties (1015 W and beyond). Among the "blue-ribbon" initiatives under discussion were projects like HiPER (the High Power Laser Energy Research project), ELI (the Extreme Light Infrastructure), and the European X-Ray Laser project (XFEL).
"International infrastructures attract the best research scientists," Christian Kurrer, research programme officer at the European Commission, told delegates. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."
With access to unprecedented laser power and scientific expertise, it is easy to see why large-scale science facilities are attractive to users. In fact, some might argue that they are too good and that they will pull in users (and resources) simply because they can guarantee results where smaller national institutions can't. "Industries want facilities for reproducible, reliable results and 100% service," was the opinion of Mike Dunne, HiPER project director.
At the same time, workshop participants agreed that there's plenty of work to do to ensure that stakeholders in research and industry are in position to maximize their interactions with "big science". "They [the laser facilities] have the scientific experts and we bring the industrial methods where the networks can really make a difference," said Federico Canova of Amplitude Technologies, a French laser manufacturer.
New European Union member states might also question the economic returns on their investment in big science, not least because the planned locations for all of these big laser facilities are in western Europe. Kurrer, however, prefers to view such challenges as opportunities. "While distribution [of projects] may never be good, the key will be to break down the borders. Europe is all about talking to each other and overcoming barriers."
Europe's new generation of high-energy laser facilities form part of an ambitious big-science roadmap coordinated by the European Strategy Forum on Research Infrastructures (ESFRI). The ESFRI roadmap covers capital and operational investments running to tens of billions of euros in strategic research areas like energy, environmental science and advanced materials.
Marking 50 years since the invention of the laser, the workshop was intended to open debate among senior figures from planned pan-European petawatt laser facilties (1015 W and beyond). Among the "blue-ribbon" initiatives under discussion were projects like HiPER (the High Power Laser Energy Research project), ELI (the Extreme Light Infrastructure), and the European X-Ray Laser project (XFEL).
"International infrastructures attract the best research scientists," Christian Kurrer, research programme officer at the European Commission, told delegates. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."
With access to unprecedented laser power and scientific expertise, it is easy to see why large-scale science facilities are attractive to users. In fact, some might argue that they are too good and that they will pull in users (and resources) simply because they can guarantee results where smaller national institutions can't. "Industries want facilities for reproducible, reliable results and 100% service," was the opinion of Mike Dunne, HiPER project director.
At the same time, workshop participants agreed that there's plenty of work to do to ensure that stakeholders in research and industry are in position to maximize their interactions with "big science". "They [the laser facilities] have the scientific experts and we bring the industrial methods where the networks can really make a difference," said Federico Canova of Amplitude Technologies, a French laser manufacturer.
New European Union member states might also question the economic returns on their investment in big science, not least because the planned locations for all of these big laser facilities are in western Europe. Kurrer, however, prefers to view such challenges as opportunities. "While distribution [of projects] may never be good, the key will be to break down the borders. Europe is all about talking to each other and overcoming barriers."
Europe's new generation of high-energy laser facilities form part of an ambitious big-science roadmap coordinated by the European Strategy Forum on Research Infrastructures (ESFRI). The ESFRI roadmap covers capital and operational investments running to tens of billions of euros in strategic research areas like energy, environmental science and advanced materials.
Thursday, April 23, 2009
Shaking the Fundamentals of Physics: At the Limits of the Photoelectric Effect
By way of the classical photoeffect, Einstein proved in 1905 that light also has particle character. However, with extremely high light intensities, remarkable things happen in the process. Scientists of the Physikalisch-Technische Bundesanstalt (Germany) have found this out with colleagues at FLASH in Hamburg, the first free-electron laser (FEL) for soft X-rays worldwide.
The current models based on Einstein's idea are simply described in such a way: A photon knocks an external electron out of an atom, provided that the photon energy is high enough. However, with wavelengths of only 13 nanometers and high radiation intensities of several petawatt per square centimeter something else - at least with some atoms - happens: With xenon, a whole light-wave packet immediately seems to knock out a huge number of internal electrons. This effect is strongly dependent on the material and not only on the characteristics of the exciting radiation, as accepted before. The work, which is currently published in the journal Physical Review Letters, has significance for future experiments of materials research at the new large X-ray laser facilities of the world.
The scientists actually wanted to develop methods for the radiometric characterization of X-ray lasers. They irradiated different gases to derive the laser strength from the ionization effect. The aim: with the laser well characterized was, for example, the testing of EUV lithography mirrors. The EUV lithography (EUV stands for extreme ultraviolet) at wavelengths in the range of 13 nanometers is considered as the future technology for the production of ever smaller computer chips.
However, during their experiments at FLASH, the new free-electron laser (FEL) in Hamburg, which currently allows the generation of EUV radiation and soft X-rays of the highest intensity in the world, they unexpectedly discovered things which concern the fundamentals of physics.
With the classical photoelectric effect (a), a single light particle (photon) of sufficient energy interacts with a single electron of the material. The process is energetically described by the Einstein equation (1905) and demonstrates the quantum structure of light. Only at very high intensities, does the multiphoton ionization occur, a process which is described in the extreme case of highly intensive ultra-short light flashes as emitted by long-wave femtosecond lasers, again, in the wave picture of light (b).
Nevertheless, the suitable theoretical models fail in the short-wave X-ray regime as shown by the experiments in Hamburg in which, for the first time, soft X-ray irradiance levels of several petawatts per square centimeter were achieved by strong beam focusing. The comparative quantitative studies prove that the degree of light-matter interaction and, thereby, the nature of the X-ray light are decisively determined by the structure of the atom and correlations in, above all, inner electron shells.
In the extreme case (xenon), a whole wave packet of photons seems to lead to the simultaneous emission of several inner electrons (c).
More information: Extreme ultraviolet laser excites atomic giant resonance. M. Richter et al., Phys. Rev. Lett. (2009) - online publication expected: April 27, 2009.
Photoelectric effect at ultra-high intensities. A. A. Sorokin et al., Phys. Rev. Lett. 99, 213002 (2007)
Source: Physikalisch-Technische Bundesanstalt
The current models based on Einstein's idea are simply described in such a way: A photon knocks an external electron out of an atom, provided that the photon energy is high enough. However, with wavelengths of only 13 nanometers and high radiation intensities of several petawatt per square centimeter something else - at least with some atoms - happens: With xenon, a whole light-wave packet immediately seems to knock out a huge number of internal electrons. This effect is strongly dependent on the material and not only on the characteristics of the exciting radiation, as accepted before. The work, which is currently published in the journal Physical Review Letters, has significance for future experiments of materials research at the new large X-ray laser facilities of the world.
The scientists actually wanted to develop methods for the radiometric characterization of X-ray lasers. They irradiated different gases to derive the laser strength from the ionization effect. The aim: with the laser well characterized was, for example, the testing of EUV lithography mirrors. The EUV lithography (EUV stands for extreme ultraviolet) at wavelengths in the range of 13 nanometers is considered as the future technology for the production of ever smaller computer chips.
However, during their experiments at FLASH, the new free-electron laser (FEL) in Hamburg, which currently allows the generation of EUV radiation and soft X-rays of the highest intensity in the world, they unexpectedly discovered things which concern the fundamentals of physics.
With the classical photoelectric effect (a), a single light particle (photon) of sufficient energy interacts with a single electron of the material. The process is energetically described by the Einstein equation (1905) and demonstrates the quantum structure of light. Only at very high intensities, does the multiphoton ionization occur, a process which is described in the extreme case of highly intensive ultra-short light flashes as emitted by long-wave femtosecond lasers, again, in the wave picture of light (b).
Nevertheless, the suitable theoretical models fail in the short-wave X-ray regime as shown by the experiments in Hamburg in which, for the first time, soft X-ray irradiance levels of several petawatts per square centimeter were achieved by strong beam focusing. The comparative quantitative studies prove that the degree of light-matter interaction and, thereby, the nature of the X-ray light are decisively determined by the structure of the atom and correlations in, above all, inner electron shells.
In the extreme case (xenon), a whole wave packet of photons seems to lead to the simultaneous emission of several inner electrons (c).
More information: Extreme ultraviolet laser excites atomic giant resonance. M. Richter et al., Phys. Rev. Lett. (2009) - online publication expected: April 27, 2009.
Photoelectric effect at ultra-high intensities. A. A. Sorokin et al., Phys. Rev. Lett. 99, 213002 (2007)
Source: Physikalisch-Technische Bundesanstalt
Friday, April 10, 2009
Curved light bends the rules
Everyone knows that light travels in a straight line — right? A couple of years back, however, physicists discovered something very different for certain laser pulses that have one intense peak next to a series of smaller peaks. The brightest part of these lopsided "Airy" pulses, they found, appear to follow a curved trajectory.
Researchers in the US have now found that sufficiently intense Airy pulses can ionize the surrounding air molecules and create curved filaments of plasma. What's more, Airy pulses interact with air such that the pulses are continually focused and so can travel long distances without being dispersed.
The bright white light given off by the plasma filaments could be used make remote spectroscopic measurements of the atmosphere — and the bending effect itself could be exploited in new kinds of waveguide.
The bendy behaviour of Airy pulses was first discovered in 2007 by Demetrios Christodoulides and colleagues at the University of Florida. Interference between the peaks causes the intense peak to veer off in one direction, while the other peaks move in the opposite direction. Although the total momentum of the pulse travels in a straight line, its brightest part appears to follow a curved path.
Christodoulides and his colleagues have now teamed up with Pavel Polynkin and others at the University of Arizona to create curved “filaments” of plasma using Airy pulses. The key to their success, according to Jerome Kasparian of the University of Geneva who was not part of the group, is their ability to — for the first time — create Airy pulses of extremely high intensity.
The team began with an intense infrared laser pulse that is about 35 fs in duration. The initially pancake-shaped pulse, which is symmetric around its direction of propagation, is then passed through a “phase mask” and then a lens, giving it a chevron shape with an intense peak at the vertex (see figure). This Airy pulse then travels about 1 m through air to a fluorescent screen where the light is detected.
As well as confirming that extremely intense Airy pulses appear to curve, the pulses also produced curved filaments of plasma by ionizing nearby molecules in the air.
Although physicists have long known that symmetric laser pulses can create such filaments, the process has proved very difficult to study. This is because symmetric laser pulses travel in the same direction as the white light given off by the plasmas they create, which means that any device that attempts to detect this light is dazzled or even destroyed by the pulse.
With Airy pulses, however, Polynkin, Christodoulides and colleagues discovered that the plasma light travels in straight lines tangentially to the curvature of the bright peak. The plasma light can therefore be detected — and perhaps even be used as a source of white light for spectroscopy.
Firing intense and long-range pulses into the air, for example, could allow researchers to make remote spectroscopic measurements of the atmosphere.
Polynkin also speculates that intense pulses could be fired into thunderclouds to create filaments that "guide" lightning to safe locations on the ground.
Studying the plasma light itself could even help physicists gain a better understanding of the complicated non-linear optics that define how intense laser beams travel through air. These include a “self-healing” effect whereby the beam is continually refocused by the plasma — rather than being dispersed — allowing intense pulses to travel very long distances.
The team are now studying the creation of curved filaments in water rather than air.
Researchers in the US have now found that sufficiently intense Airy pulses can ionize the surrounding air molecules and create curved filaments of plasma. What's more, Airy pulses interact with air such that the pulses are continually focused and so can travel long distances without being dispersed.
The bright white light given off by the plasma filaments could be used make remote spectroscopic measurements of the atmosphere — and the bending effect itself could be exploited in new kinds of waveguide.
The bendy behaviour of Airy pulses was first discovered in 2007 by Demetrios Christodoulides and colleagues at the University of Florida. Interference between the peaks causes the intense peak to veer off in one direction, while the other peaks move in the opposite direction. Although the total momentum of the pulse travels in a straight line, its brightest part appears to follow a curved path.Christodoulides and his colleagues have now teamed up with Pavel Polynkin and others at the University of Arizona to create curved “filaments” of plasma using Airy pulses. The key to their success, according to Jerome Kasparian of the University of Geneva who was not part of the group, is their ability to — for the first time — create Airy pulses of extremely high intensity.
The team began with an intense infrared laser pulse that is about 35 fs in duration. The initially pancake-shaped pulse, which is symmetric around its direction of propagation, is then passed through a “phase mask” and then a lens, giving it a chevron shape with an intense peak at the vertex (see figure). This Airy pulse then travels about 1 m through air to a fluorescent screen where the light is detected.
As well as confirming that extremely intense Airy pulses appear to curve, the pulses also produced curved filaments of plasma by ionizing nearby molecules in the air.
Although physicists have long known that symmetric laser pulses can create such filaments, the process has proved very difficult to study. This is because symmetric laser pulses travel in the same direction as the white light given off by the plasmas they create, which means that any device that attempts to detect this light is dazzled or even destroyed by the pulse.
With Airy pulses, however, Polynkin, Christodoulides and colleagues discovered that the plasma light travels in straight lines tangentially to the curvature of the bright peak. The plasma light can therefore be detected — and perhaps even be used as a source of white light for spectroscopy.
Firing intense and long-range pulses into the air, for example, could allow researchers to make remote spectroscopic measurements of the atmosphere.
Polynkin also speculates that intense pulses could be fired into thunderclouds to create filaments that "guide" lightning to safe locations on the ground.
Studying the plasma light itself could even help physicists gain a better understanding of the complicated non-linear optics that define how intense laser beams travel through air. These include a “self-healing” effect whereby the beam is continually refocused by the plasma — rather than being dispersed — allowing intense pulses to travel very long distances.
The team are now studying the creation of curved filaments in water rather than air.
Tuesday, April 07, 2009
Soliton laser offers broad tunability
A femtosecond soliton source with fast and broad spectral tunability has been developed by researchers in Argentina. The source, which comprises a Ti:sapphire laser and a highly nonlinear photonic-crystal fibre, can be tuned from 850 nm to 1000 nm with nearly constant pulse width and average power (Optics Letters 34 842).
The key to the laser's tuning performance is the use of solitons generated in the photonic-crystal fibre. At the low-power coupling regime, solitons can be tuned over a broad range of wavelengths from 850 to 1000 nm. The solitons generated in the fibre maintain almost constant pulse and spectral widths regardless of input power.
In the set-up, a photonic-crystal fibre measuring 75 cm in length is pumped with a Ti:sapphire laser that provides 37 fs pulses at a repetition rate of 94 MHz and a wavelength of 830 nm. Average power ranging from 1 to 10 mW is pumped into the fibre, controlled by an acousto-optic modulator (AOM).
The key to the laser's tuning performance is the use of solitons generated in the photonic-crystal fibre. At the low-power coupling regime, solitons can be tuned over a broad range of wavelengths from 850 to 1000 nm. The solitons generated in the fibre maintain almost constant pulse and spectral widths regardless of input power.
In the set-up, a photonic-crystal fibre measuring 75 cm in length is pumped with a Ti:sapphire laser that provides 37 fs pulses at a repetition rate of 94 MHz and a wavelength of 830 nm. Average power ranging from 1 to 10 mW is pumped into the fibre, controlled by an acousto-optic modulator (AOM).
Sunday, March 15, 2009
World's highest-energy laser to create mini-stars
To produce the temperatures and pressures needed for fusion, the facility will aim all of its 192 laser beams simultaneously on a hydrogen target. This all happens inside this 10-metre-diameter chamber, which weighs 130 tonnes. The sphere is made up of 18 aluminium sections that are each 10 centimetres thick.The square openings are for the lasers, and the round openings are used to accommodate nearly 100 pieces of diagnostic equipment.
This is a view of the target chamber from the inside. The laser beams enter through ports in the chamber to deliver almost 500 trillion watts of power to the tip of the positioner (right), which will hold the target for each experiment. When all of its beams are fully operational, NIF will focus nearly 2 million joules of ultraviolet laser energy at that tiny target, delivering 60 times more energy than any previous laser system.
All 192 lasers that enter the National Ignition Facility chamber will be trained on this pencil-eraser-sized cylinder. This capsule will hold the pea-sized target, which for fusion experiments will be a pellet of frozen hydrogen. Laser beams will enter through openings at each end to compress and heat the hydrogen in the hopes of creating a self-sustaining fusion reaction.
As laser beams hit the interior of the gold-plated capsule, they will create intense X-rays that can squeeze the pea-sized pellet of hydrogen down to a speck about the width of a human hair and heat it to some 3 million °C. The burst of laser light will last just billions of a second, but physicists hope the intense pulse will force hydrogen atoms to combine to form helium, releasing enough energy to fuse all other neighbouring hydrogen atoms until the fuel is spent.
Before reaching the chamber, laser light must be converted from infrared light to ultraviolet light, which is more effective at heating the target.This conversion is accomplished with plates sliced from large potassium dihydrogen phosphate (KDP) crystals.
This crystal, which weighed about 360 kilograms, started out from a seed crystal and grew to its pictured size inside a 2-metre-tall vat of solution over a period of two months. Each crystal is sliced into plates measuring 40 cm2. More than 600 of these plates are needed for the National Ignition Facility. (Image: Lawrence Livermore National Security, LLC/Lawrence Livermore National Laboratory/Department of Energy)
Tuesday, March 10, 2009
World's largest laser gears up for ignition experiments
(PhysOrg.com) -- Construction of the National Ignition Facility (NIF), the world's largest and highest-energy laser system, was essentially completed on Feb. 26, when technicians at Lawrence Livermore National Laboratory (LLNL), where the laser is located, fired the first full system shot to the center of the NIF target chamber.The test was the first time all 192 laser beams converged simultaneously in the 10-meter-diameter chamber. NIF has met all of its project completion criteria except for official certification of project completion by the U.S. Department of Energy, due by March 31.
An average of 420 joules of ultraviolet laser energy, known as 3-omega, was achieved for each beamline, for a total energy of more than 80 kilojoules (a joule is the energy needed to lift a small apple one meter against the Earth's gravity).
The energy level will be increased during the next several months, and when all NIF lasers are fired at full energy, they will deliver 1.8 megajoules of ultraviolet energy to a BB-sized target in a 20-nanosecond shaped laser pulse, generating 500 trillion watts of peak power -- more than the peak electrical generating power of the entire United States. This is considered more than enough energy to fuse the hydrogen isotopes of deuterium and tritium in the target into helium nuclei (alpha particles) and yield considerably more energy in the process than was required to initiate the reaction.
The last of NIF's 6,206 various optical-mechanical and controls system modules, called "line replaceable units" or LRUs, was installed on Jan. 26. The first LRU, a flashlamp, was installed on Sept. 26, 2001.
Workers have aligned and tuned NIF's final optical assemblies, which focus and convert the frequency of the project's 192 laser beams as they enter the target chamber and converge on the tiny target. Experimental systems and diagnostics are also being installed. Software for the integrated computer control system, which handles shot automation, has been completed.
Thursday, January 08, 2009
KBBF crystal gives direct access to DUV
Researchers in China have created a tunable all-solid-state laser that emits milliwatt power levels in the deep ultraviolet (DUV). Applications requiring light around the 200 nm mark, such as photoemission spectroscopy and photolithography, could benefit from this work (Applied Physics B 93 323).
"Our source tunes from 175 to 210 nm via fourth harmonic generation from a Ti:sapphire laser," Zuyan Xu of the Chinese Academy of Sciences told optics.org. "The highest output power is 2.23 mW at 193 nm but the power is above 1 mW between 182 and 210 nm. This is the first demonstration of a milliwatt-level widely tunable all-solid-state laser below 200 nm by direct second harmonic generation."
The team's set-up can essentially be broken into three stages: the initial nanosecond-pulsed Ti:sapphire laser, optics to generate the second harmonic in the UV and additional components to generate the fourth harmonic in the DUV.
The output from the Ti:sapphire (more than 3W across the range of 690–840 nm) is focused into a set of BBO crystals to generate UV light between 340 and 415 nm. This light is then passed into the KBBF crystal to generate the DUV wavelengths.
When it comes to producing tunable DUV light, one alternative approach is sum-frequency mixing. This however uses two laser beams making the system complex and of limited practical use. To remove this complexity, Xu and colleagues use a KBBF crystal that offers a direct route to DUV light below 200 nm using just one beam.
"Our source tunes from 175 to 210 nm via fourth harmonic generation from a Ti:sapphire laser," Zuyan Xu of the Chinese Academy of Sciences told optics.org. "The highest output power is 2.23 mW at 193 nm but the power is above 1 mW between 182 and 210 nm. This is the first demonstration of a milliwatt-level widely tunable all-solid-state laser below 200 nm by direct second harmonic generation."
The team's set-up can essentially be broken into three stages: the initial nanosecond-pulsed Ti:sapphire laser, optics to generate the second harmonic in the UV and additional components to generate the fourth harmonic in the DUV.
The output from the Ti:sapphire (more than 3W across the range of 690–840 nm) is focused into a set of BBO crystals to generate UV light between 340 and 415 nm. This light is then passed into the KBBF crystal to generate the DUV wavelengths.
When it comes to producing tunable DUV light, one alternative approach is sum-frequency mixing. This however uses two laser beams making the system complex and of limited practical use. To remove this complexity, Xu and colleagues use a KBBF crystal that offers a direct route to DUV light below 200 nm using just one beam.
Saturday, December 20, 2008
Light Bends Glass
Light gives a push rather than a pull when it exits an optical fiber, according to experiments reported in the 12 December Physical Review Letters. The observations address a 100-year-old controversy over the momentum of light in a transparent material: Is it greater or smaller than in air? In the experiments, a thin glass fiber bends as light shines out the end, apparently a recoil in response to the light gaining momentum as it passes from glass to air. But the many experimental subtleties mean that the issue is unlikely to be settled soon.
Light moves slower inside a material than it does in air or vacuum. In 1908 German mathematician Hermann Minkowski suggested that the momentum of light goes up as its speed goes down. A year later, German physicist Max Abraham claimed the exact opposite, that the momentum goes down with decreasing speed.
Abraham might appear to be correct, since the momentum of ordinary objects always goes down with decreasing speed. But Minkowski seems to be favored by quantum mechanics, which says that a photon's momentum goes up as the light's wavelength decreases--and the wavelength always shortens as light enters a material from air. Many theoretical arguments appear to point to an Abraham momentum, but most of the experimental evidence to date argues for Minkowski. The experimental difficulty is that in most cases, both formulations lead to the same predicted forces, after one accounts for the momenta of both the light and the medium. So experiments must be carefully designed to isolate the effect of the light's momentum and avoid other phenomena, such as thermal effects, that can mask the light-induced force.
In their experiment, Weilong She of Zhongshan University in Guangzhou, China, and his colleagues used a filament of silica half a micron wide and 1.5 millimeters long. As the fiber dangled vertically, the researchers shined 270-millisecond laser pulses at a wavelength of 650 nanometers down the fiber. As the light pulses exited out the bottom, a gain in momentum (à la Abraham) would cause the fiber to recoil back like a gun, whereas a loss (à la Minkowski) would pull the fiber straight down. "When I began this experiment, I was really unsure which one is correct," She recalls. The fiber bowed outward with each pulse, which the researchers say is a sign that it's recoiling as Abraham would predict.
The researchers performed a second experiment with a longer fiber and continuous--rather than pulsed--laser light and found similar results. The tip of the hanging fiber moved sideways like a pendulum by about 30 microns, which agreed with the tiny force (less than a billionth of a Newton) that they predicted. The team also verified that thermal effects, such as heat expansion, would be too small to influence the fiber's movement.
The researchers performed a second experiment with a longer fiber and continuous--rather than pulsed--laser light and found similar results. The tip of the hanging fiber moved sideways like a pendulum by about 30 microns, which agreed with the tiny force (less than a billionth of a Newton) that they predicted. The team also verified that thermal effects, such as heat expansion, would be too small to influence the fiber's movement.
Light moves slower inside a material than it does in air or vacuum. In 1908 German mathematician Hermann Minkowski suggested that the momentum of light goes up as its speed goes down. A year later, German physicist Max Abraham claimed the exact opposite, that the momentum goes down with decreasing speed.
Abraham might appear to be correct, since the momentum of ordinary objects always goes down with decreasing speed. But Minkowski seems to be favored by quantum mechanics, which says that a photon's momentum goes up as the light's wavelength decreases--and the wavelength always shortens as light enters a material from air. Many theoretical arguments appear to point to an Abraham momentum, but most of the experimental evidence to date argues for Minkowski. The experimental difficulty is that in most cases, both formulations lead to the same predicted forces, after one accounts for the momenta of both the light and the medium. So experiments must be carefully designed to isolate the effect of the light's momentum and avoid other phenomena, such as thermal effects, that can mask the light-induced force.
In their experiment, Weilong She of Zhongshan University in Guangzhou, China, and his colleagues used a filament of silica half a micron wide and 1.5 millimeters long. As the fiber dangled vertically, the researchers shined 270-millisecond laser pulses at a wavelength of 650 nanometers down the fiber. As the light pulses exited out the bottom, a gain in momentum (à la Abraham) would cause the fiber to recoil back like a gun, whereas a loss (à la Minkowski) would pull the fiber straight down. "When I began this experiment, I was really unsure which one is correct," She recalls. The fiber bowed outward with each pulse, which the researchers say is a sign that it's recoiling as Abraham would predict.
The researchers performed a second experiment with a longer fiber and continuous--rather than pulsed--laser light and found similar results. The tip of the hanging fiber moved sideways like a pendulum by about 30 microns, which agreed with the tiny force (less than a billionth of a Newton) that they predicted. The team also verified that thermal effects, such as heat expansion, would be too small to influence the fiber's movement.
The researchers performed a second experiment with a longer fiber and continuous--rather than pulsed--laser light and found similar results. The tip of the hanging fiber moved sideways like a pendulum by about 30 microns, which agreed with the tiny force (less than a billionth of a Newton) that they predicted. The team also verified that thermal effects, such as heat expansion, would be too small to influence the fiber's movement.
Saturday, December 06, 2008
Direct diode-pumped laser produces terawatt powers
A team of researchers from Germany has published details of what it believes is the first direct diode-pumped laser to produce terawatt peak powers. The system relies on a ytterbium-doped calcium fluoride (Yb:CaF2) crystal to amplify femtosecond pulses to the terawatt level, a milestone of particular interest to the laser fusion community (Optics Letters 33 2770).
Alternative ways of reaching the terawatt regime are high-energy Nd:glass or short-pulse Ti:Sapphire laser systems, although both of these methods rely on mature flash-lamp technology.
The heart of the system is a ten-pass amplifier based on Yb:CaF2, which is pumped by two diode laser stacks emitting at 940 nm. The amplifier itself is seeded by either a two-stage chirped pulse Yb:glass MOPA (which the team refers to as the pre-amplifiers of POLARIS or the POLARIS front end) or a Q-switched nanosecond Yb:YAG MOPA.
The team produced 192 femtosecond pulses with a pulse energy of 197 mJ (corresponding to a peak power of 1 TW) using the POLARIS front end. It was also able to amplify nanosecond pulses from the Q-switched MOPA to the joule level.
Alternative ways of reaching the terawatt regime are high-energy Nd:glass or short-pulse Ti:Sapphire laser systems, although both of these methods rely on mature flash-lamp technology.
The heart of the system is a ten-pass amplifier based on Yb:CaF2, which is pumped by two diode laser stacks emitting at 940 nm. The amplifier itself is seeded by either a two-stage chirped pulse Yb:glass MOPA (which the team refers to as the pre-amplifiers of POLARIS or the POLARIS front end) or a Q-switched nanosecond Yb:YAG MOPA.
The team produced 192 femtosecond pulses with a pulse energy of 197 mJ (corresponding to a peak power of 1 TW) using the POLARIS front end. It was also able to amplify nanosecond pulses from the Q-switched MOPA to the joule level.
Friday, December 05, 2008
Drift-free femtosecond timing synchronization of remote optical and microwave sources
Researchers at MIT, US, are joining forces with MenloSystems to commercialize a set of large-scale synchronization techniques that maintain sub-10-femtosecond timing accuracy over 10 hours and a distance of 300 m and more. This is said to be the first demonstration of such high-precision, robust timing synchronization (Nature Photonics 2 733).
According to Franz Kaertner, principle investigator of the project, this result will benefit the design and operation of seeded free-electron lasers, which require extremely high timing accuracy and may be applicable to the synchronization of large-scale phased-array antennas for radio astronomy.
"Just a few years ago, people thought this level of precision could not be achieved for such a long period of time," he commented. "Our result will enable scientists and engineers in different fields to really think about how to solve their problems or enhance performance by introducing the capabilities that we have shown."
Femtosecond modelocked lasers simultaneously carry extremely low jitter optical and microwave signals. Owing to their ultralow jitter properties, they have been expected to clock large-scale scientific facilities requiring extremely high timing accuracy that conventional electronic clock distribution cannot provide. However, lack of long-term stable synchronization techniques has hindered the realization of this pervasive clocking idea.
"The timing signal needs to be detected with both high timing detection sensitivity and high thermal stability," explained Kim. "Conventionally, this timing detection has been performed in the electronic domain using high-speed photodetection of optical pulse trains followed by phase-detection with microwave mixers. However, excess noise and thermal drift has seriously limited the stability that could be achieved."
To overcome this problem, Kaertner and colleagues shifted the timing detection from the electronic to the optical domain. Extensive details of the methods used can be found in the paper. In summary, the group uses ultralow-noise optical pulse trains generated by modelocked lasers as the timing signals, then distributes them by means of timing-stabilized fibre links and, finally, synchronizes the delivered timing signals with the optical and microwave sources being targeted.
The MIT team is optimistic that due to the scalable nature of its techniques, further improvements in precision and distance are possible. "The next milestone is attosecond-precision ultrafast photonics, which will open up more applications and opportunities that require even higher timing precision," concluded Kim.
According to Franz Kaertner, principle investigator of the project, this result will benefit the design and operation of seeded free-electron lasers, which require extremely high timing accuracy and may be applicable to the synchronization of large-scale phased-array antennas for radio astronomy.
"Just a few years ago, people thought this level of precision could not be achieved for such a long period of time," he commented. "Our result will enable scientists and engineers in different fields to really think about how to solve their problems or enhance performance by introducing the capabilities that we have shown."
Femtosecond modelocked lasers simultaneously carry extremely low jitter optical and microwave signals. Owing to their ultralow jitter properties, they have been expected to clock large-scale scientific facilities requiring extremely high timing accuracy that conventional electronic clock distribution cannot provide. However, lack of long-term stable synchronization techniques has hindered the realization of this pervasive clocking idea.
"The timing signal needs to be detected with both high timing detection sensitivity and high thermal stability," explained Kim. "Conventionally, this timing detection has been performed in the electronic domain using high-speed photodetection of optical pulse trains followed by phase-detection with microwave mixers. However, excess noise and thermal drift has seriously limited the stability that could be achieved."
To overcome this problem, Kaertner and colleagues shifted the timing detection from the electronic to the optical domain. Extensive details of the methods used can be found in the paper. In summary, the group uses ultralow-noise optical pulse trains generated by modelocked lasers as the timing signals, then distributes them by means of timing-stabilized fibre links and, finally, synchronizes the delivered timing signals with the optical and microwave sources being targeted.
The MIT team is optimistic that due to the scalable nature of its techniques, further improvements in precision and distance are possible. "The next milestone is attosecond-precision ultrafast photonics, which will open up more applications and opportunities that require even higher timing precision," concluded Kim.
Saturday, November 29, 2008
Ultraviolet pulses close in on 1 fs regime
Deep ultraviolet (DUV) pulses with a duration of just 3.7 femtoseconds have been generated by researchers at the Max-Planck-Institute for Quantum Optics in Germany. The pulses are said to be the shortest ever observed at this wavelength range and could allow the team to influence the outcome of chemical reactions by controlling the motion of electrons in molecules. (Optics Express 16 18956)Previously, the shortest pulses in the DUV (wavelengths shorter than 300 nm) had a duration of 8 femtoseconds. Much of the research effort focused on compressing the ultraviolet pulses after they had been generated, which is a formidable challenge as this requires precise dispersion control.
Here, Ulrich Graf and colleagues take a different approach. They upconvert 780 nm, 0.25 mJ pulses with a duration of 6 femtosecond pulses directly into the UV range by means of harmonic generation in a noble gas jet.
The resulting 3.7 femtosecond pulses are characterized using a dispersion-minimized SD-FROG approach and have energies in excess of 1.4µJ. The conversion efficiency is approximately 0.6%.
The group is now hoping to extend its approach to generate ultrashort pulses in the vacuum ultraviolet (VUV) spectral range (wavelengths less than 180 nm). "In the VUV, matter absorbs light even more and this spectral range will offer plenty of opportunities to explore and control the microcosm on an ultrafast scale," said Goulielmakis. "At the same time, we anticipate that these pulses will be substantially shorter and will approach the 1 femtosecond frontier."
Thursday, November 20, 2008
Ultra-short Laser Pulse Produces Positrons
More than 100 billion particles of antimatter have been created by using a short-pulse, ultraintense laser to irradiate a gold sample the size of the head of a push pin. The antimatter, also known as positrons, shoots out of the target in a cone-shaped plasma "jet."
This new ability to create a large number of positrons in a small laboratory opens the door to several avenues of antimatter research, including an understanding of the physics underlying various astrophysical phenomena such as black holes and gamma ray bursts. Antimatter research also could reveal why more matter than antimatter survived the Big Bang at the start of the universe.
In the experiment, the laser ionizes and accelerates electrons, which are driven right through the gold target. On their way, the electrons interact with the gold nuclei, which serve as a catalyst to create positrons. The electrons give off packets of pure energy, which decays into matter and antimatter, following the predictions by Einstein's famous equation that relates matter and energy. By concentrating the energy in space and time, the laser produces positrons more rapidly and in greater density than ever before in the laboratory.
Particles of antimatter are almost immediately annihilated by contact with normal matter, and converted to pure energy (gamma rays). There is considerable speculation as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter, and what might be possible if antimatter could be harnessed. Normal matter and antimatter are thought to have been in balance in the very early universe, but due to an "asymmetry" the antimatter decayed or was annihilated, and today very little antimatter is seen.
Over the years, physicists have theorized about antimatter, but it wasn't confirmed to exist experimentally until 1932. High-energy cosmic rays impacting Earth's atmosphere produce minute quantities of antimatter in the resulting jets, and physicists have learned to produce modest amounts of antimatter using traditional particle accelerators. Antimatter similarly may be produced in regions like the center of the Milky Way and other galaxies, where very energetic celestial events occur.
The presence of the resulting antimatter is detectable by the gamma rays produced when positrons are destroyed when they come into contact with nearby matter. Laser production of antimatter isn't entirely new either. Livermore researchers detected antimatter about 10 years ago in experiments on the since-decommissioned Nova petawatt laser -- about 100 particles. But with a better target and a more sensitive detector, this year's experiments directly detected more than 1 million particles. From that sample, the scientists infer that around 100 billion positron particles were produced in total.
This new ability to create a large number of positrons in a small laboratory opens the door to several avenues of antimatter research, including an understanding of the physics underlying various astrophysical phenomena such as black holes and gamma ray bursts. Antimatter research also could reveal why more matter than antimatter survived the Big Bang at the start of the universe.
In the experiment, the laser ionizes and accelerates electrons, which are driven right through the gold target. On their way, the electrons interact with the gold nuclei, which serve as a catalyst to create positrons. The electrons give off packets of pure energy, which decays into matter and antimatter, following the predictions by Einstein's famous equation that relates matter and energy. By concentrating the energy in space and time, the laser produces positrons more rapidly and in greater density than ever before in the laboratory.Particles of antimatter are almost immediately annihilated by contact with normal matter, and converted to pure energy (gamma rays). There is considerable speculation as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter, and what might be possible if antimatter could be harnessed. Normal matter and antimatter are thought to have been in balance in the very early universe, but due to an "asymmetry" the antimatter decayed or was annihilated, and today very little antimatter is seen.
Over the years, physicists have theorized about antimatter, but it wasn't confirmed to exist experimentally until 1932. High-energy cosmic rays impacting Earth's atmosphere produce minute quantities of antimatter in the resulting jets, and physicists have learned to produce modest amounts of antimatter using traditional particle accelerators. Antimatter similarly may be produced in regions like the center of the Milky Way and other galaxies, where very energetic celestial events occur.
The presence of the resulting antimatter is detectable by the gamma rays produced when positrons are destroyed when they come into contact with nearby matter. Laser production of antimatter isn't entirely new either. Livermore researchers detected antimatter about 10 years ago in experiments on the since-decommissioned Nova petawatt laser -- about 100 particles. But with a better target and a more sensitive detector, this year's experiments directly detected more than 1 million particles. From that sample, the scientists infer that around 100 billion positron particles were produced in total.
Saturday, November 15, 2008
Short fibre creates ultrafast OPO
A fibre optical parametric oscillator (FOPO) based on a 4.2 cm length of microstructured fibre that emits 70 fs pulses has been unveiled by researchers at the US universities of Cornell and California Merced. The system is said to deliver the shortest optical pulses reported for any FOPO and is a significant step towards making the technology commercially viable. (Optics Express 16 18050)
"The majority of OPOs are not portable and occupy a large footprint on an optical table," Jay Sharping of Merced's School of Natural Sciences told optics.org. "My motivation is to generate tunable pulsed light of sufficient output power in a portable fibre platform. This result is a step in that direction as it explores the generation of ultrafast laser light with tens of mW of average power."
Sharping and colleagues start with a commercially available microstructured fibre that has been drawn down to a reduced core size in order to modify the fibre's dispersion profile. They place the 4.2 cm length of fibre in a Fabry-Perot cavity and pump it using a ytterbium-doped fibre laser emitting at 1032 nm. The end result is 70 fs, 0.4 nJ pulses at 880 nm with an output peak power for 5kW for a pump peak power of 22 kW.
"The majority of OPOs are not portable and occupy a large footprint on an optical table," Jay Sharping of Merced's School of Natural Sciences told optics.org. "My motivation is to generate tunable pulsed light of sufficient output power in a portable fibre platform. This result is a step in that direction as it explores the generation of ultrafast laser light with tens of mW of average power."
Sharping and colleagues start with a commercially available microstructured fibre that has been drawn down to a reduced core size in order to modify the fibre's dispersion profile. They place the 4.2 cm length of fibre in a Fabry-Perot cavity and pump it using a ytterbium-doped fibre laser emitting at 1032 nm. The end result is 70 fs, 0.4 nJ pulses at 880 nm with an output peak power for 5kW for a pump peak power of 22 kW.
Thursday, November 13, 2008
Optical oscilloscope is fit for high-speed studies
Physics in the US have made an oscilloscope that can take snapshots of optical waveforms at a resolution fives times better than current devices. Based on an all-optical rather than an electronic design, the oscilloscope should be able to accurately profile modern telecommunications signals and various ultrafast chemical and physical phenomena.Oscilloscopes are used to trace graphs of signals over time. Conventional models are based on microelectronics and, using photodetectors, can take snapshots of optical signals at as low as 30 ps resolution.
But as telecommunication data transmission gets faster and faster, and as scientists want to probe more high-speed systems, oscilloscopes based on microelectronics are being stretched to the limit. This is because they can only cope with a relatively narrow frequency spread or “bandwidth”, which holds back their resolution.
All-optical circuits, on the other hand, can process much wider bandwidths. Although optical techniques already exist — indeed, with resolutions going down to a few femtoseconds — these have only been able to take snapshots of small segments of waveforms, and take a long time to update.
A team led by Alexander Gaeta at Cornell University in New York has found a way to exploit the fine resolution of optical techniques for longer waveforms. The researchers make use of the fact that electromagnetic waves have a space–time duality, in that there is a link between their spatial and temporal wavefunctions. This means that the researchers can use a lens to convert the temporal profile of a dispersed snapshot into a detailed, spectral output via a so-called Fourier transformation.
In the Cornell team’s device, an input waveform enters an optical fibre and mixes with a pump laser pulse, which ensures the waveform matches the focal length of the lens. As the waveform travels through the fibre it stretches out or “disperses”. Then, at the end of the fibre the lens — a nano-scale silicon waveguide — converts the waveform into a spectrum that can be measured with a spectrometer (Nature 456 81).
The device can record an input waveform at a resolution of 220 fs over lengths greater than 100 ps, giving the largest length-to-resolution ratio (more than 450) of any snapshot oscilloscope technique. Moreover, the technique uses components that can easily be integrated on chips.
Monday, November 10, 2008
Watching Electrons with Lasers
(PhysOrg.com) -- A team of researchers from the Stanford PULSE Institute for Ultrafast Energy Science at SLAC National Accelerator Laboratory has recently moved a step closer to visualizing the motions of electrons in molecules using a technique called high harmonic generation, or HHG.
Understanding these movements may help scientists better understand the early stages of chemical reactions. Electrons fuel chemical reactions. When chemicals react, electrons move between the molecules, building and breaking the connections, or bonds, that link atoms.
But in the world of quantum mechanics, electrons aren't easy to pin down. Physicists and chemists create mathematical descriptions called orbitals to illustrate the chance of finding an electron at a specific location of a molecule. Representations of these orbitals look like balloons attached to an atom's nucleus, the center of the atom.
SLAC researcher Markus Guehr and the PULSE team used HHG to learn about the electron orbitals of nitrogen gas molecules. In an HHG experiment, the researchers use molecules as tiny accelerator light sources. A laser beam is focused onto a stream of cooled nitrogen gas. The electric field of the laser tears an electron from a nitrogen molecule. As the laser field oscillates, the electron is accelerated back into the molecule and recombines with its orbital. Once the electron returns to the molecule, its energy is converted into light in the extreme ultraviolet range.
The spectrum of the light emanating from the molecule depends on the nature of the orbital the electron hits. By analyzing the number of photons at particular energies produced by this molecular laser, the team can characterize a specific orbital in the molecule.
But to understand how electrons move within a molecule over time, physicists need to characterize multiple orbitals.
In a report published in Science Express on October 30, the PULSE team, which also included Brian McFarland, Joseph Farrell and PULSE director Philip Bucksbaum, described the first evidence of HHG light signals from two different orbitals. Before these experiments, scientists had observed only light generated from electrons colliding with an orbital called the highest occupied molecular orbital, or HOMO. This orbital is the highest energy orbital that contains an electron. Physicists had theorized that detecting other orbitals was possible, but no one had observed multiple signals in an experiment.
The PULSE team reported detecting light from another orbital called the HOMO-1, which is one energy level lower than the HOMO. To detect light from the HOMO-1, the researchers had to align the nitrogen molecules perpendicular to the laser's electric field, to produce more efficient collisions between electrons and the orbital.
Science article: http://www.sciencemag.org/cgi/rapidpdf/1162780.pdf
Understanding these movements may help scientists better understand the early stages of chemical reactions. Electrons fuel chemical reactions. When chemicals react, electrons move between the molecules, building and breaking the connections, or bonds, that link atoms.
But in the world of quantum mechanics, electrons aren't easy to pin down. Physicists and chemists create mathematical descriptions called orbitals to illustrate the chance of finding an electron at a specific location of a molecule. Representations of these orbitals look like balloons attached to an atom's nucleus, the center of the atom.
SLAC researcher Markus Guehr and the PULSE team used HHG to learn about the electron orbitals of nitrogen gas molecules. In an HHG experiment, the researchers use molecules as tiny accelerator light sources. A laser beam is focused onto a stream of cooled nitrogen gas. The electric field of the laser tears an electron from a nitrogen molecule. As the laser field oscillates, the electron is accelerated back into the molecule and recombines with its orbital. Once the electron returns to the molecule, its energy is converted into light in the extreme ultraviolet range.
The spectrum of the light emanating from the molecule depends on the nature of the orbital the electron hits. By analyzing the number of photons at particular energies produced by this molecular laser, the team can characterize a specific orbital in the molecule.
But to understand how electrons move within a molecule over time, physicists need to characterize multiple orbitals.
In a report published in Science Express on October 30, the PULSE team, which also included Brian McFarland, Joseph Farrell and PULSE director Philip Bucksbaum, described the first evidence of HHG light signals from two different orbitals. Before these experiments, scientists had observed only light generated from electrons colliding with an orbital called the highest occupied molecular orbital, or HOMO. This orbital is the highest energy orbital that contains an electron. Physicists had theorized that detecting other orbitals was possible, but no one had observed multiple signals in an experiment.
The PULSE team reported detecting light from another orbital called the HOMO-1, which is one energy level lower than the HOMO. To detect light from the HOMO-1, the researchers had to align the nitrogen molecules perpendicular to the laser's electric field, to produce more efficient collisions between electrons and the orbital.
Science article: http://www.sciencemag.org/cgi/rapidpdf/1162780.pdf
Saturday, November 08, 2008
Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks
(PhysOrg.com) -- “When a laser goes through a plasma,” John Cary tells PhysOrg.com, “it pushes electrons away. Then when it snaps back, it generates an electric wake behind the laser pulse, picking the electrons up and carrying them along.” Cary is a physics professor at the University of Colorado in Boulder, as well as the founder of Tech-X Corporation, a company that specializes in computational physics and simulation software. He is a member of a collaboration that wanted to see if it was possible to accelerate heavy ions with a laser.
“Accelerating electrons is easier, because they are light,” he says. “Instead, we wanted to see if there could be the possibility of doing this with protons and heavier nuclei.” The collaboration, a team from the Shanghai Institute of Optics and Fine Mechanics in China and Cary, produced a simulation outlining possibilities. The results of the simulation are reported in Physical Review Letters: “Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks.”
Cary says that the information found in the simulation may have a variety of applications. “But the most exciting application, and the one that many people are looking to use,” he points out, “is for use in cancer therapy.”
The simulation shows that for heavier ions, it is possible to accelerate them, as well as control what is known as the Bragg Peak. “When you have a small charge to mass ratio,” Cary explains, “as an ion beam travels through matter, it deposits energy. At the end, just before it comes to rest, there is a very sharp peak of energy deposition.” This Bragg Peak is used in proton therapy to concentrate the energy on cancerous tumors to destroy them.
But there can be a problem: “If the beam is not monoenergetic, the peak smears out, potentially overlapping healthy tissue, which can then be damaged,” Cary says. “Researchers are trying to narrow this peak so that it is more precise, destroying the tumor but not the surrounding healthy cells.” This new simulation implies that this could be possible: “We found that carbon may have what is needed. The configuration seems to have nice properties, with a small energy spread and a fair amount of beam.”
More information: Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks, Phys. Rev. Lett. 101, 164802 (2008)
“Accelerating electrons is easier, because they are light,” he says. “Instead, we wanted to see if there could be the possibility of doing this with protons and heavier nuclei.” The collaboration, a team from the Shanghai Institute of Optics and Fine Mechanics in China and Cary, produced a simulation outlining possibilities. The results of the simulation are reported in Physical Review Letters: “Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks.”
Cary says that the information found in the simulation may have a variety of applications. “But the most exciting application, and the one that many people are looking to use,” he points out, “is for use in cancer therapy.”
The simulation shows that for heavier ions, it is possible to accelerate them, as well as control what is known as the Bragg Peak. “When you have a small charge to mass ratio,” Cary explains, “as an ion beam travels through matter, it deposits energy. At the end, just before it comes to rest, there is a very sharp peak of energy deposition.” This Bragg Peak is used in proton therapy to concentrate the energy on cancerous tumors to destroy them.
But there can be a problem: “If the beam is not monoenergetic, the peak smears out, potentially overlapping healthy tissue, which can then be damaged,” Cary says. “Researchers are trying to narrow this peak so that it is more precise, destroying the tumor but not the surrounding healthy cells.” This new simulation implies that this could be possible: “We found that carbon may have what is needed. The configuration seems to have nice properties, with a small energy spread and a fair amount of beam.”
More information: Generating Monoenergetic Heavy-Ion Bunches with Laser-Induced Electrostatic Shocks, Phys. Rev. Lett. 101, 164802 (2008)
Friday, November 07, 2008
Single laser traps and dissects cells
(Optics.org)--Researchers have shown for the first time that a single near-infrared laser can be used to both trap and dissect living cells.
A team of researchers from the Philippines and Japan has shown that a single laser can be switched between continuous wave (cw) and femtosecond pulsed mode to trap and penetrate yeast cells respectively. The researchers say that using a single laser with two operational modes simplifies the trapping system and allows complex and intricate manipulation of cell components (Review of scientific instruments 79 103705).
"As far as we know, this is the first time that the cw-mode of an ultrafast laser is functionally used in a system," Vincent Ricardo Daria, a researcher at the University of the Philippines, told optics.org. "While optical trapping and optical surgery is not new, it is the combination of both functionalities using a single laser that makes this work unique."
Optical trapping requires a low energy laser to avoid damaging the sample, while the opposite is required for optical surgery. Until now, two separate lasers were necessary to carry out each function - one for trapping and another for surgery. Daria and colleagues have now developed a single 780 nm Ti:Sa laser that when operated in cw mode enables non-invasive trapping of cells and by switching to femtosecond pulsed mode, precise surgery can be performed.
"When the laser is in cw mode using around 10 mW of power, we can perform the trapping part of the experiment," explained Daria. "By modulating the intensity of the incident laser at a repetition rate of 80 MHz, at the same power level, we can attain incision sizes smaller than the diffraction limit of light."
The team switched between cw and pulsed mode by adjusting the output of the pump laser. "Ultrafast lasers normally come with an electronic control module to trigger fs-pulse mode," explained Daria. "CW-mode is achieved by disturbing the fs-laser cavity by temporarily shutting-off the pump laser."
In the setup, the laser is introduced into an inverted laser-scanning microscope and brought to focus inside the sample using a water immersion objective lens. A dichroic mirror is used to direct the NIR laser to the objective lens while allowing images of cells at the focus of the same objective lens to be viewed on a CCD camera.
According to Daria the potential of such a setup goes beyond trapping and surgery of yeast cells alone. The combined system could also be used for yeast cell growth analysis, tissue and cell engineering and micro-manipulation for reproductive medicine.
A team of researchers from the Philippines and Japan has shown that a single laser can be switched between continuous wave (cw) and femtosecond pulsed mode to trap and penetrate yeast cells respectively. The researchers say that using a single laser with two operational modes simplifies the trapping system and allows complex and intricate manipulation of cell components (Review of scientific instruments 79 103705).
"As far as we know, this is the first time that the cw-mode of an ultrafast laser is functionally used in a system," Vincent Ricardo Daria, a researcher at the University of the Philippines, told optics.org. "While optical trapping and optical surgery is not new, it is the combination of both functionalities using a single laser that makes this work unique."
Optical trapping requires a low energy laser to avoid damaging the sample, while the opposite is required for optical surgery. Until now, two separate lasers were necessary to carry out each function - one for trapping and another for surgery. Daria and colleagues have now developed a single 780 nm Ti:Sa laser that when operated in cw mode enables non-invasive trapping of cells and by switching to femtosecond pulsed mode, precise surgery can be performed.
"When the laser is in cw mode using around 10 mW of power, we can perform the trapping part of the experiment," explained Daria. "By modulating the intensity of the incident laser at a repetition rate of 80 MHz, at the same power level, we can attain incision sizes smaller than the diffraction limit of light."
The team switched between cw and pulsed mode by adjusting the output of the pump laser. "Ultrafast lasers normally come with an electronic control module to trigger fs-pulse mode," explained Daria. "CW-mode is achieved by disturbing the fs-laser cavity by temporarily shutting-off the pump laser."
In the setup, the laser is introduced into an inverted laser-scanning microscope and brought to focus inside the sample using a water immersion objective lens. A dichroic mirror is used to direct the NIR laser to the objective lens while allowing images of cells at the focus of the same objective lens to be viewed on a CCD camera.
According to Daria the potential of such a setup goes beyond trapping and surgery of yeast cells alone. The combined system could also be used for yeast cell growth analysis, tissue and cell engineering and micro-manipulation for reproductive medicine.
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