Femtolaser Pulse Creates 3-D Nanostructures

A new fabrication process using femtosecond lasers creates 3-D nanostructures in materials, an essential step toward creating invisibility cloaks and other advanced materials that bend light in unusual ways.

Researchers in Eric Mazur's laboratory at the Harvard School of Engineering and Science (SEAS) fired a femtosecond laser, which releases incredibly bright flashes of light that last 5 x 10^-14, s, at a glass slide coated with a mixture of silver nitrate, water and PVP, a water-soluble polymer. The laser blast changes the electrical, physical and optical properties of the slide, and photoreduces the silver ions on the slide into nanocrystals of silver metal suspended on the polymer.

Previous attempts to create a 3-D structure failed because the coating was not quite right. When the researchers used only the silver nitrate and water, there was no lattice support for the silver atoms, they said.

"Normally, when people use femtosecond lasers in fabrication, they’re creating a woodpile structure: something stacked on something else, being supported by something else. If you want to make an array of silver dots, however, they can’t float in space," Mazur said. Ethanol and the PVP polymer were added to the solution to provide support to the structure, but reactions were fast and uncontrollable. Removing the ethanol solved the problem entirely.

"What was most surprising about it was how simple it is. It was a matter of using less," Mazur said.

The new fabrication process advances nanoscale metal lithography into three dimensions, and does it at a resolution high enough to be practical for metamaterials.

"This work demonstrates that we can create silver dots that are disconnected in X, Y, and Z," said Kevin Vora, a graduate student working on the project. "There’s no other technique that feasibly allows you to do that."

The work, which was supported by the Air Force Office of Scientific Research, is described in Applied Physics Letters.

Physicists cool semiconductor by laser light

Researchers at the Niels Bohr Institute have combined two worlds – quantum physics and nano physics, and this has led to the discovery of a new method for laser cooling semiconductor membranes. Semiconductors are vital components in solar cells, LEDs and many other electronics, and the efficient cooling of components is important for future quantum computers and ultrasensitive sensors. The new cooling method works quite paradoxically by heating the material. Using lasers, researchers cooled membrane fluctuations to minus 269 degrees C. The results are published in the scientific journal, Nature Physics.

The experiments themselves are carried out in this vacuum chamber. When the laser light hits the membrane, some of the light is reflected and some is absorbed and leads to a small heating of the membrane. The reflected light is reflected back again via a mirror in the experiment so that the light flies back and forth in this space and forms optical resonator (cavity). Changing the distance between the membrane and the mirror leads to a complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances and you can control the system so as to cool the temperature of the membrane fluctuations.

From gas to solid

Laser cooling of atoms has been practiced for several years in experiments in the quantum optical laboratories of the Quantop research group at the Niels Bohr Institute. Here researchers have cooled gas clouds of cesium atoms down to near absolute zero, minus 273 degrees C, using focused lasers and have created entanglement between two atomic systems. The atomic spin becomes entangled and the two gas clouds have a kind of link, which is due to quantum mechanics. Using quantum optical techniques, they have measured the quantum fluctuations of the atomic spin.

"For some time we have wanted to examine how far you can extend the limits of quantum mechanics – does it also apply to macroscopic materials? It would mean entirely new possibilities for what is called optomechanics, which is the interaction between optical radiation, i.e. light, and a mechanical motion," explains Professor Eugene Polzik, head of the Center of Excellence Quantop at the Niels Bohr Institute at the University of Copenhagen.

But they had to find the right material to work with.

In 2009, Peter Lodahl (who is today a professor and head of the Quantum Photonic research group at the Niels Bohr Institute) gave a lecture at the Niels Bohr Institute, where he showed a special photonic crystal membrane that was made of the semiconducting material gallium arsenide (GaAs). Eugene Polzik immediately thought that this nanomembrane had many advantageous electronic and optical properties and he suggested to Peter Lodahl's group that they use this kind of membrane for experiments with optomechanics. But this required quite specific dimensions and after a year of trying they managed to make a suitable one.

"We managed to produce a nanomembrane that is only 160 nanometers thick and with an area of more than 1 square millimetre. The size is enormous, which no one thought it was possible to produce," explains Assistant Professor Søren Stobbe, who also works at the Niels Bohr Institute.

Basis for new research

Now a foundation had been created for being able to reconcile quantum mechanics with macroscopic materials to explore the optomechanical effects.

Koji Usami explains that in the experiment they shine the laser light onto the nanomembrane in a vacuum chamber. When the laser light hits the semiconductor membrane, some of the light is reflected and the light is reflected back again via a mirror in the experiment so that the light flies back and forth in this space and forms an optical resonator. Some of the light is absorbed by the membrane and releases free electrons. The electrons decay and thereby heat the membrane and this gives a thermal expansion. In this way the distance between the membrane and the mirror is constantly changed in the form of a fluctuation.

"Changing the distance between the membrane and the mirror leads to a complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances and you can control the system so as to cool the temperature of the membrane fluctuations. This is a new optomechanical mechanism, which is central to the new discovery. The paradox is that even though the membrane as a whole is getting a little bit warmer, the membrane is cooled at a certain oscillation and the cooling can be controlled with laser light. So it is cooling by warming! We managed to cool the membrane fluctuations to minus 269 degrees C", Koji Usami explains.

"The potential of optomechanics could, for example, pave the way for cooling components in quantum computers. Efficient cooling of mechanical fluctuations of semiconducting nanomembranes by means of light could also lead to the development of new sensors for electric current and mechanical forces. Such cooling in some cases could replace expensive cryogenic cooling, which is used today and could result in extremely sensitive sensors that are only limited by quantum fluctuations," says Professor Eugene Polzik.

Plasmonics produces extreme UV light

An international team of researchers has invented a simple way of creating ultrashort pulses of extreme ultraviolet (EUV) light. The system uses a new 3D metallic waveguide, or "nanofunnel", that coverts pulses of infrared light to EUV.

EUV light has a wavelength of around 5–50 nm, which is about 100–10 times shorter than that of visible light. As a result, ultrashort pulses of EUV light are ideal for studying fundamental physics phenomena – such as how electrons move in atoms, molecules and solids.

However, it is difficult to produce EUV radiation using conventional methods that rely on using amplified light pulses from an oscillator (a source of laser light) to ionize noble gas atoms. The electrons liberated during this process are accelerated in the light field and their surplus energy is freed as attosecond (10–18 s) pulses of light of different wavelengths. The shortest wavelengths of light can then be "filtered out" to produce a single EUV pulse – a complicated process.
Simpler way of making pulses

Now, researchers at the Korea Advanced Institute of Science and Technology (KAIST), the Max Planck Institute of Quantum Optics (MPQ) in Germany and Georgia State University (GSU) in the US have come up with a different – and much simpler – way of doing things.

The new technique works by converting femtosecond (10–15 s) infrared pulses into femtosecond EUV pulses. The process exploits surface-plasmon polaritons (SPPs), which are particle-like collective oscillations that occur when light interacts with a metal's conduction electrons.

The nanofunnel made by the KAIST-MPQ-GSU team was devised so that it concentrated incident infrared light pulses into a spot that is smaller than the wavelength of the incident light. The funnel is a metallic nanostructure made of silver that contains a hollow hole shaped like a tapered cone. The cone is just a few micrometres long and filled with xenon gas. The tip of the funnel is around 100 nm across.

The researchers sent infrared light pulses (at a rate of 75 MHz) into the funnel, which is designed so that it contains patches of metal that are positively charged, followed by patches that are negatively charged. This arrangement produces electromagnetic fluctuations on the inside walls of the funnel, which result in the creation of SPPs. These particles then travel towards the tip, where the conical shape of the funnel concentrates their fields.

"The field on the inside of the funnel can become a few hundred times stronger than the field of the incident infrared light," explains Mark Stockman of GSU. "This enhanced field results in the generation of EUV light in the Xe gas."

An important feature of the nanofunnel is that it can be produced at frequencies of up to about 75 MHz. Seung-Woo Kim, team leader at KAIST, where the experiments were carried out, adds: "Due to their short wavelength and potentially short pulse duration, EUV light pulses can be an important tool for exploring electron dynamics in atoms, molecules and solids. Electrons move very fast – on the attosecond timescale – and light flashes that are shorter than attoseconds long are therefore needed to image these particles. Although scientists routinely use attosecond light flashes for such studies, they have much lower frequencies. Our new nanofunnel could change all this."

The results are detailed in Nature Photonics.

Ultra high speed film: Nano-scientists take snapshots of electronic states

German scientists in the team of Professor Michael Bauer, Dr. Kai Roßnagel and Professor Lutz Kipp from the Institute of Experimental and Applied Physics, together with colleagues from the University of Kaiserslautern and the University of Colorado in Boulder, U.S.A., are following the course of electronic switching processes which occur within fractions of a second (femtoseconds). The results of their research may trigger future developments of custom-made and ultra fast opto-electronic components in order to increase data transmission rates or to accelerate optical switches, to name just one example of potential areas of application.
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Two still frames recorded from the newly developed imaging method. The time interval betweenthe two frames is only 0.00000000000007 seconds. Recording: Rohwer et al., Copyright: CAU
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"These techniques that we have developed enables us to record films of extremely fast processes in a much more comprehensive manner than it was previously possible with similar techniques", Bauer explains. "We are able to, for example, directly track phase transitions in solids or catalytic reactions on surfaces."

To record the films, the Kiel scientists used ultra short flashes of light in the soft x-ray spectral region generated with a specific laser system. Bauer: "The amount of information gained from our pictures when played back in slow motion is vast. We will get completely new insights into most relevant electronic properties of solids which are important for a variety of current and future technologies, for example, in telecommunications."

More information: http://www.nature. … /nature09829

Ultrafast lasers give researchers a snapshot of electrons in action

(PhysOrg.com) -- In the quest to slow down and ultimately understand chemistry at the level of atoms and electrons, University of Colorado at Boulder and Canadian scientists have found a new way to peer into a molecule that allows them to see how its electrons rearrange as the molecule changes shape.

Understanding how electrons rearrange during chemical reactions could lead to breakthroughs in materials research and in fields like catalysis and alternative energy, according to CU-Boulder physics professors and JILA fellows Margaret Murnane and Henry Kapteyn, who led the research efforts with scientist Albert Stolow of the Canadian National Research Council's Steacie Institute for Molecular Sciences.

To be able to chart a chemical reaction, scientists need to be able to see how bonds are formed or broken between atoms in a molecule during chemical reactions. But only extremely limited tools are available to view the rapidly changing electron cloud that surrounds a molecule as the atoms move around, Murnane said. Changes in the electron cloud can happen on timescales of less than a femtosecond, or one quadrillionth of a second, representing some of the fastest processes in the natural world.

In a paper to appear in the Oct. 30 issue of Science Express, the online version of the journal Science, the CU team describes how they shot a molecule of dinitrogen tetraoxide, or N2O4, with a short burst of laser light to induce very large oscillations within the molecule. They then used a second laser to produce an X-ray, which was used to map the electron energy levels of the molecule, and most importantly, to understand how these electron energy levels rearrange as the molecule changes its shape, according to Kapteyn.

The researchers describe their process of stretching the N2O4 molecule as being similar to pulling on a Slinky toy and then letting it go and watching it vibrate. They used the N2O4 molecule because it vibrates more slowly compared to other molecules, allowing them to observe the physical processes under way.

In many ways, molecules are like tiny masses connected by tiny springs of differing strengths, Murnane said. These springs are the chemical bonds, made up of shared electrons, which hold all matter together. In this experiment they used ultrafast laser pulses to "twang" these springs, making the nanoscale molecular Slinkies vibrate. However, unlike real springs, when researchers vibrate the molecules their properties can change, she said.

Being able to watch and understand why the electrons did what they did is very useful in fields like alternative energy, according to the researchers.

Laser Microscalpel Created

Femtosecond lasers have just become more accurate and versatile, thanks to Adela Ben-Yakar, mechanical engineering assistant professor at The University of Texas at Austin.

By nature, Femtosecond lasers produce extremely brief, high-energy light pulses that can sear a targeted cell so quickly and accurately that the lasers’ heat has no time to escape and damage nearby healthy cells.

However, the very same laser systems, typically used in LASIK and other eye surgeries, have been too bulky - until now. Ben-Yakar’s laboratory has developed a femtosecond laser microscope system that includes a tiny, flexible probe that focuses light pulses to a spot size smaller than a human cell.

Ben-Yakar dubbed her creation the Microscalpel.

The Microscalpel can destroy a single cell while leaving nearby cells intact, which could improve the precision of surgeries for cancer, epilepsy and other diseases.

"You can remove a cell with high precision in 3-D without damaging the cells above and below it," Ben-Yakar says. "And you can see, with the same precision, what you are doing to guide your microsurgery."

As a result, the medical community envisions the lasers' use for more accurate destruction of many types of unhealthy material. These include small tumors of the vocal cords, cancer cells left behind after the removal of solid tumors, individual cancer cells scattered throughout brain or other tissue and plaque in the arteries.

Within a few years, Ben-Yakar expects to shrink the probe's 15-millimeter diameter by three-fold, so it would match endoscopes used today for laparoscopic surgery. The probe tip she has developed also could be made disposable -- for use operating on people who have infectious diseases or destroying deadly viruses and other biomaterials.

To develop the miniature laser-surgery system, Ben-Yakar worked with co-author
Olav Solgaard at Stanford University's Electrical Engineering Department to incorporate a miniaturized scanning mirror. Ben-Yakar and her graduate student Chris Hoy, another co-author, also used a novel fiber optic cable that can withstand intense light pulses traveling from an infrared, femtosecond laser.

To make the intensity more manageable, they stretched the light pulses into longer, weaker pulses for traveling through the fiber. Then they used the fiber's unique properties to reconstruct the light into more intense, short light pulses before entering the tissue.

For the study, Ben-Yakar directed laser light at breast cancer cells in three-dimensional biostructures that mimic the optical properties of breast tissue. She has since studied laboratory-grown, layered cell structures that mimic skin tissue and other tissues.

Ben-Yakar is also investigating the use of nanoparticles to focus the light energy on targeted cells. In research published last year, she demonstrated that gold nanoparticles can function as nano-scale magnifying lenses, increasing the laser light reaching cells by at least an order of magnitude, or ten-fold.

"If we can consistently deliver nanoparticles to cancer cells or other tissue that we want to target, we would be able to remove hundreds of unwanted cells at once using a single femtosecond laser pulse," Ben-Yakar says. "But we would still be keeping the healthy cells alive while photo-damaging just the cells we want, basically creating nanoscale holes in a tissue."

Ben-Yakar's experimental system is described in the June 23 issue of Optics Express.

Grants from the National Science Foundation and the National Institute of Health funded the research.

Extreme UV light made easy

A new system to generate coherent extreme-ultraviolet (EUV) light has been developed by researchers in Korea. The device, based on a nanostructure made of bow-tie shaped gold "antennas" on a sapphire substrate, is smaller and cheaper than existing systems and might allow an EUV source the size of a laptop computer to be made. Potential applications for the source include high-resolution biological imaging, advanced lithography of nanoscale patterns and perhaps even "X-ray clocks".

EUV light has a wavelength of between around 5 and 50 nm (100–10 times shorter than that of visible light). It can thus be used to etch patterns at tiny length scales and is ideal for spectroscopic applications because the wavelength is the same as that of many atomic transitions.

However, EUV radiation is currently produced in a very complicated process involving the use of amplified light pulses from an oscillator (a source of laser light) to ionize noble gas atoms. The electrons freed during this process are accelerated in the light field and their surplus energy is freed as attosecond (10^–18 s) pulses of light of different wavelengths. The shortest wavelengths of light can then be "filtered out" to produce a single EUV pulse.

Scientists would ideally like to produce EUV light directly from the oscillator without the need for expensive and bulky amplifiers. In this way, EUV-light generation could be simplified and the size of the source significantly reduced to tabletop dimensions. In contrast, current devices usually measure around 2–3 m across. Now, Seung-Woo Kim of KAIST in Daejeon and colleagues have shown that this might be possible.

The researchers report that a bow-tie nanostructure of gold – measuring around 20 nm across – can enhance the intensity of femtosecond laser light pulses by two orders of magnitude. This is high enough to generate EUV light with a wavelength of less than 50 nm directly from a small pulse with an energy of 10^11 W/cm^2 injected into argon gas (Nature 453 757). The energy needed is about 100 times less than in traditional approaches.

Surface plasmons

The technique works thanks to "surface plasmons" (surface excitations that involve billions of electrons) in the "gap" of the bow-tie gold nanostructures (see figure). When illuminated with the correct frequency of laser light, the surface plasmons can begin to resonate in unison, greatly increasing the local light field intensity. This phenomenon, known as resonant plasmon field enhancement, is already exploited in imaging techniques, such as surface-enhanced Raman scattering, which is sensitive enough to detect individual molecules on a metal surface.

Immediate applications include high-resolution imaging of biological objects, advanced lithography of nanoscale patterns and making X-ray clocks. These exploit a frequency-stabilized femtosecond laser and are being investigated worldwide to replace the current caesium atomic clocks for better time precision.

"This new method of short-wavelength light generation will open doors in imaging, lithography and spectroscopy on the nanoscale," commented Mark Stockman of the Georgia State University in a related article (Nature 453 731). The spatially coherent, laser-like light could have applications in many areas: spectroscopy; screening for defects in materials; and, if extended to X-ray or gamma-wavelengths, detecting minute amounts of fissile materials for public security and defence.

The team now plans to improve the conversion efficiency of the generated light by modifying the design of their nanostructure – for example, by making 3D cones with sharper tips. These will not only enable higher local field enhancement but also better interaction of the femtosecond light pulses with injected gas atoms. The team will also test the spatial and temporal coherence of the generated EUV light.

Source: Physicsworld.com; Phtonics.com; Optics.org

Femtosecond laser creates subsurface structures

Reported by optic.org

German researchers have used an ultrashort pulsed laser to create subsurface nanostructures in a sapphire crystal. The team believes that the techniques could be used to fabricate microfluidic devices as well as 3D photonic structures. (Optics Express 16 1517.)

SEM images of the entrance of the modified and etched channel directly after etching (left) and cross section of hollow nanoplanes in 500 μm depth of the same track. Laser beam propagated from top to bottom, three parallel scans with an offset of 3 μm, focused with NA=0.55, f=500 kHz, P=450 mW.

Femtosecond laser produced the colored metals

A tabletop femtosecond laser has been used to change the surface properties of metals to reflect a specific color or combination of colors. Silver, platinum, gold, and other metals have been turned colors such as blue, gray, black, and purple.

Today Photonics.com and Physorg.com reported Unversity of Rochester's Professor Guo's recent research achievement.

The intense blast forces the surface of the metal to form nanostructures -- pits, globules and strands that response incoming light in different ways depending on the way the laser pulse sculpted the structures. Since the structures are smaller than the wavelength of light, the way they reflect light is highly dependent upon their specific size and shape, Guo said. Varying the laser intensity, pulse length, and number of pulses, allows Guo to control the configuration of the nanostructures, and hence control what color the metal reflects.