Pioneer of lasers and optics Orazio Svelto receives Julius Springer Prize for Applied Physics 2011

The research activities of Orazio Svelto focus on physics oflaserresonators and techniques of mode selection, laser applications in biology and biomedicine, and physics of solid-state lasers, in particular ultrashort laser pulse generation.

Orazio Svelto is Professor of Physics of Matter at the Polytechnic Institute of Milan. He is also responsible for the Institute of Photonics and Nanotechnologies of the Italian National Research Council− Milan section. His research has covered a wide range of activity in the field of laser physics andphotonics, starting from the early beginning of these disciplines in 1962. He is the author of more than 200 scientific papers and holds three patents. Svelto is the recipient of several awards, including the Quantum Electronics Prize of the European Physical Society and the Charles H. Townes Award of the Optical Society of America.

At Springer, Professor Svelto has published his book Principles of Lasers (5th Ed., 2009) which has currently been adopted at several universities in Europe and the United States. The book's previous editions were translated into Russian, Chinese, Greek, Arabic and Farsi.

The Julius Springer Prize for Applied Physics recognizes researchers who have made an outstanding and innovative contribution to the fields of applied physics. It has been awarded annually since 1998 by the Editors-in-Chief of the Springer journalsApplied Physics A– Materials Science&ProcessingandApplied PhysicsB– Lasers and Optics.

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Karlsruhe invisibility cloak: Disappearing visibly

In invisibility cloaks,light wavesare guided by the material such that they leave theinvisibility cloakagain as if they had never been in contact with the object to be disguised. Consequently, the object is invisible to the observer. The exotic optical properties of the camouflaging material are calculated using complex mathematical tools similar to Einstein'stheory of relativity.

These properties result from a special structuring of the material. It has to be smaller than the wavelength of the light that is to be deflected. For example, the relatively large radio or radar waves require a material"that can be produced using nail scissors,"says Wegener. At wavelengths visible to the human eye, materials have to be structured in the nanometer range.

The minute invisibility cloak produced by Fischer and Ergin is smaller than the diameter of a human hair. It makes the curvature of a metal mirror appear flat, as a result of which an object hidden underneath becomes invisible. The metamaterial placed on top of this curvature looks like a stack of wood, but consists of plastic and air. These"logs"have precisely defined thicknesses in the range of 100 nm. Light waves that are normally deflected by the curvature are influenced and guided by these logs such that the reflected light corresponds to that of a flat mirror.

"If we would succeed again in halving the log distance of the invisibility cloak, we would obtain cloaking for the complete visible light spectrum,"says Fischer.

Last year, the Wegener team presented the first 3D invisibility cloak in the renowned journal Science. Until that time, the only invisibility cloaks existed in waveguides and were of practically two-dimensional character. When looking onto the structure from the third dimension, however, the effect disappeared. By means of an accordingly filigree structuring, the Karlsruhe invisibility cloak could be produced for wavelengths from 1500 to 2600 nm. This wavelength range is not visible to the human eye, but plays an important role in telecommunications. The breakthrough was based on the use of the direct laser writing method (DLS) developed by CFN. With the help of this method, it is possible to produce minute 3D structures withoptical propertiesthat do not exist in nature, so-called metamaterials.

In the past year, the KIT scientists continued to improve the already extremely fine direct laser writing method. For this purpose, they used methods that have significantly increased the resolution in microscopy. With this tool, they then succeeded in refining the metamaterial by a factor of two and in producing the first 3D invisibility cloak for non-polarized visible light in the range of 700 nm. This corresponds to the red color.

"The invisibility cloak now developed is an attractive object demonstrating the fantastic possibilities of the rather new field of transformation optics and metamaterials. The design options that opened up during the last years had not been deemed possible before,"emphasizes Ergin."We expect dramatic improvements of light-based technologies, such as lenses, solar cells, microscopes, objectives, chip production, and data communication."

The Way towards the Karlsruhe Invisibility Cloak

The"small improvement"of the Karlsruhe metamaterial with a high effect results from a series of development steps that appeared impossible a few years ago. Until the early 21st century, it was deemed infeasible to develop a material, by means of which light can be manipulated such that the material acts like an invisibility cloak. In 2006, the fundamentals of an invisibility cloak were described for the first time by the theory of transformation optics.

Based on theoretical calculations, first attempts were started to produce such a material artificially. Sir John B. Pendry (Imperial College, London, U.K.) and David R. Smith (Duke University, Durham, NC, USA and Imperial College, London, U.K.) published their results obtained for an invisibility cloak for radar waves in 2006. In 2008, Jensen Li (City University of Hong Kong, China) and Sir John B. Pendry presented the theoretical idea of a carpet invisibility cloak. In 2010, Wegener and his team from KIT, Karlsruhe, presented their first 3D invisibility cloak. In 2011, the effects of the Karlsruhe invisibility cloak are also visible to the bare eye.

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Stanford engineers create a tiny, energy-efficient laser for optical communication systems

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To theSilicon Valleymantra of"faster, smaller"semiconductors, you can now add"more efficient."The electrical data interconnections inside the computers of America's massive datacenters consume huge amounts of electricity, and there is a technological drive afoot to reduce that consumption.

To that end, Stanford researchers have unveiled a tiny, highly efficient semiconductor laser that could herald a new era in low-energy data interconnects that communicate withlightas well aselectrons.

"Today's electricaldata transmissioncircuits require a lot of energy to transmit a bit of information and are, relatively speaking, slow,"said Jelena Vuckovic, an associate professor ofelectrical engineeringat Stanford working on the new generation of nanoscale lasers.

She and her team– including Stanford graduate students Bryan Ellis and Gary Shambat, in collaboration with the research groups of James Harris at Stanford and Eugene Haller at the University of California-Berkeley– introduced their laser in a paper just published inNature Photonics.

Crossing the threshold

Vuckovic is working on a type of data transmitter known as a photonic-crystal laser. These lasers are particularly promising, not just for their speed and size, but because they operate at low thresholds– they don't use much energy.

"We've produced a nanoscale optical data transmitter– a laser– that uses 1,000 times less energy and is 10 times faster than the very best laser technologies in commercial use today,"said the professor."Better yet, we believe we can improve upon those numbers."

While others have created low-threshold lasers, Vuckovic said, the most promising have required a second laser to inject them with the energy they need to work– known as"pumping"– hardly an ideal solution.

"We really needed a laser pumped withelectricity, not light,"she said. The only available electrically pumped photonic-crystal laser was inefficient and difficult to fabricate, making it commercially impractical. Now, for the first time, Vuckovic has demonstrated an electrically pumped laser that is both easy to manufacture and delivers dramatically reduced energy consumption.

To create the laser, the researchers first"grow"a wafer of gallium arsenide, a semiconductor crystal, using a beam that sprays molecules to build layers one by one. At certain points in the layering process, they shuffle in three thin layers of a second crystal– indium arsenide. A cross-section reveals that the indium arsenide appears like little bumps or hills– quantum dots– within the wafer.

A deck of cards

When done, the wafer resembles a sort of nanophotonic deck of cards a mere 220 nanometers thick. Thick, however, is a relative term. It would take more than 1,000 of Vuckovic's wafers stacked atop one another to equal the thickness of a single playing card.

Next, the engineers"dope"two discrete areas on top of the wafer with ions. On one side, the researchers seed ions of silicon, and on the other they implant ions of beryllium.

These two regions are faintly visible on the surface, widening toward each other, approaching but never quite meeting at the center of the wafer. These ion-infused regions help focus the current flow to a very precise area at the core of the wafer where light is emitted, improving the performance of the laser.

Finally, with the basic wafer fabricated, the researchers have yet one more trick up their engineering sleeves. They finish by etching a precise honeycomb pattern of circular holes through the wafer.

The size and positioning of these holes is critical to the success of the laser. If the holes are too small or too large, spaced too closely or too far apart, the laser will not perform optimally– in some cases, it won't perform at all.

"These holes are almost perfectly round with smooth interior walls and are very important to the laser's function. They act like a hall of mirrors to reflect photons back toward the center of the laser,"said Vuckovic.

Here, in the heart of the wafer, the photons are concentrated and amplified into a tiny ball of light– a laser– which can be modulated up to 100 billion times per second, 10 times the best data transmitters now in use. Thus the light becomes binary data– light on, 1; light off, 0.

Real-world possibilities

At one end of a semiconductor circuit is a laser transmitter beaming out 1s and 0s as blasts of light. At the other end is a receiver that turns those blasts of light back into electrical impulses. All that is needed is a way to connect the two.

To do this, the researchers heat and stretch a thin fiberoptic filament, hundreds of times thinner than a human hair. The light from the laser travels along the fiber to the next junction in the circuit.

All this happens in a layer so thin hundreds of these nanophotonic transmitters could be arranged on a single layer, and many layers could then be stacked into a single chip.

Before Vuckovic's laser interconnect becomes commonplace, however, certain questions will need to be resolved. The new laser operates at relatively cold temperatures, 150 degrees Kelvin and below– about 190 degrees below zero Fahrenheit– but Vuckovic is confident and pressing forward.

"With improvements in processing,"she said,"we can produce alaserthat operates at room temperature while maintaining energy efficiency at about 1,000 times less than today's commercial technologies. We can see a light on the horizon."

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The secret behind NIST's new gas detector? Chirp before sniffing

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According to the NIST investigators, the new sensor overcomes many of the difficulties associated with tracegasdetection, a technique also used widely in industry to measure contaminants and ensure quality in manufacturing. A trace level of a particular gas can indicate a problem exists nearby, but manysensorsare only able to spot a specific type of gas, and some only after a long time spent analyzing a sample. The NIST sensor, however, works quickly and efficiently.

“This new sensor can simultaneously detect many different trace gases at very fast rates and with high sensitivity,” says NIST chemist Kevin Douglass.“It’s also built from off-the-shelf technology that you can carry in your hands. We feel it has great commercial potential.”

The key to the new sensor is the use of radiation at“terahertz” frequencies—between infrared and microwaves. Terahertz waves can make gas molecules rotate at rates unique to each type of gas, which implies the waves hold great promise for identifying gases and measuring how much gas is present. The NIST team has developed the technology to rotate the molecules“in phase”—imagine synchronized swimmers—and detect the spinning molecules easily as they gradually fall out of phase with each other.

A major hurdle the new technology overcomes is that it is now possible to look at nearly all possible gas molecules instantly using terahertz frequencies. Previously, it was necessary to exposemoleculesto a vast range of terahertz frequencies—slowly, one after another. Because no technology existed that could run through the entire frequency band quickly and easily, the NIST team had to teach their off-the-shelf equipment to“chirp.”

“The sensor sends a quick series of waves that run the range from low frequency to high, sort of like the‘chirp’ of a bird call,” says Douglass.“No other terahertz sensor can do this, and it’s why ours works so fast. Teaching it to chirp in a repeatable way has been one of our team’s main innovations, along with the mathematical analysis tools that help it figure out what gas you’re looking at.”

The NIST team has applied for a patent on its creation, which can plug into a power outlet and should be robust enough to survive in a real-world working environment.

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New lasing technique inspired by brightly colored birds

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In traditional lasers,lightis bounced back and forth (trapped) between mirrors with a so-called gain material between them that amplifies the light until it is of sufficient strength to pass through one end of the semi-transparentmirror, producing the beam. More recently however, optics researchers have found that another way to hold on to the light is to drill air holes in a material that causes the light to become trapped as it moves between the holes.

The air holes in the material can be placed either in a clear ordered fashion, producing just one strong wavelength, or in random fashion which allows for multiple wavelengths but isn’t very efficient; something that grows in importance as the laser power desired grows and uses more energy when it is produced.

The new technique falls somewhere in-between, in that at first glance the air holes appear to be random, but upon closer inspection, turn out to be ordered after all. This is where the brightly colored birds come in; nature has given them feathers with air pockets that at first glance appear to be randomly spaced, but under closer scrutiny it’s revealed that there is in fact, order underneath; the result is some light is trapped and bounced around inside and between them, allowing the amount of light to build up before ultimately escaping and giving the birds their brilliant hues.

To recreate the effect in the lab, the research team drilled holes in a 190 nanometer slice of gallium arsenide, a particularly good plastic for lasers, 235 to 275 nanometers apart, and which also had a layer of quantum dots that shine brilliantly when struck with just one photon. As suspected, when the wafer was lit up, it produced a laser of about 1,000 nanometers, which made it far more efficient than random lasers; after more tests were made it was found that thewavelengthproduced could be changed by altering the amount of space between the holes.

Though it’s not yet clear how the new type of laser will be used, it does seem likely the new approach will be used to help bring down the costs of lasers, and perhaps more importantly, the amount of energy needed to rum them.

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New calculations on blackbody energy set the stage for clocks with unprecedented accuracy

Precision timekeeping is one of the bedrock technologies of modern science and technology. It underpins precise navigation on Earth and in deep space, synchronization of broadband data streams, precision measurements of motion, forces and fields, and tests of the constancy of the laws of nature over time.

"Using our calculations, researchers can account for a subtle effect that is one of the largest contributors to error in modern atomic timekeeping,"says lead author Marianna Safronova of the University of Delaware, the first author of the presentation."We hope that our work will further improve upon what is already the most accurate measurement in science: the frequency of thealuminumquantum-logic clock,"adds co-author Charles Clark, a physicist at the Joint Quantum Institute, a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland.

The paper was presented today at the 2011 Conference on Lasers and Electro-Optics in Baltimore, Md.

The team studied an effect that is familiar to anyone who has basked in the warmth of a campfire: heat radiation. Any object at any temperature, whether the walls of a room, a person, the Sun or a hypothetical perfect radiant heat source known as a"black body,"emits heat radiation. Even a completely isolated atom senses the temperature of its environment. Like heat swells the air in a hot-air balloon, so-called"blackbody radiation"(BBR) enlarges the size of the electron clouds within the atom, though to a much lesser degree—by one part in a hundred trillion, a size that poses a severe challenge to precision measurement.

This effect comes into play in the world's most precise atomic clock, recently built by NIST researchers. This quantum-logic clock, based on atomicenergy levelsin the aluminumion, Al+, has an uncertainty of 1 second per 3.7 billion years, translating to 1 part in 8.6 x 10-18, due to a number of small effects that shift the actual tick rate of the clock.

To correct for the BBR shift, the team used the quantum theory of atomic structure to calculate the BBR shift of the atomic energy levels of the aluminum ion. To gain confidence in their method, they successfully reproduced the energy levels of the aluminum ion, and also compared their results against a predicted BBR shift in a strontium ion clock recently built in the United Kingdom. Their calculation reduces the relative uncertainty due to room-temperature BBR in the aluminum ion to 4 x 10-19 , or better than 18 decimal places, and a factor of 7 better than previous BBR calculations.

Current aluminum-ion clocks have larger sources of uncertainty than the BBR effect, but next-generation aluminum clocks are expected to greatly reduce those larger uncertainties and benefit substantially from better knowledge of the BBR shift.

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Beetle bling: Researchers discover optical secrets of 'metallic' beetles

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Today, the brilliant gold- (Chrysina aurigans) and silver-colored (Chrysina limbata) beetles have given optics researchers new insights into the way biology can recreate the appearance of some of nature's most precious metals, which in turn may allow researchers to produce new materials based on the natural properties found in the beetles' coloring.

A team of researchers at the University of Costa Rica has found that the beetles' metallic appearance is created by the unique structural arrangements of many dozens of layers of exo-skeletal chitin in the elytron, a hardened forewing that protects the delicate hindwings that are folded underneath. A paper about the discovery appears in the first issue of the Optical Society's (OSA) newest open access journal,Optical Materials Express, which launched this month.

The beetles were captured in the University of Costa Rica's Alberto Brenes Mesén Biological Reserve, a tropical rainforest environment."The metallic appearance of these beetles may allow them to be unnoticed, something that helps them against potential predators,"says physicist and study leader William E. Vargas. The surface of their elytra"reflects light in a way that they look as bright spots seen from any direction,"he explains."In a tropical rainforest, there are many drops of water suspended from the leaves of trees at ground level, along with wet leaves, and these drops and wet leaves redirect light by refraction and reflection respectively, in different directions. Thus, metallic beetles manage to blend with the environment."

To interpret the cause of this metallic look, Vargas and his team assumed that a sequence of layers of chitin appears through the cuticle, with successive layers having slightly different refractive indices.. In these beetles, the cuticle, which is just 10 millionths of a meter deep, has some 70 separate layers of chitin—a nitrogen-containing complex sugar that creates the hard outer skeletons of insects, crabs, shrimps, and lobsters. The chitin layers become progressively thinner with depth, forming a so-called"chirped"structure.

"Because the layers have different refractive indices,"Vargas says,"light propagates through them at different speeds. The light is refracted through—and reflected by—each interface giving, in particular, phase differences in the emerging reflected rays. For several wavelengths in the visible range, there are many reflected rays whose phase differences allow for constructive interference. This leads to the metallic appearance of the beetles."

This is similar to the way in which a prism breaks white light into the colors of the rainbow by refraction, but in the case of these beetles, different wavelengths, or colors of light are reflected back more strongly by different layers of chitin. This creates the initial palette of colors that enable the beetles to produce their distinctive hues. The mystery the researchers still needed to understand in more detail, however, was how the beetles could so perfectly create the structure causing the brilliant metallic tones of silver and gold.

Using a device they specially designed to measure the reflection of light when it strikes the curved surface of the beetles' elytra, Vargas and his colleagues found that as light strikes the interface between each successive layer (the first interface being the boundary between the outside air and the top chitin layer), some of its energy is reflected and some is transmitted down to the next interface.

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Device used to carry out the direct reflectance measurements under normal incidence of non-polarized light on the elytron (forewing) of a beetle. The allowed displacements and rotations of the probe holder allow the researchers to focus the beam on the beetle’s elytron perpendicularly. Credit: Photo courtesy Optics Express/University of Costa Rica.

"This happens through the complete sequence of interfaces,"Vargas says.

Because a portion of the light is reflected, it combines with light of the exact same wavelength as it passes back through layer upon layer of chitin, becoming brighter and more intense. Ocean waves can exhibit the same behavior, combining to produce rare but powerful rogue waves. In the case of the beetles, this"perfect storm"of light amplification produces not only the same colors but also the striking sheen and glimmer that we normally associate with fine jewelry.

In the two beetle species, interference patterns are produced by slightly different wavelengths of light, thus producing either silver or gold colors."For the golden-like beetle, the constructive interference is found for wavelengths larger than 515 nm, the red part of the visible wavelength range,"Vargas says,"while for the silver-like beetle it happens for wavelengths larger than 400 nm—that is, for the entire visible wavelength range."

"The detailed understanding of the mechanism used by thebeetlesto produce this metallic appearance opens the possibility to replicate the structure used to achieve it,"Vargas says,"and thus produce materials that, for example, might look like gold or silver but are actually synthesized from organic media."

This potentially could lead to new products or consumer electronics that can perfectly mimic the appearance of precious metals. Other products could be developed for architectural applications that require coatings with a metallic appearance. Vargas notes that in the solar industry, for example, chirped multilayer reflectors could be used as back layers supporting the active or light-absorbing medium, to improve the absorption of the back-reflectedlight.

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Diamonds shine in quantum networks: Researchers hitch precious stone's impurities onto nano-resonators

Researchers at the University of Calgary and Hewlett Packard Labs in Palo Alto, California, have come up with a way to useimpuritiesin diamonds as a method of creating a node in aquantum network. In addition to making powerful and secure networks, this discovery may also help sensitive measurements of magnetic fields and create new powerful platforms useful for applications in biology.

"Impurities in diamonds have recently been used to store information encoded onto theirquantum state, which can be controlled and read out using light. But coming up with robust way to create connections needed to pass on signals between these impurities is difficult,"says Dr. Paul Barclay, who recently moved to Calgary to start labs at the University of Calgary in the Institute for Quantum Information Science and at the National Institute for Nanotechnology in Edmonton.

"We have taken an important step towards achieving this,"adds Barclay.

Barclay and colleagues Dr. Andrei Faraon, Dr. Kai-Mei Fu, Dr. Charles Santori and Dr. Ray Beausoleil from Hewlett Packard have published a paper on their research in the journalNature Photonics.

Impurities in diamonds are responsible for slightly altering the material's colour, typically adding a slight red or yellow tint. The"NV center"impurity, which consists of anitrogen atomand a vacancy in otherwise perfect diamond carbon lattice, hasquantum propertiesthat researchers are learning to exploit for useful applications.

In principle, individual particles of light, photons, can be used to transfer this quantum information between impurities, each of which could be a node in a quantum network used for energy efficient and powerful information processing. In practice, this is challenging to demonstrate because of the small size of the impurities (a few nanometers) and the experimental complexity that comes along with studying and controlling several nanoscale quantum systems at once.

Researchers at Hewlett Packard Labs and Barclay, who worked on this research at HP and is now a professor in the Department on Physics and Astronomy at the University of Calgary, have created photonic"microring resonators"on diamond chips. These microrings are designed to efficiently channel light between diamond impurities, and an on-chip photonic circuit connected to quantum impurities at other locations on the chip.

In future work, this microring will be connected to other components on the diamond chip, and light will be routed between impurities.

"This work demonstrates the important connection between fundamental physics, blue sky applications, and near-term problem solving. It involves many of the same concepts being pushed by companies such as HP, IBM, and Intel who are beginning to integrate photonics with computer hardware to increase performance and reduce the major problem of heat generation,"says Barclay.

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Proposal for optical transistor uses light to control light

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The researchers, Ayhan Demircan and Shalva Amiranashvili from the Weierstrass Institute for Applied Analysis and Stochastics in Berlin, and Günter Steinmeyer from the Max Born Institute in Berlin, have published their study in a recent issue of Physical Review Letters. It is the first proposal that meets all the criteria for a useful all-optical transistor that were laid out in a previous paper inNature Photonicsby D.A.B. Miller, et al.

“What we would like to do is to‘switch’ an optical soliton (signal pulse) from one intensity to another,” the researchers toldPhysOrg.com.“The usual‘mechanical’ way to do so is to change the properties of the fiber. More elegant‘all-optical switching’ uses an additional pulse (control pulse). This should lead to some kind of‘optical circuit’ which is operated completely by light.”

As the scientists explained, when optical pulses collide, they typically do not change very much. Therefore, the control pulse usually has to be very large for it to have any effect on the signal pulse.

Here, the scientists have described a way to use a weak dispersive pulse to control a strong signal pulse, where the dispersive control pulse is seven times weaker than the signal pulse. In the new concept, both pulses travel in the same direction in a non-linear medium at different frequencies but at almost identical velocities. If one pulse can come from behind and catch up to the other, the pulses can interact.

Interestingly, from the perspective of the control pulse, the signal pulse looks like an opticalevent horizonsimilar to that of a white hole, which marks the boundary at which matter on the outside cannot enter. In the proposed concept, the control pulse and the signal pulse are temporarily locked in an optical event horizon for a long enough time to allow the control pulse to modify the properties of the signal pulse. For example, by changing the signal pulse’s intensity, frequency, speed or shape, the control pulse can effectively“switch” the signal pulse as part of a transistor.

“In short: a signal pulse can exchange energy with a control pulse if the latter is affected by the event horizon created by the former,” the researchers wrote.“Whatever that event horizon may possibly be, the energy transfer indeed occurs if both pulses have very close velocities.”

The signal pulse could also be switched repeatedly, making the scheme practical.

“The most important thing about our all-optical switching is that one may use several control pulses and thus switch the signal pulse many times, either increasing or decreasing its intensity,” the researchers wrote.“The signal pulse may then serve as a key element for an all-optical circuit.”

This novel concept for an all-optical transistor could overcome some of the challenges facing the design of these transistors, specifically cascadability and fan-out. Because the strong pulse does not dispersively spread or break up into multiple pulses, the output can be used for the input of the next switching action, which makes the switching scheme cascadable. In addition, the new design has the ability for fan-out, meaning that the output can be used for multiple inputs.

Since no all-optical transistor has been demonstrated that, among other things, can be cascaded in several stages, the new concept takes an important step in developing a practical optical transistor. Because photons travel much faster thanelectrons, optical transistors should have much faster switching speeds than current transistors.

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Proposed gamma-ray laser could emit 'nuclear light'

“Photons in a normallaserare emitted by atoms, by ions, and so on,” Tkalya toldPhysOrg.com.“In the nuclear gamma-ray laser, the photons are emitted by atomicnuclei.”

In the study, which is published in a recent issue ofPhysical Review Letters, Tkalya explains that a nuclear gamma-ray laser has to overcome at least two basic problems: accumulating a large amount of isomeric nuclei (nuclei in a long-lived excited state) and narrowing down the gamma-ray emission line. The new proposal fulfills these requirements by taking advantage of thorium’s unique nuclear structure, which enables some of the photons from an external laser to interact directly with thorium’s nuclei rather than its electrons.

Tkalya’s proposal uses a lithium-calcium-aluminum-fluoride (LiCaAlF₆) compound, in which some of the calcium is replaced with thorium. After a sufficient amount of isomeric thorium nuclei have been excited by an external laser, the nuclei can interact with a surrounding electric or magnetic field to create a population inversion, so that the system contains more excited nuclei than unexcited nuclei. (In a regular laser, a population inversion usually involves getting more electrons in a higher energy level than a lower energy level.) Then, Tkalya showed that the nuclei can emit or absorb photons without recoil, allowing them to produce light without losing energy.

“The nuclear gamma-ray laser considered in my article can emit‘visible’ (vacuum ultraviolet {VUV}) light (orgamma-raysof the optical range) only,” Tkalya said.

As Tkalya explained, a nuclear gamma-ray laser could open up several interesting applications, although he has not thoroughly investigated them yet. One possibility is that the gamma-ray emission of the excitedthoriumnuclei is in the optical range called“nuclear light.”

“In my opinion, it is interesting to see a‘nuclear light,’” he said.“An application of nuclear light is the nuclear metrological standard of frequency, or the‘nuclear clock.’”

In addition, the device could be used to test many fundamental properties of nature, such as the exponentiality of the decay law and the effect of the variation of the fine structure constant.

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Nuclear photonics: Gamma rays search for concealed nuclear threats

These gamma rays, called MEGa-rays (for mono-energetic gamma rays), are made by using a beam of fast-movingelectronsto convert laser photons (light at a lesser energy) into the gamma ray part of thespectrum. The incoherent gamma rays can be tuned to a specific energy so that they predominantly interact with only one kind of material.

A beam of MEGa-rays, for example, might be absorbed by thenuclear fueluranium-235 while passing through other substances including the more common (but less dangerous) isotope uranium-238. That sort of precision opens the door to“nuclear photonics,” the study of nuclei with light.“It is kind of like tunable laser absorption spectroscopy but withgamma-rays,” says Chris Barty of Lawrence Livermore National Laboratory, who will present on MEGa-rays at this year's Conference on Lasers and Electro Optics (CLEO: 2011, May 1- 6 in Baltimore).

In the last couple of years, MEGa-ray prototypes have identified elements like lithium and lead hidden behind metal barriers. The next-generation of MEGa-ray machines, which should come on-line in a couple of years, will be a million times brighter, allowing them to see through thick materials to locate specific targets in less than a second.

Barty will present several MEGa-ray applications in use today and will describe the attributes of next-generation devices. Work is under way on a MEGa-ray technology that could be placed on a truck trailer and carried out into the field to check containers suspected of having bomb material in them. At nuclear reactors, MEGa-rays could be used to quickly identify how enriched a spent fuel rod is inuranium-235. They could also examine nuclear waste containers to assess their contents without ever opening them up. MEGa-ray technology might also be employed in medicine to track drugs that carry specific isotope markers.

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Paging Han Solo: Researchers find more efficient way to steer laser beams

"In many cases, it is much easier to redirect alaser beamat a target than to steer the laser itself. We intended to develop a way to do this efficiently and without moving anything,"says Dr. Michael Escuti, an associate professor of electrical engineering at NC State and co-author of a paper on the research."We also wanted to be able to steer the beams over a wide range of angles, which is important for practical applications."

The key to the Escuti team's success was the use of"polarization gratings,"which consist of athin layerof liquid crystal material on a glass plate. The researchers created a device that allows a laser beam to pass through a stack of thesepolarizationgratings. Researchers manipulated theoptical propertiesof each grating, and were able to steer the laser beams by controlling how each individual grating redirects the light."Because each individual grating is very good at redirecting light in the desired directions with almost no absorption, the stack of gratings do not significantly weaken thelaser power,"Escuti says.

Another advantage of the system, Escuti explains, is that"every grating that we add to the stack increases the number of steerable angles exponentially. So, not only can we steer lasers efficiently, but we can do it with fewer components in a more compact system.

"Compared to other laser steering technologies, this is extremely cost-effective. We're taking advantage of materials and techniques that are already in widespread use in the liquid crystal display sector."

The technology has a variety of potential applications. For example, free space communication uses lasers to transfer data between platforms– such as between satellites or between an aircraft and soldiers on the battlefield. This sort of communication relies on accurate and efficient laser-beam steering. Other technologies that could make use of the research include laser weapons and LIDAR, orlaserradar, which uses light for optical scanning applications– such as mapping terrain.

Escuti's team has already delivered prototypes of the technology to the U.S. Air Force, and is currently engaged in additional research projects to determine the technology's viability for a number of other applications.

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EU to build most powerful laser ever in Prague

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The purpose of the Extreme LightInfrastructure(ELI), as its known, is first and foremost to serve as a research tool. Such alasercould be used to develop new cancer diagnosis and treatments as well as possible ways to deal with nuclear waste. In addition, the simple existence and experimentation with such a powerful laser could expand knowledge of nanoscience and molecular biology.

The ELI project was not easily won, as there were five countries lobbying to have it in their home states, and thereafter there was some bit of contention among the commissioners regarding feasibility and financing of the project. With the win, though, the Czech Republic will be sit at the forefront of optic and photonic research, adding to its already impressive résumé; for the past ten years, Prague has hosted Precision Automated Laser Signals (PALS), one of the premier laser systems in all of Europe. The installation will signal another milestone as well; the ELI venture will be the first big research project funded by the EU that will be located in an Eastern European country.

Slated to become operational by 2015, and located in Dolní Břežany, near Prague, the superlaser will operate using super-short pulses of very high energy particle and radiation beams, with each pulse lasting just 1.5 x 10^-14of a second, more than enough time to conduct high energy research experiments.

The installation in Prague will be followed up by projects in Hungary and then Romania, with each specializing in different areas of research; all of which will culminate in the development of a fourth super-super laser in an as yet to be decided location, which is expected to have twice the power of the original three lasers (though current plans have it comprised of 10 beans) which should add up to 200 petawatts of power; the theoretical limit for lasers.

The project is expected to cost in the neighborhood of€700 million.

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Through a Sensor, Holographically

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Enter Prof. Aydogan Ozcan, associate professor of electrical engineering at UCLA's Henry Samueli School of Engineering and Applied Science. Ozcan and his team– notably lead researchers Serhan Isikman and Dr. Waheb Bishara– have created a lens-free chip and image processing algorithm that utilizesoptical sensors,holographyand digital tomography combination to render high-resolution, high-contrast images while avoiding the limitations of standard lens-basedoptical microscopy.“The sensor,” Ozcan notes,“is an inexpensive five megapixel CMOS chip, 5MP with a 2.2 micrometer pixel size. It’s almost the same sensor that we have at the back of a Blackberry or iPhone, except that it’s monochrome rather than RGB.”

One of the biggest challenges facing the team was reducing noise artifacts resulting from spatial and temporal coherence due to illuminating the sample with lasers– especially at oblique angles. This coherence-induced noise appears as speckling patterns that obscure images of the actual sample structure. The team addressed the issue by replacing laser illumination withpartially-coherent lightthat emanates from a large aperture of ~0.05-0.1mm diameter with a bandwidth of 1-10 nm, finding that recording in-line holograms using partial coherence provided a gating function which allowed the device to filter noise beyond a defined resolution level.

Using partially-coherent light provided a high signal-to-noise ratio (SNR) that dramatically improved clarity and legibility of fine structural details. Moreover, the team developed a sample illumination approach that rotates the partially-coherent light source around the sample, rather than requiring the sample platform to be rotated within the illumination field, which is rather inconvenient to achieve practically, especially for large sample volumes.

Ozcan comments that, of the many innovations in this lens-free optical tomographic microscope, three are key:partially-coherent illumination with unit-magnification; pixel superresolution to achieve deeply subpixel lateral resolution;anddual-axis tomographic illumination. In their setup, dual-axis illumination is achieved by rotating the light source using a motorized stage; alignment is not sensitive and robustness is maintained. At every illumination angle, a series of subpixel shifted holograms are recorded for implementing pixel superresolution, such that submicron lateral resolution can be achieved even under unit fringe-magnification.“The chip uses dual-axis illumination to mitigate our limited angles of illumination such that a decent axial resolution can be achieved. The spatial frequencies that are collected from each axis is merged together to fill in some gaps in the 3D Fourier spectra of our objects. Moreover,” he adds,“this is the first time that dual-axis illumination has been applied in optical computed tomography schemes.”

This pixel superresolution approach effectively increases the sensor pixel density without physically adding additional pixels or sacrificing the imaging field of view (FOV). This is accomplished by capturing different images resulting from motion of either the illumination source or the sample and subsequently merging these lens-free frames to synthesize a higher spatial resolution holographic image. In microfluidic applications, for example, the fluidic motion of objects flowing by the sensor array can be used to generate high-resolution holograms.

Ozcan acknowledges that a fundamental challenge to transmission optical microscopy in general (whether lens-based or lens-free) is photon scattering when imaging thick tissue samples.“However,” he adds,“I expect that this limitation can be partially released if the scattering properties of tissue were to be reduced through some sample preparation steps– at least for certain class of objects. There is some very promising work in the literature around this major issue and researchers are working hard with various innovative schemes toward this end.”

Ozcan sees the primary applications of lens-free microscopy being in cell and developmental biology– especially inmicrofluidic integration.“Microfluidic integration would permit rather interestinglab-on-a-chipdevices that could dooptofluidic microscopy and tomography(also referred to asholographic optofluidic microscopy, or HOM) on the same chip. This way the compact and cost-effective platform of lab-on-a-chip devices could be coupled with high-resolution 3D micro-analysis tools on the same platform.”

In fact, Ozcan has done previous work in lens-free opto-fluidic microscopy, last year publishing a paper with Dr. Waheb Bishara and Dr. Hongying Zhu entitled Holographic Opto-Fluidic Microscopy (Optics Express18:27499–27510, 20 December 2010, Vol. 18, No. 26). In that paper, the authors note that their HOM platform does not involve complicated fabrication processes or precise alignment, nor does it require a highly uniform flow of objects within microfluidic channels. Relatively recently the Ozcan group has also demonstrated, for the first time, optofluidic tomography, soon to be published inApplied Physics Letters.

Of great interest is that when asked if further miniaturization and integration– such as an on-chip partially-coherent light source– could potentially enablein vivoapplications, Ozcan did not rule out the possibility.“Over the last decade microfluidics has created a versatile platform that has significantly advanced the ways in which microscale organisms and objects are controlled, processed and investigated, by improving the cost, compactness and throughput aspects of analysis. Microfluidics has also expanded into optics to create reconfigurable and flexible optical devices such as reconfigurable lenses, lasers, waveguides, switches, and on-chip microscopes.”

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