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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Researchers create terahertz invisibility cloak

Though this design can't translate into aninvisibility cloakfor thevisible spectrum, it could have implications in diagnostics, security, and communication.

The cloak, designed by Cheng Sun, assistant professor of mechanical engineering at Northwestern's McCormick School of Engineering and Applied Science, uses microfabricated gradient-index materials to manipulate the reflection and refraction of light. Sun's results will be presented May 4 at CLEO: 2011, the annual Conference on Lasers and Electro-Optics.

Humans generally recognize objects through two features: their shape and color. To render an object invisible, one must be able to manipulate light so that it will neither scatter at an object's surface nor be absorbed or reflected by it (the process which gives objects color).

In order to manipulate light in theterahertzfrequency, which lies between infrared and microwaves, Sun and his group developed metamaterials: materials that are designed at theatomic level. Sun's tiny, prism-shaped cloaking structure, less than 10 millimeters long, was created using a technique called electronic transfer microstereolithography, where researchers use a data projector to project an image on a liquid polymer, then use light to transform the liquid layer into a thin solid layer. Each of the prism's 220 layers has tiny holes that are much smaller than terahertz wavelengths, which means they can vary the refraction index of the light and render invisible anything located beneath a bump on the prism's bottom surface; the light then appears to be reflected by a flat surface.

Sun says the purpose of the cloak is not to hide items but to get a better understanding of how to design materials that can manipulate light propagation.

"This demonstrates that we have the freedom to design materials that can change the refraction index,"Sun said."By doing this we can manipulate light propagation much more effectively."

The terahertz range has been historically ignored because the frequency is too high for electronics. But many organic compounds have a resonant frequency at the terahertz level, which means they could potentially be identified using a terahertz scanner. Sun's research into terahertz optics could have implications in biomedical research (safer detection of certain kinds of cancers) and security (using terahertz scanners at airports).

Next Sun hopes to use what he's learned through the cloak to create its opposite: a terahertz lens. He has no immediate plans to extend his invisibility cloak to visible frequencies.

"That is still far away,"he said."We're focusing on one frequency range, and such a cloak would have to work across the entire spectrum."

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Etched quantum dots shape up as single photon emitters

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The conventional way to build quantum dots—at NIST and elsewhere—is to grow them like crystals in a solution, but this somewhat haphazard process results in irregular shapes. The new, more precise process was developed by NIST postdoctoral researcher Varun Verma when he was a student at the University of Illinois. Verma uses electron beam lithography and etching to carve quantum dots inside asemiconductorsandwich (called a quantum well) that confines particles in two dimensions. Lithography controls the dot's size and position, while sandwich thickness and composition—as well as dot size—can be used to tune the color of the dot'slightemissions.

Some quantum dots are capable of emitting individual, isolated photons on demand, a crucial trait for quantum information systems that encode information by manipulating single photons. In new work reported inOptics Express, NIST tests demonstrated that the lithographed and etched quantum dots do indeed work as sources of single photons. The tests were performed on dots made of indium gallium arsenide. Dots of various diameters were patterned in specific positions in square arrays. Using a laser to excite individual dots and a photon detector to analyze emissions, NIST researchers found that dots 35 nanometers (nm) wide, for instance, emitted nearly all light at a wavelength of 888.6 nm. The timing pattern indicated that the light was emitted as a train of single photons.

NIST researchers now plan to construct reflective cavities around individual etched dots to guide their light emissions. If each dot can emit most photons perpendicular to the chip surface, more light can be collected to make a more efficient single photon source. Vertical emission has been demonstrated with crystal-grown quantum dots, but these dots can't be positioned or distributed reliably in cavities. Etched dots offer not only precise positioning but also the possibility of making identical dots, which could be used to generate special states of light such as two or morephotonsthat are entangled, a quantum phenomenon that links their properties even at a distance.

Thequantum dotstested in the experiments were made at NIST. A final step was carried out at the University of Illinois, where a crystal layer was grown over the dots to form clean interfaces.

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Breakthrough for photons in the microwave frequency range

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Whenphotonsreleased by a conventional light or radiation source hit a detector, they trigger anelectrical signalcomparable to a single“click” of a Geiger counter, which rattles when radioactive particles strike it. However, unlike optical photons, until now there have been no detectors that can detect single photons at particularly low frequencies, such as the microwave frequency range. The intensity of these microwave photons is much too weak for this. The research group led by ETH Zurich Professor Andreas Wallraff from the Department of Physics has now been able to characterise such low-intensity photons even without any“clicking” detectors by using a special apparatus and method. Physicists need techniques of this kind, for example to research the fundamental principles of quantum mechanics or to enable efficient information transmission in optical data communication.

Detector and emitter in one

In the interdisciplinary collaboration between Wallraff’s group, the Master student Deniz Bozyigit from the Department of Information Technology and Electrical Engineering and Canadian scientists, the researchers integrated a light source that generates single microwave photons onto a microchip. The frequency range of these photons is a few gigahertz, similar to the electromagnetic radiation of mobile phones and microwave ovens. With this innovative arrangement, a single photon is generated with an accuracy controlled to within a few nanoseconds and at a total rate of two million times per second. The scientists also constructed a highly sensitive and efficient measuring device, similar to that used for optical photons, on the same microchip in order to show that this apparatus really does generate only single photons.

Only one way possible

In the case of optical photons, a half-mirrored beam splitter is used for this purpose. The photons can either be reflected or pass through the beam splitter, with equal probability. For each of these two alternatives there is a detector which identifies the reflected or transmitted photon. According to the laws of quantum mechanics, an individual photon generated by a photon source can never be half-reflected and half-transmitted. It must opt for one of the two alternatives. In this case the light is no longer described by its wave properties as in classical physics; the photons behave like particles. If only one of the detectors“clicks”, this proves that the source really does generate only single photons.

Wallraff explains how single photons have now been successfully recorded in this way even in the microwave frequency range:“Until now it was thought that these correlations– characterizing the properties of the photons and how they behave at a beam splitter– could be measured only if the detector detects the photons with a“click”, like a particle. Since the intensity of photons in the microwave frequency range is too weak for this, we amplify the signals of the incoming photons and then measure the amplitude of the field generated by the photons”. To cope with the immense volume of data from these measurements, amounting to more than 1,000 gigabytes per hour, the researchers also needed to develop an innovative system which can process this data in real time. From the statistics thus obtained, and by using the newly developed method, they were able to prove that only a single photon is emitted and, as expected, this took either one or the other of the possible routes.

Very promising for information processing

If it proves possible to manipulate and measure individual photons in the microwave frequency range, this measuring unit could become a component of a quantum computer: single photons strike a beam splitter and transmit information quantum mechanically across an integrated microchip, and are then detected. According to the physicist, this could allow information processing procedures to be carried out more efficiently in the future.

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Researchers fabricate first large-area, full-color quantum dot display

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The researchers, Tae-Ho Kim and coauthors from various institutes in South Korea, have published their study on the first four-inch, full-color quantum dot display in a recent issue ofNature Photonics. The display consists of a film printed with trillions of the tinyquantum dots(an average of 3 trillion per cm²). The quantum dots emit light at a specific wavelength (color) that can be tuned by changing the size of the quantum dots.

Previous attempts to make full-color quantum dot displays have faced challenges in that image quality tended to decrease with the size of the display. To overcome this challenge, the researchers in the current study used a different method for applying the quantum dots to the film’s surface. Instead of spraying the quantum dots onto the film, the researchers created an“ink stamp” out of a patterned silicon wafer. They used the stamp to pick up strips of size-selected quantum dots, and then stamp them onto the substrate. Unlike the spraying methods, this method does not require the use of a solvent, which previously reduced color brightness.

As the results showed, the new quantum dot display has a greater density and uniformity of quantum dots, as well as a brighter picture and higher energy efficiency than previous quantum dot displays. The new display is also flexible, so applications could include roll-up portable displays or flexible lighting applications. The technology could also be used in photovoltaic devices, which would especially benefit from quantum dots’ high energy efficiency.

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Single photon management for quantum computers advanced by NIST

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In principle, quantum computers can perform calculations that are impossible or impractical using conventional computers by taking advantage of the peculiar rules ofquantum mechanics. To do this, they need to operate on things that can be manipulated into specific quantum states. Photons are among the leading contenders.

The new NIST papers address one of the many challenges to a practical quantum computer: the need for a device that produces photons in ready quantities, but only one at a time, and only when the computer's processor is ready to receive them. Just as garbled data will confuse a standard computer, an information-bearingphotonthat enters a quantum processor together with other particles—or when the processor is not expecting it—can ruin a calculation.

The single-photon source has been elusive for nearly two decades, in part because no method of producing these particles individually is ideal."It's a bit like playing a game of whack-a-mole, where solving one problem creates others,"says Alan Migdall of NIST's Optical Technology Division."The best you can do is keep all the issues under control somewhat. You can never get rid of them."

The team's first paper addresses the need to be certain that a photon is indeed coming when the processor is expecting it, and that none show up unexpected. Many kinds of single-photon sources create a pair of photons and send one of them to a detector, which tips off the processor to the fact that the second, information-bearing photon is on its way. But since detectors are not completely accurate, sometimes they miss the"herald"photon—and its twin zips into the processor, gumming up the works.

The team effort, in collaboration with researchers from the Italian metrology laboratory L'Istituto Nazionale di Ricerca Metrologica (INRIM), handled the issue by building a simple gate into the source. When a herald photon reaches the detector, the gate opens, allowing the second photon past."You get a photon when you expect one, and you don't get one when you don't,"Migdall says."It was an obvious solution; others proposed it long ago, we were just the first ones to build it. It makes the single photon source better."

In a second paper, the NIST team describes a photon source to address two other requirements. Quantum computers will need many such sources working in parallel, so sources must be able to be built in large numbers and operate reliably; and so that the computer can tell the photons apart, the sources must create multiple individual photons, but all at different wavelengths. The team outlines a way to create just such a source out of silicon, which has been well-understood by the electronics industry for decades as the material from which standard computer chips are built.

"Ordinarily a particular material can produce only pairs in a specific pair of wavelengths, but our design allows production of photons at a number of regular and distinct wavelengths simultaneously, all from one source,"Migdall says."Because the design is compatible with microfabrication techniques, this accomplishment is the first step in the process of creating sources that are part of integrated circuits, not just prototype computers that work in the hothouse of the lab."

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A guide star lets scientists see deep into human tissue

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Lihong Wang, PhD, the Gene K. Beare Distinguished Professor of Biomedical Engineering at Washington University in St. Louis, has invented a guide star for biomedical rather than celestial imaging, a breakthrough that promises game-changing improvements in biomedical imaging and light therapy.

Wang's guide star is an ultrasound beam that"tags"light that passes through it. When it emerges from thetissue, the tagged light, together with a reference beam, creates ahologram.

When a"reading beam"is then shown back through the hologram, it acts as a time-reversal mirror, creatinglight wavesthat follow their own paths backward through the tissue, coming to a focus at their virtual source, the spot where the ultrasound is focused.

The technique, called time-reversed ultrasonically encoded (TRUE) optical focusing, thus allows the scientist to focus light to a controllable position within tissue.

Wang thinks TRUE will lead to more effective light imaging, sensing, manipulation and therapy, all of which could be a boon medical research, diagnostics, and therapeutics.

In photothermal therapy, for example, scientists have had trouble delivering enough photons to a tumor to heat and kill the cells. So they either have to treat the tumor for a long time or use very strong light to get enough photons to the site, Wang says. But TRUE will allow them to focus light right on thetumor, ideally without losing a single tagged photon to scattering.

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In both cases photons take random paths through tissue. Some are lost (blue) but others (green) will reach the mirror on the other side of the tissue. The mirror is a special phase conjugate mirror that turns the light around and sends it back on its original path, as though time had been reversed. Clever as this is, by itself it isn't very useful because the light scatters again as is backtracks (left). In the new method, called TRUE, ultrasound is focused into the tissue (small black ring). Light passing through the ultrasound field is tagged by it and selectively returned by the mirror to its virtual source, the ultrasound focus (right). Instead of scattering, the light is brought to a focus inside the tissue. Credit: Lihong Wang

"Focusing light into a scattering medium such as tissue has been a dream for years and years, since the beginning of biomedical optics,"Wang says."We couldn't focus beyond say a millimeter, the width of a hair, and now you can focus wherever you wish without any invasive measure."

The new method was published inNature Photonics, which appeared online Jan. 16, and has since been spotlighted by Physics Today (both online and in print) and in aNature PhotonicsBackstage Interview.

The problem

Light is in many ways the ideal form of electromagnetic radiation for imaging and treating biological tissues, but it suffers from an overwhelming drawback. Light photons ricochets off nonuniformities in tissue like a steel ball ricochets off the bumpers of an old-fashioned pinball machine.

This scattering prevents you from seeing even a short distance through tissue; you can't, for example, see the bones in your hand. Light of the right color can penetrate several centimeters into biological tissue, but even with the best current technology, it isn't possible to produce high-resolution images of objects more than a millimeter below the skin with light alone.

Ultrasound's advantages and drawbacks are in many ways complementary to those of light. Ultrasound scattering is a thousand times weaker than optical scattering.

Ultrasound reveals a tissue's density and compressibility, which are often not very revealing. For example, the density of early-stage tumors doesn't differ that much from that of healthy tissue.

Ultrasound tagging

The TRUE technique overcomes these problems by combining for the first time two tricks of biomedical imaging science: ultrasound tagging and time reversal.

Wang had experimented with ultrasound tagging of light in 1994 when he was working at the M.D. Anderson Cancer Center in Houston, Texas. In experiments using a tissue phantom (a model that mimics the opacity of tissue), he focused ultrasound into the phantom from above, and then probed the phantom with a laser beam from the side.

The laser light had only one frequency as it entered the tissue sample, but the ultrasound, which is a pressure wave, changed the tissue's density and the positions of its scattering centers. Light passing through the precise point where the ultrasound was focused acquired different frequency components, a change that"tagged"these photons for further manipulation.

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A conventional mirror (bottom) does not correct the distortion of a wavefront produced by the water-filled bottle in this illustration. A time reversal, or phase conjugating, mirror (top), on the other hand, produces a wavefront that precisely retraces the path of the light, as if time were going backward. It reverses the distortions introduced by the water, producing a perfect image of the tiger. Credit: Wikimedia Commons

By tuning a detector to these frequencies, it is possible to sort photons arriving from one spot (the ultrasound focus) within the tissue and to discard others that have bypassed the ultrasonic beam and carry no information about that spot. The tagged photons can then be used to paint an image of the tissue at the ultrasound focus.

Ultrasound modulation of light allowed Wang to make clearer images of objects in tissue phantoms than could be made with light alone. But this technology selects only photons that have traversed the ultrasound field and cannot focus light.

Time reversal

While Wang was working on ultrasound modulation of optical light, a lab at the Langevin Institute in Paris led by Mathias Fink, was working on time reversal of sound waves.

No law of physics is violated if waves run backward instead of forward. So for every burst of sound (or light) that diverges from a source, there is in theory a set of waves that could precisely retrace the path of the sound back to the source.

To make this happen, however, you need a time-reversal mirror, a device to send the waves backward along exactly the same path by which they arrived. In Fink's experiments, the mirror consisted of a line of transducers that detected arriving sound and fed the signal to a computer.

Each transducer then played back its sound in reverse— in synchrony with the other transducers. This created what is called the conjugate of the original wave, a copy of the wave that traveled backward rather than forward and refocused on the original point source.

The idea of time reversal is so remote from everyday experience it is difficult to grasp, but as Scientific American reported at the time, if you stood in front of Fink's time-reversal"mirror"and said"hello,"you would hear"olleh,"and even more bizarrely, the sound of the"olleh,"instead of spreading throughout the room from the loudspeakers, would converge onto your mouth.

In a 1994 experiment, Fink and his colleagues sent sound through a set of 2000 steel rods immersed in a tank of water. The sound scattered along all the possible paths through the rods, arriving at the transducer array as a chaotic wave. These signals were time-reversed and sent back through the forest of rods, refocusing to a point at the source location.

In effect, time reversal is a way to undo scattering.

Combining the tricks

Wang was aware of the work with time reversal, but at first couldn't see how it might help solve his problem with tissue scattering.

In 2004, Michael Feld, a physicist interested inbiomedical imaging, invited Wang to give a seminar at the Massachusetts Institute of Technology."At dinner we talked about time reversal,"Wang says."Feld was thinking about time reversal, I was thinking about time reversal, and so was another colleague dining with us."

"The trouble was, we couldn't figure out how to use it. You know, if you send light through a piece of tissue, the light will scatter all over the place, and if you capture it and reverse it, sending it back, it will still be scattered all over the place, so it won't concentrate photons."

"And then 13 years after the initial ultrasound-tagging experiments, I suddenly realized I could combine these two techniques.

"If you added ultrasound, then you could focus light into tissue instead of through tissue. Ultrasound tagging lets you reverse and send back only those photons you know are going to converge to a focus in the tissue."

"Ultrasound provides a virtual guide star, and to make optical time reversal effective you need a guide star,"Wang says.

A time-reversal mirror for light

It's much easier to make a time-reversal mirror for ultrasound than for light. Because sound travels slowly, it is easy to record the entire time course of a sound signal and then broadcast that signal in reverse order.

But a light wave arrives so fast it isn't possible to record a time course with sufficient time resolution. No detector will respond fast enough. The solution is to record an interference pattern instead of a time course.

The beam that has gone through the tissue and a reference beam form an interference pattern, which is recorded as a hologram by a special photorefractive crystal.

Then the wavefront is reconstructed by sending a reading beam through the crystal from the direction opposite to that of the reference beam. The reading beam reconstitutes a reversed copy of the original wavefront, one that comes to a focus at the ultrasound focus.

Unlike the usual hologram, the TRUE hologram is dynamic and constantly changing. Thus it is able to compensate for natural motions, such as breathing and the flow of blood, and it adapts instantly when the experimenter moves the ultrasonic focus to a new spot.

More photons to work with

Wang expects the TRUE technique for focusing light within tissue will have many applications, including optical imaging, sensing, manipulation and therapy. He also mentions its likely impact on the emerging field of optogenetics.

In optogenetics, light is used to probe and control living neurons that are expressing light-activatable molecules or structures. Optogenetics may allow the neural circuits of living animals to be probed at the high speeds needed to understand brain information processing.

But until now, optogenetics has suffered from the same limitation that plagues optical methods for studying biological tissues. Areas of the brain near the surface can be stimulated with light sources directly mounted on the skull, but to study deeper areas, optical fibers must be inserted into the brain.

TRUE will allow light to be focused on these deeper areas without invasive procedures, finally achieving the goal of making tissue transparent at optical frequencies.

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Engineers grow nanolasers on silicon, pave way for on-chip photonics

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They describe their work in a paper to be published Feb. 6 in an advanced online issue of the journalNature Photonics.

"Our results impact a broad spectrum of scientific fields, including materials science, transistor technology,laserscience, optoelectronics andoptical physics,"said the study's principal investigator, Connie Chang-Hasnain, UC Berkeley professor of electrical engineering and computer sciences.

The increasing performance demands of electronics have sent researchers in search of better ways to harness the inherent ability oflight particlesto carry far more data thanelectrical signalscan. Optical interconnects are seen as a solution to overcoming the communications bottleneck within and betweencomputer chips.

Becausesilicon, the material that forms the foundation of modern electronics, is extremely deficient at generating light, engineers have turned to another class of materials known as III-V (pronounced"three-five") semiconductors to create light-based components such as light-emitting diodes (LEDs) and lasers.

But the researchers pointed out that marrying III-V with silicon to create a single optoelectronic chip has been problematic. For one, the atomic structures of the two materials are mismatched.

"Growing III-V semiconductor films on silicon is like forcing two incongruent puzzle pieces together,"said study lead author Roger Chen, a UC Berkeley graduate student in electrical engineering and computer sciences."It can be done, but the material gets damaged in the process."

Moreover, the manufacturing industry is set up for the production of silicon-based materials, so for practical reasons, the goal has been to integrate the fabrication of III-V devices into the existing infrastructure, the researchers said.

"Today's massive silicon electronics infrastructure is extremely difficult to change for both economic and technological reasons, so compatibility with silicon fabrication is critical,"said Chang-Hasnain."One problem is that growth of III-V semiconductors has traditionally involved high temperatures– 700 degrees Celsius or more– that would destroy the electronics. Meanwhile, other integration approaches have not been scalable."

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Shown is a schematic (left) and various scanning electron microscope images of nanolasers grown directly on a silicon surface. The achievement could lead to a new class of optoelectronic chips. Credit: Connie Chang-Hasnain Group

The UC Berkeley researchers overcame this limitation by finding a way to grow nanopillars made of indium gallium arsenide, a III-V material, onto asilicon surfaceat the relatively cool temperature of 400 degrees Celsius.

"Working at nanoscale levels has enabled us to grow high quality III-V materials at low temperatures such that silicon electronics can retain their functionality,"said Chen.

The researchers used metal-organic chemical vapor deposition to grow the nanopillars on the silicon."This technique is potentially mass manufacturable, since such a system is already used commercially to make thin film solar cells and light emitting diodes,"said Chang-Hasnain.

Once the nanopillar was made, the researchers showed that it could generate near infrared laser light– a wavelength of about 950 nanometers– at room temperature. The hexagonal geometry dictated by the crystal structure of the nanopillars creates a new, efficient, light-trapping optical cavity. Light circulates up and down the structure in a helical fashion and amplifies via this optical feedback mechanism.

The unique approach of growing nanolasers directly onto silicon could lead to highly efficient silicon photonics, the researchers said. They noted that the miniscule dimensions of the nanopillars– smaller than one wavelength on each side, in some cases– make it possible to pack them into small spaces with the added benefit of consuming very little energy

"Ultimately, this technique may provide a powerful and new avenue for engineering on-chip nanophotonic devices such as lasers, photodetectors, modulators and solar cells,"said Chen.

"This is the first bottom-up integration of III-V nanolasers onto silicon chips using a growth process compatible with the CMOS (complementary metal oxide semiconductor) technology now used to make integrated circuits,"said Chang-Hasnain."This research has the potential to catalyze an optoelectronics revolution in computing, communications, displays and optical signal processing. In the future, we expect to improve the characteristics of these lasers and ultimately control them electronically for a powerful marriage between photonic and electronic devices."

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Scientists study processes using high-intensity ultrashort X-ray pulses

The generation of X-ray flashes that are only a few femtoseconds (quadrillionths of a second) long has been possible for some years. Such flashes can be produced by free-electron lasers (FEL), such as FLASH at the DESY research centre in Hamburg, LCLS in Stanford (USA) and the X-ray laser European XFEL currently under construction. So far, however, experiments only reached time resolutions of typically around one hundred femtoseconds– i.e., two orders of magnitude worse than the actual pulse durations. The problem was to determine precisely when the X-ray pulse arrived at the experiment.

A research group from the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB), DESY, the European XFEL GmbH and the Helmholtz Institute Jena has now found a way to measure the arrival time of the X-ray pulses with a precision of less than ten femtoseconds. The method is based on a so-called cross-correlation.

The new method was developed at the free-electron laser FLASH for so-called pump-probe processes. As an example: a first ultrashort pump pulse triggers a photochemical reaction. A second X-ray radiation pulse takes a“photograph” of how the reaction proceeds. For the first time, researchers are now able to determine exactly at what time the picture produced by the second pulse is created. For this new method, they make use of a side effect of the X-ray pulse generation. Indeed, the electron bunch accelerated in FLASH emits both an X-ray flash and an intense terahertz flash at the same time. The researchers separate the two flashes using a perforated, gold-coated mirror. As both pulses are created at the same time and from the same electron bunch, theterahertzflash can be used as a temporal“marker” of the X-ray flash. Using this method, the researchers were able to determine the time at which the X-ray pulse arrived at the sample with a precision of seven femtoseconds.

The new method can be used at all existing and planned new FEL sources given only very slight modifications. In combination with appropriate experiments, it opens up the possibility to fully exploit the potential of these large-scale facilities. For the first time, phenomena can now be studied withX-rayson the relevant femtosecond time scale– something scientists have long been waiting for.

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