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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