Unpeeling Atoms and Molecules from the Inside Out

Nora Berrah [photo courtesy: Western Michigan University]

The first published scientific results from the world's most powerful hard X-ray laser -- Linac Coherent Light Source (LCLS) -- located at the Department of Energy's SLAC National Accelerator Laboratory, show its unique ability to control the behaviors of individual electrons within simple atoms and molecules by stripping them away, one by one—in some cases creating hollow atoms.

These early results describe in great detail how the Linac Coherent Light Source's intense pulses of X-ray light change the very atoms and molecules they are designed to image. Controlling those changes will be critical to achieving the atomic-scale images of biological molecules and movies of chemical processes that the LCLS is designed to produce.

In a report published June 22 in Physical Review Letters [1], a team led by physicist Nora Berrah of Western Michigan University—the third group to conduct experiments at the LCLS—describes the first experiments on molecules. Her group also created hollow atoms, in this case within molecules of nitrogen gas, and found surprising differences in the way short and long laser pulses of exactly the same energies stripped and damaged the nitrogen molecules.

"We just introduced molecules into the chamber and looked at what was coming out there, and we found surprising new science," said Matthias Hoener, a postdoctoral researcher in Berrah's group at WMU and visiting scientist at Lawrence Berkeley National Laboratory who was first author of the paper. "Now we know that by reducing the pulse length, the interaction with the molecule becomes less violent. "

Linda Young [photo courtesy: Argonne National Laboratory]

In another report published in the July 1 issue of Nature [2], a team led by Argonne National Laboratory physicist Linda Young describes how they were able to tune LCLS pulses to selectively strip electrons, one by one, from atoms of neon gas. By varying the photon energies of the pulses, they could do it from the outside in or—a more difficult task—from the inside out, creating so-called "hollow atoms."

"Until very recently, few believed that a free-electron X-ray laser was even possible in principle, let alone capable of being used with this precision," said William Brinkman, director of DOE's Office of Science. "That's what makes these results so exciting."

Young, who led the first experiments in October with collaborators from SLAC and five other institutions, said, "No one has ever had access to X-rays of this intensity, so the way in which ultra-intense X-rays interact with matter was completely unknown. It was important to establish these basic interaction mechanisms."

SLAC's Joachim Stöhr, director of the LCLS, said, "When we thought of the first experiments with LCLS ten years ago, we envisioned that the LCLS beam may actually be powerful enough to create hollow atoms, but at that time it was only a dream. The dream has now become reality."

The world's first hard X-ray free-electron laser started operation with a bang. First experiments at SLAC National Accelerator Laboratory's Linac Coherent Light Source stripped electrons one by one from neon atoms (illustrated above) and nitrogen molecules, in some cases removing only the innermost electrons to create "hollow atoms." Understanding how the machine's ultra-bright X-ray pulses interact with matter will be critical for making clear, atomic-scale images of biological molecules and movies of chemical processes. (Artwork by Gregory Stewart, SLAC)

While the first experiments were designed to see what the LCLS can do and how its ultra-fast, ultra-bright pulses interact with atoms and molecules, they also pave the way for more complex experiments to come. Its unique capabilities make the LCLS a powerful tool for research in a wide range of fields, including physics, chemistry, biology, materials and energy sciences.

The LCLS forms images by scattering X-ray light off an atom, molecule or larger sample of material. Yet when the LCLS X-rays are tightly focused by mirrors, each powerful laser pulse destroys any sample it hits. Since certain types of damage, like the melting of a solid, are not instantaneous and only develop with time, the trick is to minimize the damage during the pulse itself and record the X-ray snapshot with a camera before the sample disintegrates.

Both teams found that the shorter the laser pulse, the fewer electrons are stripped away from the atom or molecule and the less damage is done. And both delved into the detailed mechanisms behind that damage.

Atoms are a little like miniature solar systems, with their electrons orbiting at various distances from the nucleus in a sort of quantum fuzz. To make things simpler, scientists describe the electrons as orbiting in "shells" at specific distances from the nucleus. The innermost shell contains up to two electrons, the next one up to eight, the third one up to 18, and so on.

Since they're closest to the positively charged nucleus, the two innermost electrons are generally the hardest to wrest away. But they also most readily absorb photons of X-ray light, and so are the most vulnerable to getting stripped away by intense X-rays.

Although previous experiments with intense optical lasers had stripped neon atoms of most of their electrons, Young's was the first to discover how ultra-intense X-ray lasers do this. At low photon energies, the outer electrons are removed, leaving the inner electrons untouched. However, at higher photon energies, the inner electrons are the first to be ejected; then the outer electrons cascade into the empty inner core, only to be kicked out by later parts of the same X-ray pulse. Even within the span of a single pulse there may be times when both inner electrons are missing, creating a hollow atom that is transparent to X-rays, Young said.

"This transparency associated with hollow atoms could be a useful property for future imaging experiments, because it decreases the fraction of photons doing damage and allows a higher percentage of photons to scatter off the atom and create the image," Young said. She said application of this phenomenon will also allow researchers to control how deeply an intense X-ray pulse penetrates into a sample.

Berrah's team bombarded puffs of nitrogen gas with laser pulses that ranged in duration from about four femtoseconds, or quadrillionths of a second, to 280 femtoseconds. No matter how short or long it was, though, each pulse contained the same amount of energy in the form of X-ray light; so you might expect that they would have roughly the same effects on the nitrogen molecules.

But to the team's surprise, that was not the case, Hoener said. The long pulses stripped every single electron from the nitrogen molecules, starting with the ones closest to the nucleus; the short ones stripped off only some of them.

Their report attributes this to the "frustrated absorption effect": Since the molecule's electrons are preferentially stripped from the innermost shells, there is simply not enough time during a short pulse for the molecule's outermost electrons to refill the innermost shells and get kicked out in turn.

With all this activity going on inside the atom, scientists have a new way to explore atomic structure and dynamics. Further experiments have investigated nanoclusters of atoms, protein nanocrystals and even individual viruses, with results expected to be published in coming months.

Reference
[1] M. Hoener, L. Fang, O. Kornilov, O. Gessner, S. T. Pratt, M. Gühr, E. P. Kanter, C. Blaga, C. Bostedt, J. D. Bozek, P. H. Bucksbaum, C. Buth, M. Chen, R. Coffee, J. Cryan, L. DiMauro, M. Glownia, E. Hosler, E. Kukk, S. R. Leone, B. McFarland, M. Messerschmidt, B. Murphy, V. Petrovic, D. Rolles and N. Berrah, "Ultraintense X-Ray Induced Ionization, Dissociation, and Frustrated Absorption in Molecular Nitrogen", Phys. Rev. Lett. 104, 253002 (2010). Abstract.
[2] L. Young, E. P. Kanter, B. Krässig, Y. Li, A. M. March, S. T. Pratt, R. Santra, S. H. Southworth, N. Rohringer, L. F. DiMauro, G. Doumy, C. A. Roedig, N. Berrah, L. Fang, M. Hoener, P. H. Bucksbaum, J. P. Cryan, S. Ghimire, J. M. Glownia, D. A. Reis, J. D. Bozek, C. Bostedt & M. Messerschmidt, "Femtosecond electronic response of atoms to ultra-intense X-rays", Nature 466, 56-61 (1 July 2010). Abstract.

Entanglement with Frequency Combs

David Hayes

For the first time, physicists have employed a powerful technique of laser physics – the “optical frequency comb” – to entangle two trapped atoms [1]. This form of control is a promising candidate for use as a means of quantum control for quantum computing and information-processing, and offers substantial operational advantages over other methods.

The team, led by the Joint Quantum Institute (JQI) Fellow Christopher Monroe, began by preparing two ytterbium ions, spaced about five micrometers apart in an electrical trap, in identical minimal-energy ground states. [See Figure 1] The goal of the experiment was to entangle these two ions – that is, to place them in a condition in which the quantum state of one is inextricably correlated with the state of the other – using light from a single high-speed pulsed laser.

>>Link to `Trapped Ion Quantum Information Group' led by Christopher Monroe, University of Maryland

Past 2Physics articles on the work of this group:
"Long-Distance Teleportation between Two Atoms"
"Quantum Entanglement between Single Atoms One Meter Apart!"

[Click on the figures 1-3 below to view higher resolution versions]

Entanglement is the resource that will likely be used for transferring quantum information from place to place in any future quantum computer or information-processing system. As a result, there is intense global interest in finding dependable, high-speed entanglement schemes.

The researchers entangled the ions in a two-part process in which both parts occur simultaneously, generated by the laser pulses. In part one, the ions are placed in a superposition of two hyperfine states – tiny energy differences within a single excitation level of an atom that result from electrons’ interaction with the nucleus. The hyperfine states serve as a two-level system that permits each ion to function as a quantum information bit, or “qubit,” as it exists in some combination of both states at the same time.

In part two, the laser pulses give each ion a momentum “kick” that depends on which qubit state it is in. These kicks are slight physical displacements that cause each positively charged ion to affect the other through interaction of their electrical fields. There are multiple pathways by which a sequence of photons can place each ion in a given quantum vibrational state. Owing to a peculiarity of quantum mechanics, when it is impossible to know which photon sequence put the ions in their target vibrational states, the ions are entangled. [See Figure 2]

“These sorts of procedures have been done before with continuous-wave lasers,” says Dave Hayes, first author on the paper, “but never with a frequency comb.”

An optical frequency comb is a peculiar property of pulsed, “mode-locked” lasers. In the cavity of a laser, photons with a narrow range of frequencies reflect back and forth between the two mirrors on either end. [See Figure 3] Many of those frequencies will be suppressed by destructive interference; they cancel themselves out because the round-trip distance from mirror to mirror in the cavity is not an integer multiple of their wavelengths. But many frequencies will have exactly the right wavelengths to resonate in the cavity, forming standing waves like a jump rope. Each of those standing waves is a mode of the cavity.

When all the modes have the same phase relationship to each other, they are said to be “mode-locked,” and something remarkable happens: At periodic intervals, the standing waves interfere constructively – that is, reinforce each other – forming a very brief, intense pulse with a duration in the picosecond range or even shorter. (A picosecond is one millionth of one millionth of a second.) The JQI team used a train of 1-picosecond pulses separated by 12.5 nanoseconds.

Each pulse consists of multiple frequencies, which over time build up into a pattern of sharply defined frequency spikes that are uniformly spaced like the teeth in a comb. If the energy difference between any two comb frequencies corresponds exactly with an atomic quantum transition, it will produce it. But many desired effects – including those sought by the JQI team – do not precisely match any spacing in the teeth of a single comb. So the scientists split the main pulsed laser beam into separate beams and applied slight frequency offsets to them with devices called acousto-optic modulators (AOMs).

By tuning the system with the AOMs, the team can create any frequency difference they want to produce both the target effects: generating qubit states in the ions, and entangling the ions by momentum kicks. For the latter, Hayes says, “when we can use two teeth from offset combs to match the energy of transition between motion quanta, we’re in business to entangle neighboring ions.”

“There are inherent advantages of the frequency comb method,” Hayes notes. “One consideration is cost. Mode-locked lasers are just cheaper, especially for ions because the transitions are usually in the ultraviolet. Continuous-wave lasers in that frequency range are really expensive, while it’s simple to generate uv light with these high power pulses.” The main criterion governing the type of pulsed laser to be used is the bandwidth – the range of frequencies that can be carried in the pulses.

The JQI researchers used a titanium-doped sapphire laser for this experiment. “What’s special about the beam,” Hayes says, “is that the bandwidth of a single pulse is larger than the energy difference between the two qubit states.”

The comb method is also attractive because of the potential for much higher laser powers, delivered in a much shorter time span, which could allow quantum logic gates to operate very fast. While this aspect was not part of this demonstration, it will be addressed in future experiments. But more generally, as Hayes explains, “the optical frequency comb has so many frequency markers that this system should also be useful for the quantum control of almost any optical quantum system, from other atomic species to molecules or even quantum dots.”

We thank Curt Suplee for writing this article.

Reference
[1] “Entanglement of Atomic Qubits Using an Optical Frequency Comb,” D. Hayes, D.N. Matsukevich, P. Maunz, D. Hucul, Q. Quraishi, S. Olmschenk, W. Campbell, J. Mizrahi, C. Senko and C. Monroe, Physical Review Letters, 104, 140501 (2010). Abstract.

Single Photons Observed at Seemingly Faster-than-Light Speeds

Paul Lett [Photo courtesy: Joint Quantum Institute, U. Maryland]

Researchers at the Joint Quantum Institute (JQI), a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland at College Park, can speed up photons (particles of light) to seemingly faster-than-light speeds through a stack of materials by adding a single, strategically placed layer. This experimental demonstration confirms intriguing quantum-physics predictions that light’s transit time through complex multilayered materials need not depend on thickness, as it does for simple materials such as glass, but rather on the order in which the layers are stacked. This is the first published study [1] of this dependence with single photons.

Strictly speaking, light always achieves its maximum speed in a vacuum, or empty space, and slows down appreciably when it travels through a material substance, such as glass or water. The same is true for light traveling through a stack of dielectric materials, which are electrically insulating and can be used to create highly reflective structures that are often used as optical coatings on mirrors or fiber optics.

In a follow up to earlier experimental measurements [2], the JQI researchers created stacks of approximately 30 dielectric layers, each about 80 nanometers thick, equivalent to about a quarter of a wavelength of the light traveling through it. The layers alternated between high (H) and low (L) refractive index material, which cause light waves to bend or reflect by varying amounts. After a single photon hits the boundary between the H and L layers, it has a chance of being reflected or passing through.

When encountering a stack of 30 layers alternating between L and H, the rare photons that completely penetrate the stack pass through in about 12.84 femtoseconds (fs, quadrillionths of a second). Adding a single low-index layer to the end of this stack disproportionately increased the photon transit time by 3.52 fs to about 16.36 fs. (The transit time through this added layer would be only about 0.58 fs, if it depended only upon the layer’s thickness and refractive index.) On the contrary, adding an extra H layer to a stack of 30 layers alternating between H and L would reduce the transit time to about 5.34 fs, so that individual photons seem to emerge through the 2.6-micron-thick stack at superluminal (faster-than-light) speeds.

What the JQI researchers are seeing can be explained by the wave properties of light. In this experiment, the light begins and ends its existence acting as a particle—a photon. But when one of these photons hits a boundary between the layers of material, it creates waves at each surface, and the traveling light waves interfere with each other just as opposing ocean waves cause a riptide at the beach. With the H and L layers arranged just right, the interfering light waves combine to give rise to transmitted photons that emerge early. No faster than light speed information transfer occurs because, in actuality, it is something of an illusion: only a small proportion of photons make it through the stack, and if all the initial photons were detected, the detectors would record photons over a normal distribution of times.

References
[1] N. Borjemscaia, S.V. Polyakov, P.D. Lett and A. Migdall, "Single-photon propagation through dielectric bandgaps", Optics Express, v 18, p 2279 (2010). Abstract.
[2] N. Rutter, S.V. Polyakov, P. Lett amd A. Migdall, "Photon tunneling through dielectric bandgaps and evanescent gaps", Presented at the American Physical Society March Meeting, New Orleans, La. Session: W14.00010. Abstract. News.

[We thank Joint Quantum Institute at University of Maryland and National Institute of Standards and Technology for materials used in this report]

Ultralong Lasers Cavity Length Limits Explored

Juan Diego Ania-Castañón of Instituto de Óptica (CSIC), Spain

[This is an invited article based on a recent work by the author and his collaborators from UK and Russia. -- 2Physics.com]

Author: Juan Diego Ania Castañón
Affiliation: Instituto de Óptica, CSIC, Spain

Since their inception, lasers have been considered simply as sources of light. However, ultralong lasers implemented in optical fiber can also be seen as unique transmission media opening the way to a new outlook on information transmission and secure communications.

In our recent paper in Physical Review Letters [1] we present a study of the physical mechanisms that restrict the achievable cavity length in a fiber laser cavity, achieving in the process what is to date the longest laser ever built, reaching 270 km.

P. Harper, S. Turitsyn, D. Churkin, A.E. El-Taher (Photonics Research Group, Aston University, UK)

Ultralong lasers, first proposed in 2004 [2] and experimentally demonstrated in 2006 [3], have been shown to induce virtual transparency in optical fiber, offering quasi-lossless transmission conditions which are ideal for the implementation of soliton-based systems and signal processing. Departing from previous application-oriented studies, we set out on this occasion to explore the fundamental limits of laser operation. This endeavor has resulted in the discovery of interesting new physical regimes of operation, different from those observed in traditional lasers.

S. Kablukov, E.V. Podivilov and S. Babin (Institute of Automation of Electrometry, Russia)

A typical ultra-long Raman fiber laser consists of one or more reels of optical fiber, which act as both the active medium and the potential transmission medium, one or more pump sources that inject radiation into the cavity in order to induce lasing, and a set of fiber Bragg grating reflectors (tuned to the Stokes wavelength) which delimit the cavity. These pump sources are themselves usually standard short-length Raman fiber lasers. The Raman frequency shift in optical fiber is of ~ 13 THz, which translates into roughly 100 nm at the usual infrared wavelengths used for telecommunication.

Taking advantage of the reduced attenuation offered by standard optical fiber in the telecommunication spectral window, we were able to strech cavity length up to 270 km while still retaining a resolvable cavity mode structure, confirming the formation of an ultralong standing electromagnetic wave. Our initial pump sources at 1450 nm were used to induce lasing in the cavity at the corresponding Stokes wavelength of 1550 nm.

Of course, such an enormous resonant cavity presents a similarly extraordinary number of longitudinal cavity modes [4]. Indeed, the observed spectral separation between modes clearly follows the classical formula ∆ν =c/2nL, where n is the refractive index of the fiber core, c the speed of light and L the cavity length, which for a typical grating bandwidth of 100 GHz, brings the number of modes to the hundreds of millions. These modes are broadened and eventually washed out as they interact with each other through intensity-dependent, turbulent-like four-wave mixing processes in the fibre, meaning that in order for the mode-structure to be resolvable at such extended cavity lengths, Stokes wave intensity must be kept low.

Perhaps even more interestingly, at such long cavity lengths there is an additional physical effect that contributes to the washing out of the cavity modes: Rayleigh backscattering. The random backreflection of Stokes photons by Silica molecules along the optical fiber forms a family of overlapping cavities of randomly varying length. Our calculations show that for a system such as ours, the amount of radiation reflected in these random scatterings and the amount of radiation reflected at the ultra-long laser cavity gratings themselves become comparable when the length of fiber is of the order of 250 km, in agreement with the observed experimental limit for mode resolution.

Ultralong lasers already present unique applications in a variety of areas ranging from the implementation of effectively lossless broadband transmission links, the posibility of new classical means for secure key distribution and the design of highly efficient supercontinuum sources, but most importantly, they represent a new and rich field of study that combines diverse areas of Physics such as nonlinear science, the theory of disordered systems or wave turbulence. This richness allows us to anticipate that new applications and technologies of ultralong lasers will continue to emerge in the future.

References
[1] S.K. Turitsyn, J.D. Ania-Castañón, S.A. Babin, V. Karalekas, P. Harper, D. Churkin, S.I. Kablukov, A.E. El-Taher, E. V. Podivilov, and V. K. Mezentsev, “270 km Ultralong Raman Fiber Laser”, Phys. Rev. Lett. 103 133901 (2009). Abstract.
[2] J.D. Ania-Castañón, “Quasi-lossless transmission using second-order Raman amplification and fibre Bragg gratings”, Optics Express, 12(19), 4372-4377 (2004). Abstract.
[3] Juan Diego Ania-Castañón, Tim J. Ellingham, R. Ibbotson, X. Chen, L. Zhang, and Sergei K. Turitsyn, “Ultralong Raman Fiber Lasers as Virtually Lossless Optical Media”, Phys. Rev. Lett. 96 23902 (2006). Abstract.
[4] S. A. Babin, V. Karalekas, P. Harper, E. V. Podivilov, V. K. Mezentsev, J. D. Ania-Castañón, and S. K. Turitsyn, “Experimental demonstration of mode structure in ultralong Raman fiber lasers”, Opt. Lett. 32(9) 1135-1137 (2007). Abstract.

Science Begins At The World's Most Powerful X-Ray Laser

Joachim Stöhr, Director of LCLS (Photo courtesy: Stanford Linear Accelerator Laboratory)

The first experiments are now underway using the world's most powerful X-ray laser, the Linac Coherent Light Source (LCLS), located at the Department of Energy's SLAC National Accelerator Laboratory. LCLS produces ultrafast pulses of X-rays millions of times brighter than even the most powerful synchrotron sources — pulses powerful enough to make images of single molecules. Illuminating objects and processes at unprecedented speed and scale, the LCLS has embarked on groundbreaking research in physics, structural biology, energy science, chemistry and a host of other fields.

In early October, researchers from around the globe began traveling to SLAC to get an initial glimpse into how the X-ray laser interacts with atoms and molecules. The LCLS is unique, shining light that can resolve detail the size of atoms at ten billion times the brightness of any other manmade X-ray source.

"No one has ever had access to this kind of light before," said LCLS Director Jo Stöhr. "The realization of the LCLS isn't only a huge achievement for SLAC, but an achievement for the global science community. It will allow us to study the atomic world in ways never before possible."

The LCLS is a testament to SLAC's continued leadership in accelerator technology. Over 40 years ago, the laboratory's two-mile linear accelerator, or linac, was built to study the fundamental building blocks of the universe. Now, decades later, this same machine has been revitalized for frontier research underway at the LCLS.

After SLAC's linac accelerates very short pulses of electrons to 99.9999999 percent the speed of light, the LCLS takes them through a 100-meter stretch of alternating magnets that force the electrons to slalom back and forth. This motion causes the electrons to emit X-rays, which become synchronized as they interact with the electron pulses over this long slalom course, thereby creating the world's brightest X-ray laser pulse. Each of these laser pulses packs as many as 10 trillion X-ray photons into a bunch that's a mere 100 femtoseconds long—the time it takes light to travel the width of a human hair.

Atomic, Molecular and Optical Science (AMO) Instrument (Image courtesy: Stanford Linear Accelerator Laboratory)

Currently, user-assisted commissioning is underway, with researchers conducting experiments using the Atomic, Molecular and Optical science instrument, the first of six planned instruments for the LCLS. In these first AMO experiments, researchers are using X-rays from the LCLS to gain an in-depth understanding of how the ultra-bright beam interacts with matter.

Early experiments are already revealing new insights into the fundamentals of atomic physics and have successfully proven the machine's unique capabilities to control and manipulate the underlying properties of atoms and molecules. Earlier this month, researchers used the LCLS's strobe-like pulses to completely strip neon atoms of all their electrons. Researchers also watched for two-photon ionization—an event where two photons pool their energy to eject a single electron from an atom. Normally difficult to observe at X-ray facilities, researchers at the LCLS were able to study these events using the extreme brightness of the laser beam.

Future AMO experiments will create stop-action movies of molecules in motion. The LCLS's quick, short, repetitive X-ray bursts enable researchers to take individual photos as molecules move and interact. By stringing together many such images to make a movie, researchers will for the first time have the ability to watch the molecules of life in action, view chemical bonds forming and breaking in real time, and see how materials work on the quantum level.

By 2013, all six LCLS scientific instruments will be online and operational, giving researchers unprecedented tools for a broad range of research in material science, medicine, chemistry, energy science, physics, biology and environmental science.

"It's hard to overstate how successful these first experiments have been," said AMO Instrument Scientist John Bozek. "We look forward to even better things to come."

[We thank Stanford Linear Accelerator Laboratory for materials used in this posting.]

Physics Nobel 2009 : The Masters of Light

Charles K. Kao [photo courtesy: Chinese University of Hong Kong]

This year's Nobel Prize in Physics is awarded for two scientific achievements that have helped to shape the foundations of today’s networked societies. They have created many practical innovations for everyday life and provided new tools for scientific exploration.

The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2009 with one half to Charles K. Kao of Standard Telecommunication Laboratories, Harlow, UK, and Chinese University of Hong Kong "for groundbreaking achievements concerning the transmission of light in fibers for optical communication", and the other half jointly to Willard S. Boyle and George E. Smith of Bell Laboratories, Murray Hill, NJ, USA "for the invention of an imaging semiconductor circuit – the CCD sensor".

In 1966, Charles K. Kao made a discovery that led to a breakthrough in fiber optics. He carefully calculated how to transmit light over long distances via optical glass fibers. With a fiber of purest glass it would be possible to transmit light signals over 100 kilometers, compared to only 20 meters for the fibers available in the 1960s. Kao's enthusiasm inspired other researchers to share his vision of the future potential of fiber optics. The first ultrapure fiber was successfully fabricated just four years later, in 1970.

Willard S Boyle [photo courtesy: RMC Club of Canada]

Today optical fibers make up the circulatory system that nourishes our communication society. These low-loss glass fibers facilitate global broadband communication such as the Internet. Light flows in thin threads of glass, and it carries almost all of the telephony and data traffic in each and every direction. Text, music, images and video can be transferred around the globe in a split second.

If we were to unravel all of the glass fibers that wind around the globe, we would get a single thread over one billion kilometers long – which is enough to encircle the globe more than 25 000 times – and is increasing by thousands of kilometers every hour.

A large share of the traffic is made up of digital images, which constitute the second part of the award. In 1969 Willard S. Boyle and George E. Smith invented the first successful imaging technology using a digital sensor, a CCD (Charge-Coupled Device). The CCD technology makes use of the photoelectric effect, as theorized by Albert Einstein and for which he was awarded the 1921 year's Nobel Prize. By this effect, light is transformed into electric signals. The challenge when designing an image sensor was to gather and read out the signals in a large number of image points, pixels, in a short time.

George E. Smith [photo courtesy: IEEE]

The CCD is the digital camera's electronic eye. It revolutionized photography, as light could now be captured electronically instead of on film. The digital form facilitates the processing and distribution of these images. CCD technology is also used in many medical applications, e.g. imaging the inside of the human body, both for diagnostics and for microsurgery.

Digital photography has become an irreplaceable tool in many fields of research. The CCD has provided new possibilities to visualize the previously unseen. It has given us crystal clear images of distant places in our universe as well as the depths of the oceans.

Laser Pulses Control Single Electrons in Complex Molecules

Garching and Munich researchers (L to R) Regina de Vivie-Riedle, Philipp von den Hoff, Matthias Kling, and Irina Znakovskaya. [Photograph compliments of Thorsten Naeser of Max Planck Institute of Quantum Optics]



[This is an invited article based on recent works of the authors and their team members -- 2Physics.com]



Authors: Matthias Kling and Regina de Vivie-Riedle

Affiliations: Max Planck Institute of Quantum Optics, Garching and
Ludwig-Maximilians University, Munich, Germany

Link to 'Attosecond Imaging' Group >>

Predatory fish are well aware of the problem: In a swarm of small fish it is hard to isolate the prey. A similar situation can be found in the microcosm of atoms and molecules, whose behavior is influenced by “swarms” of electrons. In order to achieve control over single electrons in a bunch, ultrashort light pulses of a few femtoseconds duration are needed.

Electrons are extremely fast. In atoms and molecules they move on attosecond timescales [1]. An attosecond is only a billionth of a billionth of a second. With light pulses that last only a few femtoseconds down to attoseconds it is possible to achieve control over these particles and to interact with them on the timescale of their motion. These short light pulses exhibit strong electric and magnetic fields influencing the charged particles. A femtosecond lasts 1000 times longer than an attosecond. In molecules with only a single electron, such as the hydrogen molecular ion and its isotopes, their control with such light pulses is relatively easy and was demonstrated by Kling et al. [2, 3]. These first studies on controlling electrons in molecules with waveform controlled light pulses sparked, however, the question, whether the steering of electrons in more complex systems is feasible.

In our recent work [4] we have now managed to control and monitor the outer electrons from the valence shell of the complex molecule carbon monoxide (CO) utilizing the electric field waveform of laser pulses. Carbon monoxide has 14 electrons. With increasing number of electrons in the molecule the control over single electrons becomes more difficult as their states lie energetically very close to each other. Control of the electric field waveform E(t) = E₀(t) cos(ωt + φ), with envelope E₀(t), and frequency ω, is achieved by stabilization and control of the carrier envelope phase (CEP) φ and constitutes a new paradigm of coherent control that can be applied to steer electrons in atomic and molecular systems [1].

We used visible (740 nm) laser pulses with only 4 femtoseconds duration. The control of electron dynamics in the system was experimentally determined via an asymmetric distribution of C⁺ and of O⁺ fragments after the breaking of the molecular bond. The measurement of C⁺ and O⁺ fragments implies a dynamic charge shift along the molecular axis in one or the other direction, controlled via the CEP of the laser pulse.

Theoretical methods based on ab initio quantum mechanics were developed for multi electron systems and applied to analyze the ionization and subsequent electron localization process. The femtosecond laser pulses initially detached an electron from a CO molecule. Subsequently the electron was driven by the laser field away from and back to the ion, where it transferred its energy in a collision. The whole process takes only ca. 1.7 femtoseconds. The collision produces an electronic wave packet which induces a directional motion of electrons along the molecular axis. The excitation and subsequent interaction with the remaining part of the intense laser pulse leads to a coupling of electron and nuclear motion and gives a contribution to the observed asymmetry.

From theoretical analysis it became clear that in the performed experiments, it was also possible to image the structure and form of the outer two electron orbitals of carbon monoxide via the ionization process. The extremely short femtosecond laser pulses allowed to explore this process in the outermost HOMO (highest occupied molecular orbitals) and HOMO-1 orbitals. The ionization of the molecules is found to take place with a distinct angular dependence with respect to the laser polarization direction. This dependence is seen in fragments from the dissociation following ionization (see Fig. 1 for recorded C+ fragments). Our observation is in good agreement with theoretical calculations (shown as black line in Fig. 1) [4]. The ionization process itself also gives a contribution to the observed asymmetry, which strongly depends on the duration of the laser pulses [4].

Fig. 1: The detachment of electrons from carbon monoxide molecules by femtosecond laser pulses leads to a characteristic angular distribution of the molecular ions and their fragments. The angular distribution resembles the structure of orbitals from which electrons have been ionized. The figure shows the angular distribution of C⁺ ions measured in the laser-induced dissociative ionization of CO via velocity-map imaging (VMI) for CEP = Π. The laser polarization is vertical and the number of observed ions is displayed in color.

Electrons are present in all important microscopic biological and technical processes. Their extremely fast motion on the attosecond timescale, determines biological and chemical processes and also the speed of microprocessors – technology at the heart of computing. With their experiments the researchers have made a further, important step towards the control of chemical reactions with light. The results are also related to basic research on lightwave electronics [5] aiming at computing speeds on attosecond timescales.

References
[1] M. F. Kling, and M. J. J. Vrakking, "Attosecond Electron Dynamics", Annu. Rev. Phys. Chem. 59, 463 (2008). Abstract.
[2] M. F. Kling, Ch. Siedschlag, A. J. Verhoef, J. I. Khan, M. Schultze, Th. Uphues, Y. Ni, M. Uiberacker, M. Drescher, F. Krausz, M. J. J. Vrakking, "Control of Electron Localization in Molecular Dissociation", Science 312, 246 (2006). Abstract.
[3] M. F. Kling, Ch. Siedschlag, I. Znakovskaya, A. J. Verhoef, S. Zherebtsov, F. Krausz, M. Lezius, M. J. J. Vrakking, "Strong-field control of electron localization during molecular dissociation", Mol. Phys. 106, 455 (2008). Abstract.
[4] I. Znakovskaya, P. von den Hoff, S. Zherebtsov, A. Wirth, O. Herrwerth, M. J. J. Vrakking, R. de Vivie-Riedle, M. F. Kling, "Attosecond Control of Electron Dynamics in Carbon Monoxide", Phys. Rev. Lett. 103, 103002 (2009). Abstract.
[5] E. Goulielmakis, V. S. Yakovlev, A. L. Cavalieri, M. Uiberacker, V. Pervak, A. Apolonski, R. Kienberger, U. Kleineberg, F. Krausz, "Attosecond Control and Measurement: Lightwave Electronics", Science 317, 769 (2007). Abstract.

Phonon Laser Demonstrated Using Trapped Ions

For decades there has been interest in phonon lasers, a device that operates in analogy to optical lasers, except replacing light with vibrational energy. Phonon lasers would amplify motion to the point of coherence through the process of stimulated emission, whereby the presence of energy quanta increases the chances that additional quanta are emitted. Now, researchers at the Max Planck Institute of Quantum Optics in Garching, Germany have realized such a device using a cold, harmonically bound, magnesium ion.

Reporting in the journal Nature Physics [1], the ion was first laser cooled to a temperature of around 1 milliKelvin in an electromagnetic trap, and then a second laser was applied, whose wavelength was precisely adjusted so as to create mechanical (not optical) amplification through the process of stimulated emission of phonons. The phonons are associated with the center-of-mass motion of the ion in the electromagnetic trap. As such, phonon stimulated emission causes the ion to oscillate, once a threshold condition is achieved (figure).

Sequence of time-averaged images showing coherent motion of the phonon laser. At the far left, the magnesium ion is cooled to approximately 1 milli-Kelvin using a red-detuned laser. In each subsequent image, an additional, blue-detuned pump laser is stepped in intensity. Beginning around the fifth trace, laser threshold is achieved at which point mechanical amplification compensates mechanical damping. Beyond this threshold pumping level, the motion is sustained by stimulated emission of phonons and stabilized by saturation of the amplification. The ions oscillate vertically in the figure, and the brighter end points in each image result from the ion slowing and eventually stopping to reverse motion.

“The possibility of phonon laser action has been considered dating back to the earliest days of the optical laser, nearly 50 years ago,” notes Kerry Vahala, who is currently a guest scientist at the Max Planck Institute of Quantum Optics in Garching, on leave from Caltech. He adds that the possible ways to realize these devices are as numerous as those used for conventional optical lasers, and ultimately, many other types of phonon lasers including those in the solid-state are possible.

“Laser-cooled ions are, however, a very good starting point to explore the physics of these intriguing devices,” notes Thomas Udem. Udem observes that they can be precisely controlled and manipulated, because of a multi-decade effort directed towards exploration of quantum phenomena using chains of cold ions. That history is also interesting in another respect.

“The physical mechanism that is responsible for amplification and stimulated emission of phonons has, over the last decades, been misunderstood as a different phenomenon referred to as heating,” says Maximilian Herrmann. He explains that the regime of heating was always associated with laser pumping using a wavelength that is blue-detuned, relative to an optical transition of the trapped ion. In stark contrast, the Garching team was able to show that the regime is, in fact, one of phonon laser action.

In the experiment, the ion was simultaneously cooled using a red-detuned laser and also pumped to create stimulated phonon amplification using the blue-detuned beam. The cooling laser also allowed the researchers to introduce a controlled amount of damping into the system. To understand the origin of amplification in the ion system, consider the so-called small-signal regime with the blue detuned pumping beam. This interaction involves a second-order quantum process in which a pump photon is absorbed, followed by emission of center-of-mass phonon.

Herein, the role of atomic transition damping is crucial. As is also true in a conventional optical laser oscillator, the transition damping is fast enough so as to quench the underlying Rabi dynamic. As a result, the rate of emission of phonons can be shown to contain a stimulated component that is proportional to the number of phonons, and also a component that is purely spontaneous (and responsible for starting the motion in the first place). The stimulated term, as in a conventional laser, produces the amplification that ultimately creates the coherent motion.

The transition from purely thermal motion to coherent motion is marked by a threshold pumping power, where stimulated amplification balances ion motional damping. As pumping is further increased through this threshold, coherence emerges and is visible in images of the ion motion (figure). Ultimately, the large signal motion of the ion saturates the stimulated amplification. The saturation process in the ion is, itself, interesting as it involves a competing phonon absorption process. In effect, as the coherent motion grows in amplitude, this phonon absorption competes more effectively with the stimulated emission and saturates the net amplification. Generally speaking, saturation of amplification is an essential feature of all regenerative oscillators, and makes possible stable coherent motion.

The mechanism of amplification through scattering of a pump photon and generation of a stimulated phonon is in some ways analogous to the process of Raman optical amplification in which a pump photon scatters to produce a stimulated (and amplified) optical Stokes wave [2]. In that process, a phonon is also produced, but is so strongly damped that it effectively serves to only assist the overall amplification of light as an intermediate process. In the ion phonon laser system, on the other hand, this situation changes dramatically.

The researchers note that the pump photon creates both a polarization excitation and a phonon; but in a peculiar twist, the phonon switches out of its conventional “supporting” role, and, because of the very low damping rate of phonons in the ion system, takes-on the lead role analogous to the optical Stokes mode in the conventional optical Raman process. This feature of the stimulated phonon process in the ionic system can be made mathematically precise; and also helps to explain, as is true in optical Raman lasers, operation without an obvious inversion.

Current research is directed towards controlling the phonon laser using tools and techniques that can be adapted from the laser world. One example is a process called injection locking whereby a weak, external control field is used to phase-synchronize the phonon laser with an external reference. The team has recently used this method to image the coherent motion of the ion as it oscillates. Also interesting is the possibility of studying excitations in chains of ions and even two-dimensional arrays of ions.

Concerning applications, Professor Theodor Hänsch notes, “the fact that the forces involved here are so weak suggests that this phenomenon might prove useful as a weak force probe.” He adds, “it is always important when a phenomenon or idea can be reduced to practice.” Along these lines, the Garching work has already inspired realization in a very different system of phonon laser action [3].

Reference
[1] K. Vahala, M. Herrmann, S. Knünz, V. Batteiger, G. Saathoff, T. W. Hänsch & Th. Udem, “A phonon laser,” Nature Physics, 5, 682 – 686 (2009). Abstract.
[2] Shen, Y. R. & Bloembergen, N. “Theory of stimulated Brillouin and Raman
scattering,” Phys. Rev. 137, 1787–1805 (1965). Abstract.
[3] Ivan S. Grudinin, O. Painter, Kerry J. Vahala, “Phonon laser action in a tunable, two-level photonic molecule,” arXiv:0907.5212v1 (2009).

Tiniest Semiconductor Laser

Xiang Zhang [Photo courtesy: Roy Kaltschmidt/ Lawrence Berkeley National Laboratory]

In an advanced online publication of the journal Nature dated Aug. 30, a team of researchers from USA and China -- led by Xiang Zhang of University of California, Berkeley -- has reported the creation of the world's smallest semiconductor laser, capable of generating visible light in a space of only 5 nanometer -- smaller than a single protein molecule. The team not only successfully squeezed light into such a tight space, but found a novel way to keep that light energy from dissipating as it moved along, thereby achieving laser action. The research was performed at the NSF Nanoscale Science and Engineering Centre of University of California -- Berkeley, the Materials Sciences Division of Lawrence Berkeley National Laboratory, and the State Key Lab for Mesoscopic Physics and School of Physics of the Peking University -- China.

The achievement helps enable the development of such innovations as nanolasers that can probe, manipulate and characterize DNA molecules; optics-based telecommunications many times faster than current technology; and optical computing in which light replaces electronic circuitry with a corresponding leap in speed and processing power.

[Image courtesy of Xiang Zhang Lab/UC Berkeley] Left: Light being compressed and sustained in the 5 nanometer gap — smaller than a protein molecule — between a nanowire and underlying silver surface. Right: Electron microscope image of the hybrid design shown in the schematic.

While it is traditionally accepted that an electromagnetic wave - including laser light - cannot be focused beyond the size of half its wavelength, research teams around the world have found a way to compress light down to dozens of nanometers by binding it to the electrons that oscillate collectively at the surface of metals. This interaction between light and oscillating electrons is known as surface plasmons.

Scientists have been racing to construct surface plasmon lasers that can sustain and utilize these tiny optical excitations. However, the resistance inherent in metals causes these surface plasmons to dissipate almost immediately after being generated, posing a critical challenge to achieving the buildup of the electromagnetic field necessary for lasing.

Recently, another team of researchers from Norfolk State University, Purdue University and Cornell University reported the creation of "spaser-based nanolasers" which were spheres 44 nanometers in diameter - more than 1 million could fit inside a red blood cell [Read 2Physics article dated August 22, 2009]. Those nanolasers are based on lasing action of gold spheres in a dye-filled, glasslike shell immersed in a solution. The dye coupled to the gold spheres could generate surface plasmons when exposed to light.

The UC Berkeley researchers used semiconductor materials and fabrication technologies that are commonly employed in modern electronics manufacturing. By engineering hybrid surface plasmons in the tiny gap between semiconductors and metals, they were able to sustain the strongly confined light long enough that its oscillations stabilized into the coherent state that is a key characteristic of a laser.

The Berkeley team took a novel approach to stem the loss of light energy by pairing a cadmium sulfide nanowire - 1,000 times thinner than a human hair - with a silver surface separated by an insulating gap of only 5 nanometers, the size of a single protein molecule. In this structure, the gap region stores light within an area 20 times smaller than its wavelength. Because light energy is largely stored in this tiny non-metallic gap, loss is significantly diminished. With the loss finally under control through this unique "hybrid" design, the researchers could then work on amplifying the light.

[Image courtesy of Xiang Zhang Lab/UC Berkeley] Left: Light being compressed and sustained in the 5 nanometer gap — smaller than a protein molecule — between a nanowire and underlying silver surface. Right: Electron microscope image of the hybrid design shown in the schematic.

"When you are working at such small scales, you do not have much space to play around with," said Rupert Oulton, the research associate in Zhang's lab who first theorized this approach last year and the study's co-lead author. "In our design, the nanowire acts as both a confinement mechanism and an amplifier. It's pulling double duty."

Trapping and sustaining light in radically tight quarters creates such extreme conditions that the very interaction of light and matter is strongly altered, the study authors explained. An increase in the spontaneous emission rate of light is a telltale sign of this altered interaction; in this study, the researchers measured a six-fold increase in the spontaneous emission rate of light in a gap size of 5 nanometers.

"Plasmon lasers represent an exciting class of coherent light sources capable of extremely small confinement," said Zhang. "This work can bridge the worlds of electronics and optics at truly molecular length scales."

"What is particularly exciting about the plasmonic lasers we demonstrated here is that they are solid state and fully compatible with semiconductor manufacturing, so they can be electrically pumped and fully integrated at chip-scale," said Volker Sorger, a Ph.D. student in Zhang's lab and a co-lead author of the paper.

Scientists hope to eventually shrink light down to the size of an electron's wavelength, which is about a nanometer, or one-billionth of a meter, so that the two can work together on equal footing.

"The advantages of optics over electronics are multifold," added Thomas Zentgraf, a post-doctoral fellow in Zhang's lab and another co-lead author of the Nature paper. "For example, devices will be more power efficient at the same time they offer increased speed or bandwidth."

Reference
"Plasmon lasers at deep subwavelength scale"
Rupert F. Oulton, Volker J. Sorger, Thomas Zentgraf, Ren-Min Ma, Christopher Gladden, Lun Dai, Guy Bartal & Xiang Zhang,
Nature advance online publication 30 August 2009 doi:10.1038/nature08364; Abstract

[Our presentation of this work is based on a write-up by Sarah Yang of University of California, Berkeley]

Tiniest Laser Developed -- Confirming 'Spaser' Concept

In a paper published in 'Nature' [1], a team of scientists reported the creation of the tiniest laser since its invention nearly 50 years ago, paving the way for a host of innovations, including superfast computers that use light instead of electrons to process information, advanced sensors and imaging. The research was conducted by Norfolk State University researchers Mikhail A. Noginov, Guohua Zhu and Akeisha M. Belgrave; Purdue University researchers Reuben M. Bakker, Vlad Shalaev and Evgenii E. Narimanov; and Cornell University researchers Samantha Stout, Erik Herz, Teeraporn Suteewong and Ulrich B. Wiesner.

Because the new device, called a "spaser" (which stands for 'Surface Plasmon Amplification by Stimulated Emission of Radiation') is the first of its kind to emit visible light, it represents a critical component for possible future technologies based on "nanophotonic" circuitry, said Vladimir Shalaev of Purdue University.

[Image courtesy: Birck Nanotechnology Center, Purdue University] The color diagram (a) shows the nanolaser's design: a gold core surrounded by a glasslike shell filled with green dye. When a light was shined on the spheres, plasmons generated by the gold core were amplified by the dye. The plasmons were then converted to photons of visible light, which was emitted as a laser. Scanning electron microscope images (b and c) show that the gold core and the thickness of the silica shell were about 14 nanometers and 15 nanometers, respectively. A simulation of the SPASER (d) shows the device emitting visible light with a wavelength of 525 nanometers.

Nanophotonics may usher in a host of radical advances, including powerful "hyperlenses" resulting in sensors and microscopes 10 times more powerful than today's and able to see objects as small as DNA; computers and consumer electronics that use light instead of electronic signals to process information; and more efficient solar collectors.

Nanophotonic circuits will require a laser-light source, but current lasers can't be made small enough to integrate them into electronic chips. Now researchers have overcome this obstacle, harnessing clouds of electrons called "surface plasmons," instead of the photons that make up light, to create the tiny spasers.

The "spaser-based nanolasers" created in the research were spheres 44 nanometers in diameter - more than 1 million could fit inside a red blood cell. The spheres were fabricated at Cornell, with Norfolk State and Purdue performing the optical characterization needed to determine whether the devices behave as lasers.

Past 2Physics article by Vlad Shalaev and his collaborators:
"Large Broadband Invisibility Cloak for Visible Light"

To act like lasers, Spasers require a "feedback system" that causes the surface plasmons to oscillate back and forth so that they gain power and can be emitted as light. Conventional lasers are limited in how small they can be made because this feedback component for photons, called an optical resonator, must be at least half the size of the wavelength of laser light.

The researchers, however, have overcome this hurdle by using not photons but surface plasmons, which enabled them to create a resonator 44 nanometers in diameter, or less than one-tenth the size of the 530-nanometer wavelength emitted by the spaser.

The findings confirm work by physicists David Bergman (Tel Aviv University) and Mark Stockman (Georgia State University), who first proposed the spaser concept in 2003 [2].

"It's fitting that we have realized a breakthrough in laser technology as we are getting ready to celebrate the 50th anniversary of the invention of the laser," Shalaev said.

The first working laser was demonstrated in 1960.

Reference
[1] "Demonstration of SPASER-based Nanolaser" , M. A. Noginov, G. Zhu, A. M. Belgrave, R. Bakker, V. M. Shalaev, E. E. Narimanov, S. Stout, E. Herz, T. Suteewong, U. Wiesner,
Nature, doi:10.1038/nature08318, (Published online 16th August, 2009), Abstract
[2] "Surface Plasmon Amplification by Stimulated Emission of Radiation: Quantum Generation of Coherent Surface Plasmons in Nanosystems", David J. Bergman and Mark I. Stockman, Phys. Rev. Lett. 90, 027402(2003). Abstract.

[Our presentation of this work is based upon a write-up by Emil Venere of Purdue University]

World’s Fastest Continuously Running Camera

Kevin K. Tsia, Bahram Jalali, and Keisuke Goda (from Left to Right)

[This is an invited article based on a recent work of the authors -- 2Physics.com]

Authors: Keisuke Goda, Kevin K. Tsia, and Bahram Jalali

Affiliation: Photonics Laboratory, Department of Electrical Engineering, University of California, Los Angeles.

Photonics.ucla.edu


Continuous real-time imaging technology with high temporal resolution is required for studying rapid dynamical phenomena such as shockwaves, chemical dynamics in living cells, neural activity, laser surgery, and microfluidics. However, traditional electronic image sensors such as CCD and CMOS cameras are unable to capture such processes with high sensitivity and resolution. This is partly due to the technological limitation that it takes time to read out the data from the sensor array, and also due to the fundamental trade-off between speed and sensitivity; at high frame rates, fewer photons are collected during each frame – a predicament that affects virtually all optical imaging systems.

Past 2Physics article by Keisuke Goda and his collaborators:
"Beating the Quantum Limit in Gravitational Wave Detectors"

To overcome these limitations, we have proposed and demonstrated a new type of imaging technique that offers at least 1000 times higher frame rates than traditional electronic image sensors such as CCDs [1]. This imaging method, which we refer to as serial time-encoded amplified imaging or microscopy (STEAM), is an extension of our ultrafast 1D imager [2]. It maps the 2D image of an object into a serial time-domain data stream and simultaneously amplifies the image in the optical domain using the technique known as amplified dispersive Fourier transformation. It captures the 2D image, not with a CCD camera, but with a single-pixel photodiode. The main attributes of the STEAM camera are the optical image amplification and the elimination of the CCD. When combined, it achieves continuous real-time imaging at a record frame rate of more than 6 MHz and a shutter speed of 440 ps. Achieving an image gain of 25 dB, the STEAM camera overcomes the fundamental trade-off between speed and sensitivity without having to resort to cooling and high-intensity illumination. The STEAM camera operates continuously and can capture ultrafast dynamical processes without any knowledge of the timing of their occurrence.

Movie 1: Animated movie that illustrates the functionality of the STEAM camera.

The details of the STEAM camera’s operation are shown in Movie 1. A broadband pulse is dispersed and separated by a pair of dispersive elements consisting of a virtually-imaged phased array and a diffraction grating to produce a 2D rainbow. When this is incident onto the object, the spatial coordinates of the object are encoded into the spectrum of the back-reflected 2D rainbow. The key innovation is what happens next. The reflected pulse is directed toward an amplified dispersive Fourier transformer which maps the image-encoded spectrum into a 1D temporal data stream, and simultaneously amplifies the image using distributed Raman amplification. The optically amplified 1D serial data is detected by a single-pixel photodiode and digitized by a real-time oscilloscope. The 2D image of the object is reconstructed by folding the 1D vector into a 2D matrix, representing the 2D image, in the digital domain.

The image amplifier in the STEAM camera is different from so-called image intensifiers. In the STEAM camera, amplification occurs in the optical domain as opposed to in the electronic domain in image intensifiers. Image intensifiers are complex devices and have a low image acquisition rate up to ~10 kHz in continuous mode – performance that is adequate for its intended use in night-vision cameras because they only need to operate at the video rate. The limited frame rate is due to the fundamental trade-off between gain and bandwidth in all electronic systems, including the image intensifier.

In scientific applications, high-speed imaging is often achieved with the time-resolved pump-probe technique [3]. Pump-probe techniques can capture the dynamics of fast events, but only if the event is repetitive. Because they do not operate in real time, they are unable to capture non-repetitive random events that occur only once or do not occur at regular intervals such as rogue events [4]. Detection of such events requires an imaging technology with fast, continuous, and real-time capability.

Another type of high-speed image sensor is the framing streak camera that has been employed for diagnostics in laser fusion, plasma radiation and combustion. This device operates in burst mode only (providing only several frames) and requires synchronization of the camera with the event to be captured, rendering streak cameras also unable to capture unknown or random events. This, along with the high cost of the camera, limits its use in practical applications.

Figure 2: [Click to see a better resolution image]
Continuous real-time images of the laser ablation experiment. Continuous real-time images captured by the STEAM camera with a temporal resolution of 163 ns and shutter speed of 440 ps. The changes in sample surface reflectivity due to the laser-induced mass ejection are evident after the ablation pulse hits the sample at t = 0 ns.

To demonstrate the STEAM camera’s ultrafast real-time imaging capability and utility, we have used it to successfully capture the dynamics of laser ablation. Laser ablation is a ubiquitous technology that is used in laser surgery, laser cutting and micromachining, and laser-induced breakdown spectroscopy. The ablation was performed with a mid-infrared pulse with 5 ns pulse width focused at an angle onto a sample consisting of a bilayer of aluminum and silicon dioxide deposited on top of a silicon-on-insulator substrate. The imaging pulse train of the STEAM camera was incident to the surface of the sample at a normal angle. Figure 2 shows the real-time sequence of the images with a frame repetition period of 163 ns. The entire frame sequence corresponding to the dynamics (laser-induced mass ejection) caused by the single ablation pulse was captured in real time.

Reference
[1] “Serial time-encoded amplified imaging for real-time observation of fast dynamic phenomena", K. Goda, K. K. Tsia, and B. Jalali, Nature 458, 1145 (2009). Abstract.
[2] “Amplified dispersive Fourier-transform imaging for ultrafast displacement sensing and barcode reading”, K. Goda, K. K. Tsia, and B. Jalali, Applied Physics Letters 93, 131109 (2008). Abstract.
[3] “Ultrafast single-shot diffraction imaging of nanoscale dynamics”, A. Barty, S. Boutet, M. J. Bogan, S. Hau-Riege, S. Marchesini, K. Sokolowski-Tinten, N. Stojanovic, R. Tobey, H. Ehrke, A. Cavalleri, S. Düsterer, M. Frank, S. Bajt, B. W. Woods, M. M. Seibert, J. Hajdu, R. Treusch & H. N. Chapman,
Nature Physics 2, 415 (2008). Abstract.
[4] “Optical rogue waves,” D. R. Solli, C. Roper, P. Koonath, and B. Jalali,
Nature 450, 1054 (2007). Abstract.

Large Broadband Invisibility Cloak for Visible Light

Vera Smolyaninova (Towson University)


[This is an invited article based on recent work of the authors. -- 2Physics.com]

Authors: Vera Smolyaninova¹ and Vlad Shalaev<sup>2

1</sup>Dept. of Physics Astronomy and Geosciences, Towson University, Towson, MD, USA

²Birck Nanotechnology Center, School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, USA


Most researchers believe that sophisticated artificially engineered materials are required to build an invisibility cloak. Such “metamaterials” exhibit high losses and work for only one color. The resulting invisibility cloaks are tiny, and cannot hide anything if another color of light is used. Our team, Igor Smolyaninov from BAE Systems, Vera Smolyaninova from Towson University, Alexander Kildishev and Vlad Shalaev form Purdue University demonstrated a different approach to cloaking.

Vlad Shalaev (Purdue University)

Instead of sophisticated metamaterials, we used a waveguide, which is curved to mimic the metamaterial properties. This approach leads to all-color invisibility cloak with much lower losses. As a result, we built a large optical cloak, which is about hundred times larger than the cloaks built previously. This “see-through” cloak bends light around itself and thus differs from the “invisibility carpet,” which camouflages bumps on a metal surface. We believe that further size increase is possible, and that the same technique may be applied to other tasks, which require the use of metamaterials, such as building new “hyperlenses” which considerably surpass the resolution limit of conventional lenses.

This work is reported in the May 29 issue of Physical Review Letters [1]. In the experiments, conducted at Towson University, electromagnetic cloaking is achieved using a specially tapered waveguide. An area with a radius ~100 times larger than the wavelengths of light shined by a laser into the device has been cloaked, an unprecedented achievement. This is the first experiment on optical cloaking performed with normal visible light.

Previous experiments with metamaterials, which require complex nanofabrication, have been limited to cloaking regions only a few times larger than the wavelengths of visible light [2,3]. The new design is a far simpler device: waveguides represent established technology - including fiber optics - used in communications and other commercial applications. Because the new method enabled us to dramatically increase the cloaked area, the technology offers hope of cloaking larger objects. All previous attempts at optical cloaking have involved very complicated nanofabrication of metamaterials containing many elements, which makes it very difficult to cloak large objects. Here, we showed that if a waveguide is tapered properly it acts like a sophisticated nanostructured material. The waveguide is inherently broadband, meaning it could be used to cloak the full range of the visible light spectrum. Unlike metamaterials, which contain many light-absorbing metal components, only a small portion of the new design contains metal.

Igor Smolyaninov (BAE Systems)

Theoretical work for the design was led by Purdue, with BAE Systems and Towson University leading work to fabricate the device and demonstrate its cloaking properties. The cloaking device is formed by two gold-coated surfaces, one a curved lens and the other a flat sheet. We cloaked an object about 50 microns in diameter, or roughly the width of a human hair, in the center of the waveguide. Instead of being reflected as normally would happen, the light flows around the object and shows up on the other side, like water flowing around a stone.

This research falls within a new field called transformation optics, which may usher in a host of radical advances, including cloaking; powerful "hyperlenses" resulting in microscopes 10 times more powerful than today's and able to see objects as small as DNA; computers and consumer electronics that use light instead of electronic signals to process information; advanced sensors; and more efficient solar collectors.

Alexander Kildishev (Purdue University)

Unlike natural materials, metamaterials are able to reduce the "index of refraction" to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. It causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material. Natural materials typically have refractive indices greater than one. Metamaterials, however, can be designed to make the index of refraction vary from zero to one, which is needed for cloaking. The precisely tapered shape of the new waveguide alters the refractive index in the same way as metamaterials, gradually increasing the index from zero to 1 along the curved surface of the lens. Previous cloaking devices have been able to cloak only a single frequency of light, meaning many nested devices would be needed to render an object invisible.

We reasoned that the same nesting effect might be mimicked with the waveguide design. Subsequent experiments and theoretical modeling proved the concept correct. We do not know of any fundamental limit to the size of objects that could be cloaked, but additional work will be needed to further develop the technique.

Recent cloaking findings reported by researchers at other institutions have concentrated on a technique that camouflages features against a background. Those works, which use metamaterials, are akin to rendering bumps on a carpet invisible by allowing them to blend in with the carpet, whereas our work concentrates on enabling light to flow around an object.

The work was funded by the ARO-MURI and the National Science Foundation.

References
[1] “Anisotropic metamaterials emulated by tapered waveguides: application to electromagnetic cloaking”, I.I. Smolyaninov, V.N. Smolyaninova, A.V. Kildishev, and V.M. Shalaev,
Phys. Rev. Letters, 102, 213901 (2009). Abstract.
[2] “Metamaterial electromagnetic cloak at microwave frequencies”, D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, , Science 314, 977-980 (2006). Abstract.
[3] “Two-dimensional metamaterial structure exhibiting reduced visibility at 500 nm”, I.I. Smolyaninov, Y.J. Hung, and C.C. Davis, Optics Letters 33, 1342-1344 (2008). Abstract.