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Optics & Photonics News Magazine
December Issue

Feature Articles

Photorefractive Polymers for Biomedical Imaging

Imaging through highly scattering media using optical radiation has recently received particular attention due to potential applications in medical diagnostics. By using optical radiation one can avoid the hazards of ionizing X-rays while obtaining high spatial resolution (potentially diffraction limited) images, offering a distinct advantage over current clinical techniques. However, optical radiation, unlike ionizing radiation, is heavily scattered in biological tissue by refractive index inhomogeneities. As a result, the transmitted light consists of photons that have experienced no scattering events (ballistic light), weakly scattered photons that emerge almost collinear with the incident radiation (so-called "snake-like" light), and highly scattered (or diffuse) photons.

by B. Kippelen, N. Peyghambarian, S.R. Marder
Highly Efficient 1.3-µm Luminescence from Rare-earth-doped Halides Prepared from Low Temperature Aqueous Solutions

The tremendous growth in photonic materials and device research results largely from the explosion in telecommunications, including the traffic requirements needed for the Internet and future interactive video and multimedia services. These trends have motivated a global search for materials suitable for optical amplification at the 1.3-μm zero dispersion wavelength, which maximizes the information carrying capacity of silica fibers. Praseodymium (Pr3+) has received the most attention as a dopant for gain at 1.3 μm and has reached some level of commercialization despite a relatively low quantum yield from the requisite nonoxide glass hosts (~10 %). Dysprosium (Dy3+) has also attracted recent interest since its absorption cross-section is larger than Pr3+ thus lessening the requisite amplifier length. In both cases, maximizing the luminescence efficiency requires hosts with low phonon energies. Accordingly, we considered the solution-synthesis of doped-lanthanum halides because of their low phonon energy and high rare-earth solubility.

by John Ballato, Richard E. Riman, Elias Snitzer
High-speed Multi-wavelength Photonic Switch

The ability to rapidly and independently switch multiple wavelengths in a light beam is useful for both WDM fiber-optic communications and for hardware compressed photonic signal processing. Key features for such a multi-wavelength switch include microsecond domain wavelength switching and excellent -50 dB optical switching isolation, leading to rapidly reconfigured high signal-to-noise ratio photonic systems. Previously, the bulk acousto-optic tunable filter (AOTF) was found to be attractive for this multi-wavelength switch application as the bulk AOTF can operate over wide optical bandwidths with high speeds and at high optical power levels. Recent improvements in AOTF device designs have lowered the high drive power requirements commonly associated with bulk AOTFs. In addition, these new devices show spectral resolutions in the 1-nm. level range, indicating possible use for high (> 32) channel count WDM fiber communications. Nevertheless, one debilitating problem associated with the inherent operation of the AOTF is its finite (e.g., 95%) diffraction efficiency that leads to its low and undesirable -20 dB level type optical crosstalk numbers.

by N.A. Riza, CREOL and The School of Optics, Univ. of Central Florida, Orlando, FL.
Wavelength Compensation of Broadband Light Diffraction

Propagation of electromagnetic waves in free space is a physical phenomenon that explicitly depends on the wavelength of the light radiation. This fact results in the chromatic dispersion of the optical field diffracted by an aperture illuminated with a broadband source. The above situation severely restricts the spectral bandwidth of the illumating source that can be used in a diffraction-based optical system. If our interest is that all the spectral components produce the same effect, broad-band-dipersion compensation is then required. The milestone of the compensation procedure lies in achieving the incoherent superposition of the monochromatic versions of a selected diffraction pattern in a single plane, with the same scale for all the wavelengths of the incident light. Achromatic diffraction systems meet the above requirement in a first-order approximation.

by P. Andrés, Dept. Óptica, Universidad de Valencia, Burjassot, Spain; V. Climent, J. Lancis, E. Tajahuerce, M. Fernández-Alonso, and G. Mínguez, Dept. Ciencias Experimentales, Universitat Jaume I, Castelló, Spain.
Artificial Dielectrics Boost Diffraction Efficiency in Diffractive Optical Elements

direct laser-beam writing offer nearly-100% efficiency for zone widths or grating periods much larger than the wavelength. In practice, however, performance decreases1 in the so-called resonance domain, i.e., when zone widths or grating periods are equal to a few wavelengths. This is due to both theoretical limitations and fabrication problems encountered when monitoring together continuous profiles and sharp vertical edges. Obtaining high diffraction efficiency with large deflection angles is a challenge in design and fabrication of tomorrow's DOEs.

by Philippe Lalanne, Simion Astilean, and Pierre Chavel, Laboratoire Charles Fabry de I'Institut d'Optique, Centre National de la Recherche Scientifique, Orsay, France; Edmond Cambril and Huguette Launois, Laboratoire de Microstructures et de Micro-électronique, Bagneux, France
Gaussian Wave Packets in Resonant Diffraction Gratings

Resonant diffraction gratings are frequently used as convenient and flexible wavelength filters since they offer a reflectance >99% for their central wavelength, whereas far off the central wavelength the reflectance can fall below 1%. The central wavelength can be adjusted by the choice of the angle of incidence, and the FWHM of the filter can be fixed over a wide range during the design process. Resonance effects occur due to the coupling of the incident wave to guided modes supported by a waveguide layer of the structure. Numerical considerations concerning resonant gratings are usually carried out under the assumption of an incident plane wave.

by Frank Schreier, Martin Schmitz, and OIof Bryngdahl, Physics Dept., Univ. of Essen, Essen, Germany.
Temporal Speckle Pattern Interferometry

In recent years, speckle techniques have been propelled from scientific to industrial environments due to rapid advancements in computer technology and its peripherals. In many ways, the limited use of speckle techniques is set by its relatively high sensitivity, thereby only displacements in the range of 5-6 μm can be measured. Recent developments to record the temporal fluctuation of speckles rather than changes in spatial distribution have rendered the possibility of measuring large displacements over 100 μm with almost the same accuracy as using phase shifting techniques.

by C. Joenathan, Dept. of Physics and Applied Optics, Rose-Hulman Institute of Tech, Terre Haute, IN; P. Haible and H.J. Tiziani, Universität Stuttgart, Institut für Technische Optik, Stuttgart, Germany.
Adaptive Compensation for Laser-based Ultrasound Detection Using Holographic Quantum Wells

Holograms that can change in time and adapt to environmental disturbances have interesting and important applications in diverse areas. The key to adaptive performance is the ability of the holographic recording process to track slow changes, such as thermal fluctuations in the optical path or mechanical vibrations. Low-frequency noise is adaptively compensated by the adapting hologram, while high-frequency signals are passed through to the detector. A classic example of an incessant problem in optics is the stability of an interferometer. With an adaptive hologram acting as a beam combiner in the interferometer, no active stabilization is necessary. Any changing path lengths are simply compensated by a corresponding shift in the interference fringes of the hologram.

by D.D. Nolte, I. Lahiri, and L.J. Pyrak-Nolte, Dept. of Physics, Purdue Univ., West Lafayette, IN; M.B. Klein and G.D. Bacher, Lasson Technologies, Pacific Palisades, CA; M.R. Melloch, School of Electrical and Computer Engin., Purdue Univ., West Lafayette, IN.
Diode-pumped Tunable Room-temperature Lasers Based on F2+ and F2- Color Centers in LiF

Pulsed room-temperature color center lasers based on F2+ and F2- centers in crystals are presently reliable sources of tunable radiation with fundamental frequencies in the 820-1340 nm spectral region. Ever since the discovery of Cr4+:forsterite and Ti3+:sapphire lasers, the LiF:F2+* and LiF:F2- color center tunable systems have attracted the attention of researchers.

by Valerii V. Ter-Mikirtychev, Faculty of Engin., Kyoto Sangyo Univ., Kyoto, Japan.
Tuning and Stabilization of Laser Diodes by a Planar Optical Wavelength Analyzer

For many applications of narrow-band light sources such as edge-emitting laser diodes or VCSELs, it is important to know the spectral properties of the optical beam, and it is required that we be able to control the spectrum actively. However, conventional solutions such as tunable lasers or thermal light sources in combination with monochromators are expensive and, in the latter case, very bulky.

by M. Wiki and R.E. Kunz, CSEM Centre Suisse d'Electronique et de Microtechnique SA, Zurich, Switzerland.
Microlasers with Chaotic Resonators and Bow-tie Lasers

High quality micro-resonators are of particular interest for semiconductor lasers as they allow a smaller volume of the active material with concomitant moderate energy of a ray in a whispering gallery mode to fluctuate in time. Eventually a ray trapped by total internal reflection impinges on the boundary below the critical angle and escapes by refraction.

by Claire Gmachl, Federico Capasso, Deborah L. Sivco, Alfred Y. Cho,
Coupled Microcavities in Light-Emitting Porous Silicon

Although porous silicon (p-Si) can be easily, rapidly, and cheaply produced in electrochemical cells without the need for lithographic or epitaxial techniques, the emitted light has an undesirably broad spectrum (FWHM ~150 nm) and long (microsecond) decay times.1 If the spectrum can be narrowed, the response time reduced, and the efficiency improved, p-Si light emitters are likely to rapidly find their way into advanced Si-based optoelectronic chips.

by P.A. Snow, E.K. Squire, and P.St.J. Russell, Optoelectronics Group, Dept. of Physics, Univ. of Bath, Bath, U.K.; L.T. Canham, A.J. Simons, and C.L. Reeves, DERA, Great Malvern, U.K.
Suppression of Multiphoton Fluorescence in Hyper-Rayleigh Scattering

Hyper-Rayleigh scattering (HRS) has become widely accepted as an experimental technique for the determination of the first hyperpolarizability (second-order nonlinear polarizability), β, of molecules in solution. Apart from being simpler both theoretically and experimentally than electric-field-induced second-harmonic generation (EFISHG)—applicable to neutral, dipolar molecules only—HRS is the sole technique that gives a β value for ionic or octopolar species. The combination of HRS and EFISHG also allows different elements of the hyperpolarizability tensor to be analyzed.

by Koen Clays, Tom Munters, Geert Olbrechts, and Andre Persoons, Laboratory of Chemical and Biological Dynamics, Dept. of Chemistry, Univ. of Leuven, Leuven, Belgium.
Backward Second-harmonic Generation in Periodically-poled LiNbO3

second-harmonic waves, some of the waves propagate in the direction opposite to the fundamental waves and are thus known as backward second-harmonic (BSH) waves. Because of the extremely large phase-mismatch that occurs in such a process, the BSH effect is negligible. Conventional phase-matching techniques using either angle or temperature tuning cannot compensate for large phase-mismatching. Thus, the backward process has been virtually ignored by most of the nonlinear optics community. While there were some early theoretical studies, it is only recently that the backward parametric processes have been analyzed theoretically in detail. In general, the backward nonlinear processes are important since they can lead to mirrorless optical parametric oscillators (OPOs) and other novel effects

by Xinhua Gu and Yujie J. Ding, Dept. of Physics and Astronomy, Centers for Photochemical and Materials Sciences, Bowling Green State Univ., Bowling Green, OH; Jin U. Kang, Naval Research Lab., Washington, DC; Jacob B. Khurgin, Dept. of Electrical and Computer Engin., Johns Hopkins Univ., Baltimore, MD
Extremely Nonlinear Methyl-red Doped Nematic Liquid Crystal Film

To date, liquid crystals remain an important electro-optics material because of their broadband birefringence. They possess a sizable birefringence, Δn ~ 0.3 spanning the visible to IR spectral regime. Because of their easy susceptibility to optical fields, they are also well known for their nonlinear optical properties associated with laser induced director axis reorientation effects.

by I.C. Khoo, B.D. Guenther, Min-Yi Shih, P. Chen, and W.V. Wood, Electrical Engin. Dept., Penn. State Univ., University Park, PA.
Photonic Time-stretch Offers Solution to Ultrafast Analog-to-digital Conversion

Analog-to-digital (A/D) conversion represents the key bottleneck in high performance radar and communication systems. The current trend in electronic receivers is to perform the conversion at microwave frequencies. In the so-called "digital receiver," the A/D conversion is performed at microwave (carrier) frequencies, thus placing stringent requirements on the sampling frequency and input bandwidth of the A/D. It is widely recognized that new concepts leading to major advances in A/D technology are a priority.

by B. Jalali, F. Coppinger, and A.S. Bhushan, Univ. of California at Los Angeles, Los Angeles, CA.
After Image
Microresonators for Integrated Optical Devices

Resonators have frequency selective properties that make them particularly suitable for optical signal processing. By cascading coupled resonators in different configurations, it is possible to synthesize a wide variety of desirable optical filter characteristics. Significantly, resonators behave as lumped elements, tuning as a unit. This is to be contrasted with many interferometric devices in which the details of the device shape are critical to the performance, e.g., in apodized evanescently coupled waveguide filters. Other novel resonator applications for integrated optics, not achievable with conventional devices, include absorption controlled WDM signal routing.

by B. Little, H. Haus, E. Ippen, G. Steinmeyer, E. Thoen, J. Foresi, L. Kimerling, Sai T. Chu, W. Greene
Vector Description of a Realistic Photonic Crystal Fiber

The most relevant property of periodic dielectric structures (i.e., photonic crystals) is the possibility of generating photonic bandgaps. A related phenomenon occurring in photonic crystal structures is light localization at defects. Although the previous phenomenon of light confinement at defects has already been analyzed in 2-D structures, the study of the guiding properties of dielectric crystals that have a 2-D periodicity in the x — y plane broken by the presence of a defect, but are continuous and infinitely long in the z direction—the so-called photonic crystal fibers (PCFs)—has not yet been performed. However, the experimental feasibility of these fibers has been recently proven. A robust single-mode structure was observed for an unusually wide range of wavelengths, a remarkable property not present in ordinary fibers.

by A. Ferrando, J.J. Miret, E. Silvestre, and P. Andres, Dept. d'Optica, Universitat de València, Spain; M.V. Andrés, Institut de Ciència dels Materials, Universitat de Valencia, Spain.
Large Mode Area Photonic Crystal Fiber

Photonic crystal fiber (PCF) is a new type of optical fiber waveguide consisting of a pure silica fiber with a regular array of small holes running through its entire length. A single missing air hole defines a region that effectively has a higher refractive index than the surrounding "holey" material. This region can form the core of a low-loss optical waveguide. Such a structure is readily fabricated by stacking a few hundred silica capillary tubes (of around 40-cm-length and 1-mm-diameter) and then drawing the stack down to a long fine fiber (at an elevated temperature) on an optical fiber drawing tower. A single solid silica rod embedded within the stack forms the fiber core.

by J.C. Knight, T.A. Birks, R.F. Cregan, and P.St.J. Russell, Optoelectronics Group, Dept. of Physics, Univ. of Bath, Bath U.K.
Efficient Polarized Laser Mirror

Waveguide-mode resonance effects in thin-film layers can be applied to realize an efficient laser mirror. We have demonstrated the application of a high-efficiency guided-mode resonance (GMR) filter as an output-coupling mirror in a dye laser. This simple two-layer device provides polarized output laser light without the use of laser Brewster windows. The experimental value of the mirror reflectance is ~98% at the resonance peak with corresponding laser transmission of ~2%. Similar experiments with an ~50% reflective corrugated-waveguide mirror have been reported in which parasitic lasing via Fresnel bulk reflections occurred simultaneously due to the low value of the mirror reflectance.

by R. Magnusson, Z.S. Liu, D. Shin, S. Tibuleac, and P.P. Young, Dept. of Electrical Engin., Univ. of Texas at Arlington, Arlington, TX.
Surface-mode-induced Rescaling of the Dipole-dipole Interaction

A surface or structure can modify the optical properties of a nearby radiator. These effects include the modification of a molecule's radiative lifetime near a metal surface and the strikingly large effect known as surface-enhanced Raman scattering (SERS). During our attempts to understand the mechanism responsible for another dipole-surface effect—nanoparticle-enhanced photodetection—we found evidence that a nearby surface can also modify the dipole-dipole interactions taking place within a layer of radiators, producing rather dramatic results.

by Howard R. Stuart and Dennis G. Hall, The Institute of Optics, Univ. of Rochester, Rochester, NY.
Unraveling the Mysteries of Intense Femtosecond Pulse Propagation

Propagation of electromagnetic pulses is of fundamental importance in pure and applied science, and the recent development of sources of intense femtosecond laser pulses has added many interesting twists to this long-standing problem. The broad spectral band-widths, high peak powers, and 4-D nature of femtosecond fields give rise to complicated linear and nonlinear dynamics that have posed significant challenges to researchers. A few years ago, it was observed that in contrast to a continuous beam of light, a femtosecond pulse having the same peak power does not collapse to a singularity under the influence of self-focusing in a nonlinear medium. Instead, the original pulse splits temporally into two pulses of lower power. However, the details of this splitting process remained unclear, and furthermore, it was unknown whether the newly split pulses would in turn undergo a secondary splitting.

by Scott A. Diddams, Hilary K. Eaton, Amelia G. Van Engen, and Tracy S. Clement, JILA, Univ. of Colorado, and NIST, Boulder, CO; Alex A. Zozulya, Dept. of Physics, Worcester Polytechnical Institute, Worcester, MA.
Long Distance Propagation in Air Due to Dynamic Spatial Replenishment

Recently, there has been considerable excitement regarding experimental demonstrations of propagation of femtosecond pulses over 102-104 m in air due to its potential applications, e.g., lightning channeling and LIDAR. To determine the utility of this phenomenon for these and other applications the underlying physics needs clarifying. The critical power for self-focusing in air is Pcr =1.7 GW. Catastrophic collapse is avoided by a combination of multi-photon ionization (MPI), and absorption and defocusing by the electron-plasma generated by MPI. Our question is: How do these mechanisms conspire to produce long distance propagation?

by M. Mlejnek, E.M. Wright, and J.V. Moloney, Arizona Center for Mathematical Sciences, and Optical Sciences Center, Univ. of Arizona, Tucson, AZ.
Seeking for New Propagation Invariant Wave-fields

The possibility of producing wave-fields (WFs) with propagation-invariant characteristics has generated wide interest in recent years. These WFs are attractive for applications such as delivery of information and energy, beam shaping, atom guiding, interferometry, and measurements. Apart from the technological interest, the physical phenomena associated with such WFs provides insight into the ways to control and use diffraction.

by Rafael Piestun, Dept. of Electrical Engin., Technion, Israel Institute of Technology, Haifa, Israel and Ginzton Lab., Stanford Univ., Stanford, CA; Joseph Shamir, Dept. of Electrical Engin., Technion, Israel Institute of Technology, Haifa, Israel.
Coherent Optical Control of the Quantum State of a Single Quantum Dot

Semiconductor quantum dots are nano-scale 3-D semiconductor heterostructures characterized by dimensions that are comparable to electronic scale lengths. In the case of excitons in GaAs this corresponds to tens of nanometers. The recent developments in nano-optical probing have resulted in the observation of many electronic and optical properties of these structures, which show great similarity to simple atomic systems including sharp line spectra and relatively simple nonlinear optical signatures.

by N.H. Bonadeo, John Erland, D. Gammon, D.S. Katzer, D. Park, and D.G. Steel, Univ. of Michigan, Ann Arbor, Ml, and Naval Research Laboratory, Washington, DC.
RF Coupled Optical Gain in a Solid Owing to Quantum Interference

The interaction of a strong resonant laser field with two levels in a three-level system can modify the absorption and refractive index of a probe field whose transition involves the third level. Particularly, the probe field is not absorbed at line center due to two-photon coherence and destructive quantum interference, so that an optically thick medium can become transparent. This is called electromagnetically induced transparency (EIT).

by Byoung S. Ham and Selim M. Shahriar, Research Lab. of. Electronics, MIT, Cambridge, MA; Philip R. Hemmer, Air Force Research Laboratory, Hanscom AFB, MA.
Optical Confinement of Bose-Einstein Condensates

Three years after the first observations of Bose-Einstein condensation (BEC) in dilute atomic gases this new field is still bustling with increasing activities. Until recently, all studies of BEC had been performed in magnetic traps. These traps, in combination with RF-induced evaporation, are ideal for cooling and trapping atoms at very low temperature and might develop into workhorses for nanokelvin atomic physics.

by Wolfgang Ketterle, Dept. of Physics and Research Laboratory of Electronics, MIT, Cambridge, MA.
Optically Induced Coherent Rotation and Vortex Creation in Trapped Bose-Einstein Condensates

The research on trapped ultracold atomic gases has become one of the most exciting areas in physics since the experimental realization of Bose-Einstein condensation (BEC) in magnetic traps. One aspect of particular current interest is whether such gaseous atomic BEC also exhibit superfluid properties as in liquid helium.

by Karl-Peter Marzlin and Weiping Zhang, School of Mathematics, Physics, Computing, and Electronics, Macquarie Univ., Sydney, Australia.
Control of Broad-area Optical Devices: Patterns to Order

Nonlinear physical systems can have multiple output states compatible with a single input state. This enables them to be used to store, process, and transmit information. Applications typically require the selection of a particular desired state out of this multiplicity, for example, the Gaussian mode in a laser. Often the desired state is, or becomes, unstable—as when a laser passes from single- to multi-mode operation. In 1998, several experiments have shown how Fourier-space filtering can persuade optical systems to produce otherwise unstable patterns. Persuade, rather than force, because with appropriate design, the system self-organizes in such a way that little or no energy is lost in the filter. This is exciting because optical systems could, in principle, be similarly persuaded to display unstable states representing images or information rather than simple patterns.

by Graeme K. Harkness, Gian-Luca Oppo, and Willie J. Firth, Dept. of Physics and Applied Physics, Univ. of Strathclyde, Glasgow, Scotland, U.K.
Three-dimensional Spatial Electro-optical Correlator

Correlation is essential in signal processing in general, and in optical image processing in particular. A spatial correlation is used extensively in various schemes of edge enhancement, pattern recognition, target tracking, and more. In most of these schemes the functions involved are at most 2-D. However, our real world is 3-D, and in some applications one needs to process 3-D objects in their natural 3-D environment. Pattern recognition and target tracking are examples of applications that can benefit from the use of 3-D correlation. In these applications one uses the information obtained from the 3-D shape of the target and learns its location in the 3-D space.

by Joseph Rosen, Dept. of Electrical and Computer Engin., Ben-Gurion Univ. of the Negev, Beer-Sheva, Israel.
Diffraction Tomography of Strongly Scattering Objects Based on Homomorphic Filtering

For very weak scattering objects the first Born approximation can be applied, which assumes ψ ψi. Then Eq. 1 identifies ψs as the Fourier transform of the object V along a circle tangent to the origin in Fourier space. The superposition of data taken with different si yields the information about a low pass filtered image of V. To extend the range of objects that can be imaged, many attempts have been made to evaluate higher-order terms of the Born series approaching the solution of Eq. 1 by means of iterative numerical methods. However, without significant further sophistication, the convergence of the Born series, and hence the validity of reconstruction methods based on it, is still limited to objects with small k0Va, with "a" being a measure of the physical extent of V.

by J.D. Sanchez, A.E. Morales-Porras, M. Testorf, and M.A. Fiddy, Dept. of Electrical & Computer Engin., Univ. of Massachusetts-Lowell, Lowell, MA.
Application of Smart Pixels to Optical Implementation of the Wavelet Transform

The concept of wavelets is based on fundamental ideas in transform domain processing, which were first expressed centuries ago in a variety of forms. However, it is only within the past decade, since the pioneering work of Daubechies demonstrated the relationship between wavelets and subband transforms, that significant progress has been made in applying wavelet theory to practical signal processing problems. Since then, there has been an explosion of interest in wavelets and subband transforms for wide-ranging applications in signal processing, communications, biomedical techniques, and many interdisciplinary fields.

by B. Shoop, D.M. Litynski, and D. Hall, U.S. Military Acad., West Point, NY; P. Das, ECSE Dept., Rensselaer Polytechnic Institute, Troy, NY; C. DeCusatis, IBM Corp., Poughkeepsie, NY.
Dark Incoherent Solitons

Dark beams are nonuniform optical beams that contain either a 1-D dark stripe or a 2-D dark hole resulting from a phase singularity or an amplitude depression in their optical field. Thus far, self-trapped dark beams (dark solitons) have been observed using coherent light only. Recently, however, we have demonstrated self-trapping of dark incoherent light beams (or, in a broader prospective, self-trapping of dark incoherent wave-packets in nature) for the first time. Both dark stripes and holes (vortices) nested in a broad partially spatially incoherent wavefront were shown to self-trap and form incoherent dark solitons in a nonlinear photorefractive crystal. These self-trapped 1-D and 2-D dark beams induce refractive-index changes akin to planar and circular dielectric waveguides, which introduces the possibility of controlling high-power laser light with low-power incoherent optical sources such as LEDs.

by Zhigang Chen, San Francisco State Univ., San Francisco, CA; Matthew Mitchell and Mordechai Segev, Princeton Univ., Princeton, NJ; Tamer H. Coskun and Demetrios N. Christodoulides, Lehigh Univ., Bethlehem, PA.
Splitting Light Beams with Light Itself

Self-guided beams (or spatial solitons) are natural building blocks for a future all-optical technology where light guides and manipulates light itself in a bulk medium.1-3 Theory shows that spatial solitons can steer each other or even be made to spiral about each other. By colliding such solitons we can create fused beams or control the birth of new beams. A number of these predictions have now been observed in the laboratory including induced optical fibers, spiraling, fusion, and soliton birth. Recently a new phenomenon has been added to this arsenal: splitting light beams with light itself. A "bright" beam can be split into two bright beams upon illumination by a "dim" beam.

by Allan W. Snyder, Optical Sciences Centre, Institute of Advanced Studies, Australian National Univ., Canberra, Australia; Alexander V. Buryak, School of Mathematics and Statistics, University College, Australian Defence Force Academy, Canberra, Australia.
Another Twist of Light: Soliton Collisions in Bulk Media

Self-guided optical beams (or spatial solitons) have attracted substantial research interest because they hold a promise of all-optical switching and controlling light by light. Typically interactions of spatial solitons have been analyzed for 2-D geometries (soliton interactions in nonlinear slab waveguides). Only recently, experimental discoveries of stable solitons in bulk nonlinear media initiated the experimental study of fully 3-D collisions between solitary beams, and some exciting results have been achieved. However, a major problem had been hampering further progress in the development of futuristic 3-D soliton switches: namely that a reliable theory of such 3-D soliton interactions had not been developed, making physical intuition and numerics the only tools for predicting experimental outcomes.

by Victoria V. Steblina, Alexander V. Buryak, Yuri S. Kivshar, C. De Angelis, A. Barthelemy and B. Bourliaguet
A New Tunable Coherent FIR-THz Source

Bloch oscillations are one of the most basic effects in solids: If an electron is put into a static field, it will accelerate until it reaches the edge of the first Brillouin zone. It is then Bragg-reflected and returns to its original position, where it starts to accelerate again. The resulting spatial oscillation has never been observed in bulk solids since it is suppressed by scattering events. Bloch oscillations have recently been observed in semiconductor superlattices. These experiments demonstrate that the frequency of the oscillations is tunable over a large range by the static electric field and that the electron oscillations lead to emission of THz radiation, which is promising for applications.

by M. Sudzius, V.G. Lyssenko, G. Valusis, F. Loser, T. Hasche, K. Leo, M.M. Dignam, K. Köhler
Frequency-resolved Optical Gating Characterization of 4.5-fs Pulses

Accurate phase and amplitude characterization of sub-5-fs laser pulses, which became available a year ago, is essential for most spectroscopic applications. For such short pulses, spectral phase corrections must be carried out across the bandwidth of several hundreds of nanometers. Therefore, a comprehensive knowledge of phase distortions is invaluable for optimization of multistage compressors to obtain the ultimate spectral-limited pulses.

by Andrius Baltuska, Maxim S. Pshenichnikov, Douwe A. Wiersma, Ultrafast Laser and Spectroscopy Laboratory, Dept. of Chemistry, Univ. of Groningen, Groningen, The Netherlands.
Adaptive Compression and Shaping of Femtosecond Pulses

Linearly chirped femtosecond pulses can be compressed efficiently by grating or prism pair compressors. Combination of these compressors can be used to remove slightly more complicated spectral phase distributions. If the spectral phases cannot be approximated by the second- and third-order dispersion terms, or if the pulses are completely uncharacterized, these compressors cannot be used to accomplish efficient compression. Consequently, they cannot be used in situations where pulses undergo slow variations in time.

by Doron Meshulach, Dvir Yelin, and Yaron Silberberg, Dept. of Physics of Complex Systems, Weizmann Institute of Science, Rehovot, Israel
Phase Matched Generation of Coherent Soft-X-rays

Nonlinear optical techniques for frequency conversion have played a pivotal role in the development of efficient coherent light sources. Most of these techniques rely on phase matching to obtain high conversion efficiency. Typically, by using birefringence, one can equalize the phase velocity of the pump light with the desired signal light, resulting in constructive interference of the signal produced through a nonlinear interaction. This allows the signal to build up rapidly. However, most techniques rely on crystals. This significantly limits extending nonlinear optics to shorter wavelengths, since no solid material is transparent to wavelengths below 110 nm. Furthermore, dispersion in solid materials makes it difficult to generate extremely short pulses at short wavelengths.

by A. Rundquist, C.G. Durfee III, 2. Chang, C. Heme, S. Backus, M.M. Murnane, and H.C. Kapteyn, Center for Ultrafast Optical Science, Univ. of Michigan, Ann Arbor, Ml.
Two-dimensional Near-field and Far-field Imaging of a Ne-like Ar Capillary Discharge Table-top Soft X-ray Laser

The observation of large soft X-ray amplification in the plasma of a capillary discharge and the subsequent demonstration of a saturated discharge pumped table-top soft X-ray laser in Ne-like Ar at 46.9 nm has established a new approach for the development of compact and practical soft X-ray lasers. In these lasers the gain medium is a hot and dense plasma column with aspect ratios approaching 1000:1, generated in a capillary channel by a fast discharge current pulse where lasing is obtained by collisional electron excitation of Ne-like ions. Knowledge of the near- and far-field spatial distribution of the output of these lasers is of both practical and basic interest.

by C.H. Moreno, M.C. Marconi, V.N. Shlyaptsev, B.R. Benware, C.D. Macchietto, J.L.A. Chilla, and J.J. Rocca, Electrical Engin. Dept., Colorado State Univ., Fort Collins, CO; A.L. Osterheld, Lawrence Livermore National Lab., Livermore, CA.