Feature Articles

Long-wavelength multiple quantum well modulators

Avariety of novel physical effects have been observed in multiple quantum wells (MQWs), which are thin layers of a low band-gap semiconductor surrounded by higher band gap material. Frequently, quantum mechanical effects are manifested in useful, macroscopic properties. Several years ago, it was discovered that large changes in the zero-point energies of particles in MQWs could be achieved by the application of an electric field perpendicular to the quantum well layers.

by Thomas H. Wood
Nonlinear optical lightwave local network

Nonlinear effects in fibers have been primarily seen as a limitation to lightwave system performance. A recently demonstrated network instead turns nonlinear optics to advantage by using stimulated Brillouin scattering to provide channel selection in a densely packed wavelength division multiplexed network.

by R. W. Tkach, A.R. Chraplryvy
Continuous wave operation of Ga/As lasers on Si substrates

Integration of optical devices (dominated by semiconductor GaAs) and electronic devices...

by H.Z. Chen, A. Ghaffari, H. Wang, H. Morkoc
External electro-optic sampling

Scientists at Bell Laboratories have developed and demonstrated a new, non-contact, picosecond, electro-optic technique for obtaining signal waveforms at internal nodes of high speed integrated circuits or from discrete devices fabricated on any substrate material.1 This technique, referred to as external electro-optic sampling, achieves sub-picosecond temporal resolution with a spatial resolution of a few microns. It is designed to operate at the wafer level on conventional wafer probing equipment without any special circuit preparation. Initial experiments have characterized GaAs SDHT prescaler circuits, and also addressed the inherent speed and loading limits of the technique. Temporal resolution of less than 300 fs has been shown.

by J. Valdmanis, S. Pei, and M. Nuss
Optical associative memories

The complimentary properties of neural network models, with their collective, iterative, nonlinear, robust, and fault-tolerant approach to information processing, and the inherent capabilities of optics (parallelism and massive interconnectivity) was first pointed out and the first optical associative memory demonstrated in 1985. Since then, work and interest in neuromorphic optical signal processing has been growing steadily. Work in this area is evolving rapidly enough to make it difficult to be aware of all relevant developments and to assess their implications for the purposes of this report. Some noteworthy work may have been left out. For that the author offers his sincere apologies and hopes that such work is included in next year's report.

by Nabil H. Farhat
Optical resonators, mode competition, and associative memory

The mathematical description of the laser is of a rather universal character that emerges in many fields of science. The same mathematics has been applied to the interaction among neurons forming a network to model brain behavior and cognitive function. That these two systems share a common mathematical heritage leads one to wonder whether one can extract useful brain-like behavior from a laser or a similar nonlinear optical system. Some recent work with holographic optical resonators has been motivated by precisely this notion.

by Dana Z. Anderson
Optical associative memory incorporating holography and phase-conjugation

The complimentary properties of neural network models, with their collective, iterative, nonlinear, robust, and fault-tolerant approach to information processing, and the inherent capabilities of optics (parallelism and massive interconnectivity) was first pointed out and the first optical associative memory demonstrated in 1985. Since then, work and interest in neuromorphic optical signal processing has been growing steadily. Work in this area is evolving rapidly enough to make it difficult to be aware of all relevant developments and to assess their implications for the purposes of this report. Some noteworthy work may have been left out. For that the author offers his sincere apologies and hopes that such work is included in next year's report.

by Nabil H. Farhat
New mechanism tor generating redshifts of spectral lines

According to current cosmological theories, the universe came into existence some 15 to 20 billion years ago. Since then, the large scale four-dimensional fabric of the world has continued to drag matter and radiation further and further apart. The available theories and observational evidence suggest that the recessional velocity of the visible and presumably also the invisible features increases linearly with the separation between the observer and the observed structures. This statement, known as Hubble's law, plays a central role in our current interpretation of the universe and in our understanding of its distant past. The observed red shift of the distant galaxies fits nicely with the picture of an expanding dynamical universe.

by Emil Wolf
Simulated annealing and lens design

Based on theoretical work done by Metropolis in the 1950s, Bohachevsky, Viswanathan, and others at the Los Alamos National Laboratories pioneered the application of simulated annealing to lens design. More recently, efforts by Yang and Hopkins, Hearn, and Weller have advanced this technology toward commercial use by designers.

by Scott W. Weller
Quantum nondemolition detection and four-mode squeezing

Typically, when a laser beam is measured with a photodetector, the light is absorbed. Thus information about the amplitude of the beam is obtained at the cost of its destruction. A portion of the beam can be diverted with a beam-splitter and measured, leaving the remainder of the beam available for further measurements. At the quantum level, this also destroys any correlation between the amplitude of the detected beam and the transmitted beam. Both of these measurements are termed "quantum demolition" measurements because they do not provide information about the quantum state of the light subsequent to the measurement event. A method of measuring light amplitude which did not alter that amplitude would be valuable to experimentalists. Such a method has now been realized using nonlinear optics.

by R. M. Shelby
Precision measurement beyond the shot-noise limit

The first applications of squeezed states to improve the precision of optical measurements beyond the limit set by the vacuum or zero-point fluctuations of the electromagnetic field have recently been reported. Improvements in the signal-to-noise ratio relative to the shot-noise limit for the detection of both amplitude and phase changes of the field have been achieved. Phase modulation in optical interferometers has been detected with an increase in sensitivity of 3dB relative to the shot-noise limit. Additionally, amplitude modulation was detected with an improvement in signal-to-noise of 2.5dB relative to the shot-noise limit. The experiments made use of squeezed light generated by an optical parametric oscillator to reduce the level of fluctuations below the level of vacuum fluctuations of the electromagnetic field.

by H. Jeff Kimble
Inertial confinement fusion and x-ray laser advances

Significant advances in both inertial confinement fusion and laboratory x-ray lasers have been made at the Nova laser at Lawrence Livermore National Laboratory in 1987. For inertial confinement fusion to succeed in producing net energy gain in the laboratory, it is necessary to implode capsules containing deuterium-tritium (DT) fuel to densities approaching 200 g/cm3 and ion temperatures of approximately 4 keV (4.4 X 10^7 °K). This represents a fuel pressure of ~ 2.7 X 10^11 atmospheres. To achieve such conditions with reasonable ablation pressures (PABL ~ 10^8 atmospheres) and with liquid DT fuel, the fuel radius must be reduced 20 to 40 fold. The ratio of the initial fuel radius to final fuel radius is commonly referred to as the convergence ratio. To achieve such high convergence implosions, stringent requirements are placed on the uniformity of the ablation pressure (pressure uniformity on the capsule surface must be better than 2-3%) and hydrodynamic instabilities.

by E. Michael Campbell
Progress toward a gamma-ray laser

Levels of nuclear excitation that might be efficiently stimulated in a gamma-ray laser are very difficult to pump directly. To have sharply-peaked cross sections for stimulated emission, such levels must have very narrow widths for interaction with the radiation field. This is a fundamental attribute that has led to the facile criticism that "absorption widths in nuclei are too narrow to permit effective pumping with x-rays."

by C.B. Collins, J. A. Anderson, F. Davanloo, C. D. Eberhard, J. F. McCoy, and S. S. Wagal
Green infrared-pumped upconversion lasers

The advent of powerful near infrared GaAlAs diode lasers has stimulated interest in the development of visible laser sources that are pumped by semiconductor diode lasers. One approach uses intra-cavity second harmonic generation in Nd3+ lasers pumped by high-power GaAlAs diode lasers. We have recently demonstrated another approach, that of laser-pumped upconversion lasers where nonlinear pumping excites green laser emission using two-step absorption or cooperative energy transfer.

by W. Lenth, R. M. MacFarlane, and A. J. Silversmith
Nondiffracting beams

Recently workers at the University of Rochester have undertaken both theoretical and experimental investigations of the properties of nondiffracting beams. In their ideal form, these beams represent a class of fields having transverse intensity distributions that are unaltered as they propagate in free space. Although not achievable in their ideal form (the field distribution would need to be perfectly mapped over an infinite plane), many interesting results have been obtained with these beams under the restriction of finite aperture. Investigation of the power transport efficiency and depth of field obtainable with these beams has revealed some interesting trade-offs in their utilization.

by J. Durnin and J. J. Miceli Jr.
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Ultrafast all-optical glass fiber switches

The promise of achieving ultrarapid all-optical signal processing is attracting considerable interest. Many of the necessary components have been available for some time, including lasers that produce subpicosecond pulses, optical fiber waveguides that support tremendous band-widths, and optical materials that have nonlinearities with femtosecond response.

by P. F. Liao
Encoding and decoding of femtosecond pulses

Diverse communication and signal processing technologies utilize specially coded signal formats in order to achieve desirable capabilities such as error correction, interference rejection, or secrecy. The ability to encode signals according to the specified format is a crucial component of such systems.

by Peter W. Smith
Terahertz-frequency quantum beats in organic molecules

Recent femtosecond measurements of the relaxation of several organic dye molecules have revealed one or more damped sinusoidal components superimposed on an exponential background. A typical experimental trace is shown in the figure for ethyl violet dissolved in ethylene glycol. The decay is dominated by a sinusoid with a period of 150 fs. Several other triphenyl methane dyes have been studied, and all of them produce an oscillating component with approximately the same period. The measured relaxation of the saturable absorber dye DODCI also exhibits a sinusoidal term with a 150-fs period, as well as several other oscillating terms.

by F. Wise and C. L. Tang
Ultrafast photorefractive effects

Recent work by collaborators at Hughes Research Laboratories and North Texas State University has demonstrated the photorefractive effect in GaAs at a wavelength of 1.06 μm using 43-ps pulses and in BaTiO3 at a wavelength of 0.53 μm using 30-ps pulses.1,2 In undoped semi-insulating GaAs, beam coupling gains up to 10% were observed with fluences that ranged up to 10 mj/cm2, while in BaTiO3 beam coupling was below the detection limits but modest diffraction efficiencies were observed in a three-pulse transient grating geometry.

by Arthur L. Smirl, K. Bohnert and Thomas F. Boggess
New wavelength femtosecond dye lasers

There is a continuing need for lasers capable of generating femtosecond optical pulses for application to investigations of ultrafast phenomena in chemistry, physics, biology, electronics, and communications. The most common source of these pulses is the passively mode-locked cw dye laser, which may be either continuously or synchronously pumped. From the inception of femtosecond spectroscopy in 1981 until very recently, such lasers operated almost exclusively using a single pair of dyes— Rhodamine 6G and DODCI—as the gain medium and the saturable absorber, respectively. This restricted direct femtosecond generation to the 600-635 nm spectral region.

by Martin D. Dawson and Thomas F. Boggess
Nonlinear optics in polymeric semiconductors

The future of photonics critically depends on new materials that possess large and fast optical nonlinearities. Conjugated polymers like the polydiacetylenes and archtypical polyacetylene have emerged as potentially important materials to fill this role

by S. James Allen
A digital optical switch

Optical fiber communications is already a well developed technology and fibers are being installed in the field at an ever growing rate. It is therefore somewhat surprising to realize that the optical network has been limited so far to point-to-point communications. Any manipulation of the signal, and in particular switching and routing, is performed after the optical signal is converted to an electrical signal. Direct switching and routing of light signals is one of the functions that the developing technology of integrated optics promises to accomplish.

by Peter W. Smith
Organic polymers as nonlinear optical materials

Defying the traditional perception of plastics as structural materials, organic polymeric systems containing conjugated structures have emerged as exciting nonlinear optical materials. The reason lies in their highly polarizable π-electron clouds, which yield the largest observed nonresonant third-order optical susceptibility (x(3) ≤ 10-9 esu) and the fastest (femtoseconds) response times. High non-resonant optical nonlinearity is desirable for wave guiding in integrated optics applications. Polymeric systems also offer the flexibility of tailoring the structure by molecular engineering whereby a rich variety of nonlinear polymeric systems can be synthesized and fabricated in various shapes such as films, fibers, slabs, etc.

by Paras N. Prasad
Optical nonlinearities with an alternate growth technique

Recently, there has been a great deal of interest in developing low power, high speed optical signal processing elements from semiconductor materials. These devices use an intensity dependent absorption or an intensity dependent index of refraction to modulate light. Bulk semiconductors have large optical nonlinearities, especially when exposed to light that is resonant with the band-gap. Two types of semiconductor materials that exhibit even larger nonlinearites are multiple quantum wells (MQWs) and doping superlattices (DS).

by A. Kost, M. Kawase, and E. Garmire
Low loss single-mode semiconductor optical waveguides

Integrated optoelectronic circuits made of GaAs/AlGaAs and InP/InGaAsP have attracted considerable interest in recent years. Such integrated circuits combine electronic circuitry with lasers, optical waveguide devices, and optical detectors, all on the same semiconductor chip. However, one of the important problems associated with integrated optoelectronics has been the relatively high propagation losses in single (spatial) mode semiconductor optical waveguides. Propagation loss coefficients were ≥ 1 dB/cm for photon energies below the semiconductor band gap. These loss values are about one order of magnitude larger than those achievable in the more well established materials used for integrated optics, most notably lithium niobate. Such high propagation losses are unacceptable for many integrated optoelectronics applications.

by T. P. Lee
Photonic band structure

The question of equivalence between photon propagation in disordered dielectric media and electron propagation in disordered conducting media has been intensively investigated within the last several years. Recently, the question of equivalence has been addressed with respect to ordered, or spatially periodic, media by Yablonovitch4 and by John5, yielding interesting new perspectives on the band theory of spin-one particles and "strong," or Anderson, localization, with potentially important implications for the development of optoelectronic devices.

by D. E. Aspnes
Decoding messages from atoms and molecules

The subject of "radiation exchange" is quite open-ended and provides the teacher with many options for presentation. It could be argued that a good pedagogical approach would be to present at least some of the material along the line of decoding messages from atoms and molecules. At this stage in the careers of most students taking introductory physics, they probably will not have given much thought to the fact that atoms and molecules are constantly communicating with us, so-to-speak, and that we want to try to be clever enough to decode their messages.

by J.H. Taylor
Japanese law prompts U.S. action on instrument standards

Standards news takes somewhat of a political flavor this month. Sid Braginsky, the U.S. leader for ISO/TC 172/SC 5, Microscopes, recently received a letter from the Secretariat for TC 172 urging rapid consideration of standards for "operation microscopes," those microscopes used by surgeons during delicate surgery.

by Robert E. Parks