December 1994

Photorefractive Dark and Vortex Solitons

Spatial solitons in photorefractive (PR) materials1 have been the object of growing interest during the last two years. Until now, three different types of PR solitons have been investigated. One type of PR soliton stems from the nonlocal nature of the photorefractive effect, as manifested in the dependence of the perturbation in the refractive index on the transverse derivatives of the light intensity distribution.

Crosstalk in Photorefractive Optical Interconnects Due to Nonlinear Mixing of Gratings

Photorefractive interconnects are used in many applications such as neural networks, information storage, clock distribution, and optical computing. In these applications, an important issue is crosstalk. Recently, we have investigated crosstalk in a Bi12SiO20(BSO) optical interconnect.1 In this device, significant crosstalk is observed due to nonlinear mixing of gratings. The nonlinear mixing of gratings is a consequence of the nonlocal response of the space charge field in the photorefractive medium, and it can be analyzed from a nonlinear solution of the Band Transport Model.

New Lights on Liquid Crystal Mystery: Orientational Photorefractivity

Interest in materials that exhibit photorefractivity has been very intense, due to their capabilities for low power optical wave mixing applications, which include phase conjugation, image and signal processing, and holographic storage. Currently, inorganic photorefractive crystals are the materials of choice. These crystals are quite expensive and hard to grow. Moreover, the spectral response of these materials is limited to the visible-1 μm range. There has also been recent work on polymer-based photorefractive materials; they, however, require very high dc fields (several tens of kilovolts/ centimeter), respond slowly, and are applicable over a limited spectral region just as inorganic crystals.

Near 100% Diffraction Efficiency and High Net-gain in a Photorefractive Polymer

Photorefractive (PR) polymers are new materials1-4 with many potential photonic applications, including dynamic holographic storage and image processing. They offer structural flexibility, ease of processing, a superior figure of merit compared with commonly used inorganic crystals, and low cost. We have developed a photorefractive polymer composite with significantly enhanced diffraction efficiency and two-beam-coupling gain.5 For the first time, near complete diffraction of an incident beam can be observed in a 105-μm thin photorefractive polymer film using a low-power laser diode. The material also exhibits a net two-beam coupling gain of more than 200 cm-1.

Acceleration of Coherent Transfer of Energy by Stimulated Emission and Absorption

In ACTESEA, modifications of the probe gain in the vicinity of the frequency of a narrowband cw beam injected into the Fabry-Perot peak of a VCSEL just above threshold are detected as changes in the broad lasing spectral profile. As shown in the figure, the emission is enhanced below the injection frequency and suppressed above.

The Quantum Cascade Laser

In semiconductor diode lasers, light is generated by the recombination of electrons and holes injected into the active layer by means of a p-n junction. In this process, electrons from filled states in the conduction band make a transition to empty states in the valence band (holes), emitting laser photons of energy equal to the energy bandgap.

Discharge Pumped Soft-X-Ray Laser

Following the first demonstrations of soft-x-ray lasing in the plasmas generated by large laser facilities in 1984,1 researchers have pursued more compact and efficient X-ray lasers that could have widespread use in applications. With this motivation, recent experiments have explored amplification schemes that use smaller drivers, leading to much progress during the past year.

Quantum Wire Microcavity Laser

High efficiency, low current threshold semiconductor lasers are attractive for many applications in communications, optical processing, and optical interconnects. Recent efforts to produce such lasers rely on either strong electronic or optical confinement to more completely couple the material states or cavity modes into the lasing process. For example, quantum confinement in one-dimensional quantum wires concentrates the electronic density of states into a narrow wavelength band.

A 5-MW Laser for Ultrafast Spectroscopy at a 10-fsec Time Scale

The Ti:sapphire laser is rapidly becoming the light source in all ultrafast laboratories providing, reliably, 10-15 fsec pulses with 4-5 nJ energy at 80 MHz repetition rate. Meanwhile, a wide range of nonlinear ultrafast experiments would greatly benefit from a Ti:sapphire laser-based light source that generates more powerful pulses at variable repetition rates, without sacrificing the ultrashort pulse duration or any other performance of a femtosecond Ti:sapphire laser. We have recently succeeded in building such a laser by incorporating a cavity dumper at a stategic position into the resonator of a self-modelocked Ti:sapphire laser.

Diffractive Optic Laser Resonator

Virtually all commercial lasers use spherical or planar mirrors to establish the laser mode. However, resonators with spherical mirrors represent only a very small subset of possible resonator configurations. Recently, the technology of diffractive optics has made it possible to fabricate high-quality mirrors with arbitrary phase reflectance, giving rise to an entirely new class of laser resonators. These new resonators can have unique and highly desirable properties such as user-designable mode shapes and profiles, high modal discrimination without a correspondingly high fundamental mode loss, and large mode volumes in short cavities.

Intrinsic Perturbations of Bloch Oscillations and Wannier-Stark Ladders in Semiconductor Superlattices

In the absence of scattering mechanisms, a crystal electron subjected to a homogeneous electric field oscillates in both real and momentum space. This periodic motion has an oscillation frequency proportional to the applied electric field, and is known as Bloch oscillation (BO). It translates into the frequency domain as a spectrum of equally spaced energy levels, the so-called Wannier-Stark ladder (WSL). The two phenomena have recently been detected in optical experiments on semiconductor superlattices. WSLs have shown promise as light modulators, and Bloch oscillators have been discussed as tunable emitters of terahertz (THz) radiation.

Nanosecond Time-scale Semiconductor Photoexcitations Probed by Scanning Tunneling Microscopy

We show that the temporal response of photoexcited charge carriers in semiconductor structures can be probed by scanning tunneling microscopy (STM) on a nanosecond time scale. The results presented here complement previous work in which static and dynamic surface photovoltage (SPV) at semiconductor surfaces were measured using an STM operating in the tunneling and capacitance modes, respectively. Note that optically induced picosecond switching of an STM that probed a metal surface has also been reported.

Photoassociation of Laser-cooled Atoms: A New Spectroscopy

Advances in laser cooling and trapping have allowed researchers to begin to explore the field of ultracold collisions. An important new development is the technique of ultracold photoassociative spectroscopy. The idea was proposed by Thorsheim et al. in 1987 but has only been realized experimentally within the last two years. In photoas-sociative spectroscopy, free atoms that are held in an optical trap collide. During the collision, the free atoms are promoted by laser excitation to bound molecular states. These excited molecules can then either be ionized with an additional photon and detected or allowed to decay into hot atoms that escape the trap. The photoassociation in this latter case is monitored by means of the increased loss of atoms from the trap. An example of the two types of spectra is shown in the figure. Extremely precise spectroscopy of the created dimers can be performed in this way.

New Lidar Reveals Seasonal Temperature Variations in a Mid-latitude Mesopause Region

It is now common knowledge that releases of trace gases from human activity may cause global climatic change. At the current rates of increase, the amount of tropospheric CO2 and CH4 are predicted to double within the next century, causing the temperature in the troposphere to be warmer and cooler in the stratosphere by a few degrees. Circulation model studies1 suggest that doubling of CO2 and CH4 may cool the upper mesosphere by 10-20 K and cool the thermosphere by as much as 50 K.

Trapping Laser-cooled Atoms With Microwave Radiation

A team of researchers from Harvard University and the National Institute of Standards and Technology recently demonstrated the trapping of laser-cooled neutral atoms by the force exerted on them by microwave radiation. Such trapping, first suggested a few years ago, is an attempt to address one of the obstacles to achieving Bose-Einstein condensation (BEC) of an atomic gas.

Quadrature-squeezed Light Detection Using a Self-generated Matched Local Oscillator

Recent quadrature-squeezing experiments have greatly reduced the amount of observed quantum noise in a system. The cw squeezed light generated from a sub-threshold optical parametric oscillator resulted in 6.0 dB (75%) of noise reduction1 and 5.1 dB (69%) of pulsed squeezing was observed in traveling-wave fiber ring experiments. However, the pulsed squeezed light generated from a single-pass traveling-wave optical parametric amplifier (OPA) has not yielded more than 2 dB (37%) of noise reduction, despite a number of attempts. This is rather puzzling because the intensity noises of the signal and idler beams at the output of the same OPA were correlated by as much as 7 dB (80%).

Optical Anyons

Recent developments in theoretical physics have shown that particles with fractional spin, anyons, can arise in two dimensional systems, since the phase accumulated upon exchanging two identical particles can assume any value, thus interpolating between bosons and fermions. Here we describe some of our recent results, showing that composite particles formed from dark-seeking atoms bound to electromagnetic vortices display the properties of anyonic quasiparticles in optical physics.

Pushing Atoms with Darkness: Adiabatic Momentum Transfer

Two groups, one at Harvard University and one at the National Institute of Standards and Technology (NIST) in Gaithersburg, recently demonstrated the transfer of linear momentum from a laser beam to an atom, with the laser frequency tuned to the atomic resonance, but without the atom ever being in the optically excited state.

High Contrast High Speed Normal Incidence Optical Modulator

Coherent time-domain optical memory (CTOM), also known as stimulated photon echo memory, offers the potential of ultra-high storage density and ultra-high data throughput rate. Similar to persistent spectral hole burning, CTOM stores information in the frequency dimension of an inhomogeneous broadened absorbing material in addition to the spatial addresses used in the conventional 2-D optical memories. However, in CTOM, stored information is the Fourier transform of a structured data pulse, instead of zeros and ones in the frequency dimension. In practice, the data pulse can be conveniently an amplitude modulated binary stream, composed of zeros and ones in the time sequence.

Polarization Switching of InGaAsP/lnP Lasers in the Gigahertz Range

Optical computing and fiber-optic communication systems need fast optical switches and memories. Potential candidates for such devices are InGaAsP/lnP semiconductor lasers that can be directly switched between the TE and TM polarization modes (electric field parallel or perpendicular, respectively, to the junction plane) with high extinction ratios by changing the injection current.

Transverse Patterns in a Nonlinear Cell with Single Mirror Feedback

Transverse pattern formation in nonlinear optics was investigated in many complex arrangements both experimentally and numerically. But, as generally both nonlinearity and diffraction act simultaneously, the analytic treatment could never be performed, and moreover patterns were out of control. In this view, a rather simple system made of a thin Kerr cell and a planar feedback mirror and exhibiting transverse effects was recently proposed.

Second-order Optical Activity of Chiral Surfaces

Chiral molecules occur in two forms that are identical in their chemical composition but differ in their handedness. Distinction between these mirror image forms, or enantiomers, is important because they can have different physiological properties.

Resonant Tunneling and the Optical Response of Conjugated Molecules

Conjugated molecules are promising organic materials for photonic switching applications because of their potentially large nonresonant nonlinear response due to the easy delocalization of the -π electrons along their backbone. In addition, the ability to process these molecules into thin films on various substrates presents a major advantage from a device engineering standpoint. To design new compounds, an accurate estimate of the molecular length at which the optical response saturates is needed. We have shown1, that treating a conjugated molecule as a set of adequately coupled quantum oscillators leads to a strikingly good description of the experimentally observed length dependencies. This model is suitable for the study of the scaling laws, the effect of conformation, and of excited-state enhancement on the optical response.

Beyond the Lorentz-Lorenz Formula

The general method of integral equations (MIE) is developed for arbitrary nonlinear and anisotropic medium, taking into account quadrupole and magnetic-dipole radiation. The main idea of this generalization stems from the supposition that microscopic (local) fields E and H must satisfy the macroscopic wave equation.

Fabrication of a Large, Thin, Off-axis Aspheric Mirror

In dentistry, one frequently must prepare artificial material, such as dentures, to match the color of existing teeth. Usually, this is done by visually matching the material to existing teeth. The result of such a comparison depends on many factors, such as color-judging experience of the observer, lighting conditions, glare, and so on. Therefore, an objective method is very useful for this color matching process.

Low-cost Dynamic Photoelasticity

Photoelasticity is the photomechanics tool that introduces engineers to whole field optical stress analysis. However, only static experiments are demonstrated due to expense and complexity of dynamic photoelastic systems. The rapid growth in low-cost, pulsed light sources and advances in digital imaging have made it possible to develop systems that are more accessible and affordable for demonstrating dynamic stress visualization and analysis.1-5 The main components in a dynamic visualization setup are the illumination and recording systems coupled to the event under study by a trigger mechanism. Illumination sources are either high voltage unwieldy multiple spark gap systems or expensive and moody pulsed laser sources. The recording system can be a stationary film or a complex moving film system. The whole system cost is very high and system setup is time consuming.

Unsupervised Learning of Temporal Features

It is easy to recognize a radio channel that carries Morse code, even if you do not know the code. Morse code consists of a small set of simple temporal features (a dot, a dash, and two pause lengths), and a Morse signal is built by their repeated occurrences. The brain quickly identifies these features as the dominant content of the received signal. We have implemented an optical system that can discover, on its own, the dominant features in a temporal signal characterized by repetitive entities. This task is a precursor to the more complex processing required for the self-organized feature extraction and recognition of audio and sonar signals.

Phase-only Fourier Transform of an Amplitude Mask

Various types of phase-only filters for correlators have been proposed. Accordingly, the Fourier transform of a phase-only object has been widely discussed in the literature. A phase-only filter produces a specific intensity pattern of its Fourier transform. On the contrary, we reported recently an optical mask that can produce a phase-only Fourier transform. Since the produced Fourier transform is a phase-only function, the intensity pattern will be uniform. The optical mask can be applied for focusing and testing optical apparatus such as telescopes and for testing the human eye's accommodation capability.

Femtosecond Switching Speed in a Current-Injected GaAs/AlGaAs Multiple Quantum-Well Nonlinear Directional Coupler

We have demonstrated ultrafast switching behavior in a current-injected GaAs/AlGaAs multiple quantum well nonlinear directional coupler at room temperature. The results show low cross-over pulse energy (6 pJ) and full recovery within 1 picosecond.

Intra-Multi-Chip Module (MCM) Optical Clock Signal Distribution

The speed limitations of the current generation of computers have led researchers to seriously consider new computing architecture based on optical interconnects.1 Thus far, optical interconnects have been based on two-dimensional (2-D) waveguide arrays suitable for intra-board bus-connections and three-dimensional (3-D) freespace interconnections suitable for board-to-board connections.

Fiber-optic Interconnects for Large-scale Parallel Data Processing

The use of high-speed fiber-optic channels for data processing has become the key to a new class of high performance computer architectures. Conventional mainframe computers use high performance silicon bipolar technology that is densely packaged and requires water cooling. As a result, the cost/performance curve for these machines has grown prohibitively steep in recent years. Because of increasing performance in CMOS-based technology, an alternative approach to achieving very high performance involves connecting a large number of microprocessors in parallel. Since the individual processors can now be air cooled, the cost is much lower to achieve equivalent or superior performance to traditional mainframes. To take advantage of this highly coupled parallel processing, high bandwidth data communication channels are required; this application is well suited to fiber-optic technology.

Don't Step on the Toys, Grandpa!

When I was about seven years old, we had to be very careful with toys on the floor, because my grandparents had trouble seeing them, and might step on them. This seemed strange to me, but then older people were a little strange in those days. Some would tilt their heads in funny ways when reading the paper and lots of others wore glasses as necklaces. Many adults would peer at fine print and bring it under a bright light to read it.

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