InTISb alloys have been grown by low-pressure metalorganic chemical vapor deposition, and characterized. Photoconductors exhibit a cutoff wavelength that can be tailored from 5.5 micrometers up to 9 micrometers by varying the thallium content. Experimental observations suggest that this can be further extended by increasing the thallium content. An InTISb photoconductor having a 9 micrometers cutoff wavelength exhibited a D* of 10⁹ cm Hz^1/2 W^-1 at 7-micrometers operating wavelength.

We report a novel approach to normal incidence multiple quantum well light modulators. The quantum-confined Stark effect is utilized to tune the polarization rotation and phase retardation created by a thermally induced in-plane anisotropic strain. An exceedingly high contrast ratio of 4800:1 is demonstrated for a normally-on device at room temperature.

In this paper, a new quantum-well IR photodetector based on bound-to-miniband transitions in a GaAs/InGaP quantum well with GaAs/AlGaAs short superlattice barriers is presented and compared with the conventional GaAs/InGaP QWIPs. Results of the theoretical calculations of the detector parameters and the preliminary fabrication results of an embedded-well to miniband GaAs/InGaP/AlGaAs quantum well/superlattice detector are presented. The advantages of the proposed design include improvement of the material quality, ability to adjust the peak wavelength in 8 to 12 micrometers range, and in the lower dark current.

GaInAsP lattice matched to GaAs in the entire bandgap range has been grown by low-pressure metalorganic chemical vapor deposition. Small mismatch and strong interference fringes in the x-ray spectrum, sharp photoluminescence (PL) peak, and high electron mobility indicate good control of the quaternary compositions, smooth epilayer interfaces, and coherent growth of the epilayers. Temperature coefficient of bandgap is measured from the temperature dependence of the PL peak to be 4.09 XT 10^-4eV/K at 300K. Anomalous temperature dependence of PL at low temperature, similar to that reported for GaInP, is reported for GaInAsP/GaAs for the first time. It questions the attribution of the uncommon behavior to the crystal defects related to the long-range ordered structure.

Quantum well heterostructures provide enhanced electrooptic effects that allow waveguide modulators with both low drive voltage requirements and small physical footprint. Compactness is important for incorporation in systems where space is at a premium or weight is an issue. Minimizing waveguide device length is also a critical factor in reducing production cost, especially when the modulator is monolithically integrated with other components for higher functionality. Finally, for electrorefractive waveguide modulators that are RC-limited, compactness is the key to obtaining high speed operation.

Thin dielectric disks fabricated from semiconductors or possibly organic materials serve as low-loss optical resonators. The disk thickness is typically only a third of an optical wavelength in the material. Whispering-gallery modes near the disk edge account for the low-loss resonator modes. Microdisk lasers based on this thin disk resonator have been demonstrated using the InGaAs/InGaAsP system at wavelengths from 1.6 to 1.0 micrometers and in the GaAs/AlGaAs system at wavelengths near 0.8 micrometers . The simplicity of the fabrication for these resonators make them attractive for a wide range of materials and wavelengths. Both optical and electrical pumping have been demonstrated with thresholds for pulsed operation at room temperature near 1 mA in the electrical case. Electrical pumping is accomplished with relatively low resistance posts located above and below the disk. The microdisk radii can be reduced to 1 micrometers while maintaining sufficiently low optical loss for laser operation. At these small dimensions there is only one low-loss mode within the luminescence spectrum and a large fraction of the spontaneous emission from the active region is emitted into this lasing mode. This results in novel threshold characteristics including a gradual increase in light output near threshold and large, power independent, laser linewidths. Various schemes for coupling the microdisk laser output into waveguides and optical fibers will be discussed along with possible applications of these microlasers in 2D arrays and photonic circuits.

We discuss circular gratings and review their applications in the fields of integrated optics and optoelectronics. A historical review is provided and recent progress is summarized. Advantages and drawbacks of circular-grating waveguide devices are mentioned.

With the growing interest in photonic switching and optical information processing, 2D optical interconnect systems have been extensively studied. The key devices for these systems are surface-emitting optical devices. As surface-emitting optical functional devices, we have developed vertical-to-surface transmission electro-photonic devices (VSTEPs) based on a concept of fusion of electronics and photonics in a device level. These VSTEPs have several functions, such as light emission, light detection, memory, and logical operation. Recently, a pnpn VSTEP with a vertical cavity (VC-VSTEP), which operates as a laser in the on-state, has been demonstrated. To obtain a highly integrated 2D array, low consumption power for each device is needed. Therefore, we have to reduce threshold current. Here, threshold reduction by photon recycling is reviewed. The sidewalls of VC-VSTEPs are covered with gold film which has high reflectivity. This gold film retains spontaneously emitted photons in the optical cavity, and a large part of spontaneously emitted photons is absorbed and the carriers are regenerated. This photon recycling of spontaneous emission can reduce the threshold current without sacrificing the light-output power in contrast to microcavity lasers.

Vertical-cavity surface-emitting lasers (VCSELs) are of increasing interest to the photonics community because of their surface-emitting structure, simple fabrication and packaging, wafer-level testability, and potential for low cost manufacture. Scaling VCSELs to higher power outputs requires increasing the device area, which leads to transverse mode control difficulties if devices become larger than about 5 microns. One approach to increasing the device size while maintaining a well controlled transverse mode profile is formation of coupled or phase-locked 2D arrays of VCSELs that are individually single-transverse mode. Such arrays have unique optical properties, not all of which are desirable. This paper covers some of the basic principles of these devices and reviews recent work on device designs, fabrication and operation. A technique for improving the far- field properties of the arrays is demonstrated and performance limitations are discussed.

Selectively excited photoluminescence in microcavity enhanced light-emitting diodes reveals that the coupling between the optical cavity mode and the quantum well exciton can be controlled by the exciting light itself. Optical pumping can also provide information on the internal electrical fields.

Colored-oxide glass filters containing CdS quantum dots were found to exhibit third-order optical nonlinearity in 1983. However, the dot sizes were somewhat large and the size distributions were wide. The sol-gel method proved to be a better alternative for CdS quantum dot materials. This paper describes the advantages of the sol-gel method, and how it has been exploited to prepare two types of CdS quantum dot materials. These are fully dense sodium borosilicate glasses and organically-modified silicates, Ormosils. Both materials give high values of third-order nonlinearity and excellent optical transparency.

Changes in the transmission of commercially available semiconductor doped glasses and porous silicon layers are studied by using picosecond pump and probe measurements. Bleaching bands attributed to the saturation of optical transitions in semiconductor nanostructures (crystallites or wires) are registered in time-resolved differential transmission spectra for both of the materials under investigation. It is found that porous silicon exhibits strong and fast optical nonlinearity (third-order nonlinear susceptibility is about 10-s esu; transmission recovery time is 30 - 40 ps) which can be used for optical switching.

A brief review of solid state and vacuum sources of submillimeter wave radiation. The physical parameters that effect the high frequency limits of these devices are identified.

A possible laser device is designed with the use of classically free quasibound electron states. An asymmetric semiconductor electron wave Fabry-Perot interference filter is designed with an upper electron state having much stronger confinement than the lower electron state. This structure also allows for direct current pumping of the upper state and rapid depletion of the lower state under the presence of a field. Spectroscopy experiments demonstrate the existence of the upper quasibound state in a test structure. This laser filter structure, designed for infrared gain with current pumping, is combined with a special injector filter for room temperature narrow energy current injection into the upper lasing state. A stack of 54 periods of this electrically pumped structure is placed within a waveguide geometry. A laser device is fabricated by etching mesa structures from 50 to 100 micrometers wide. End cleaved facets serve as reflectors for mesas from 2 to 5 mm long. Tests are performed on these devices to determine their electrical properties and suitability for lasing.

In 1928 Bloch proposed that electrons in a solid subject to an electric field will undergo periodic oscillations in the momentum and real space. These Bloch oscillations have generated considerable interest and controversy since the original proposal. Semiconductor superlattices provide an ideal system for investigating Bloch oscillations. We review recent observations of Bloch oscillations in semiconductor superlattices excited by ultrashort light pulses and observed through Four-Wave-Mixing techniques and the coherent sub-millimeter-wave radiation generated by these oscillations. Negative differential resistance has also been observed in electrically biased superlattices. We also briefly discuss the relation between these two kinds of experiments.

Recently, there is a great deal of interest in applications of intersubband transitions of quantum wells for optoelectronics. In this paper, we will focus on the application of the first order and second order susceptibilities. The use of the global to local transition in asymmetric quantum wells is shown to give rise a high Stark shift of intersubband absorption. A transfer matrix method for calculation of electronic states is used. Using AlGaAs/GaAs quantum wells, intersubband modulators based on this principle have been demonstrated. Both amplitude and phase modulators are possible. For nonlinear optics applications, we will describe another transfer matrix method for analysis of nonlinear optical processes. With these techniques, optimal designs of desired optical properties for quantum wells with arbitrary composition profiles can be performed and the required phase matching condition can be conveniently obtained.

Ultrashort pulses of electromagnetic radiation propagating through free space are used to perform coherent time-domain spectroscopy by probing the complex index of refraction of various materials, in particular thin films of high-critical- temperature superconductors and the microwave substrates that support them. The terahertz beam system utilizes Hertzian-dipole- like antennas consisting of a dc-biased photoconductive gap in a coplanar stripline as a transmitter, and an identical receiver with a photoconductive gap biased by the THz radiation. The transmitter is driven to produce the short radiation bursts by a 100-fs optical pulse from a Ti:sapphire self-mode-locked laser, while the receiver is synchronously gated by laser pulses split from the original beam. By performing measurements in the time domain and transforming data to the frequency domain, both the real and imaginary parts of the index of refraction of dielectrics and the conductivity of superconductors are determined over the entire range from approximately 200 GHz to several terahertz. This technique allows the direct broadband determination of these quantities in the mmw and sub-mmw regimes from the measurement of only a few time-domain waveforms and without the need for Kramers-Kronig analysis or complicated processing.

We have developed all-electronic integrated circuits that generate and detect picosecond pulses. We have used these circuits with integrated antennas in a system capable of free- space spectroscopy in the THz regime. With this system, we have measured magnitude and phase transmission characteristics for a variety of samples in the 200 GHz - 1 THz frequency range.

We describe the operation of a cw phase-locked doubly resonant optical parametric oscillator that is suitable as a tunable optical source for the generation of terahertz radiation. A highly stable cw optical parametric oscillator has been constructed with a tuning range of several THz in the signal- idler difference frequency. By phase locking this difference frequency to a microwave source, either directly or with the use of a terahertz optical frequency comb generator, the parametric oscillator serves as a precisely tunable difference-frequency source. In one experiment, we phase locked the parametric oscillator at a signal-idler difference frequency of 665 GHz. In a second experiment, we measured a signal-idler difference frequency of 1.45 THz with the use of a diode-pumped YAG laser.

Low-temperature-grown, non-stoichiometric GaAs is used as an optical mixer to generate coherent output radiation up to a frequency of 1.2 THz. The mixer structure consists of an epitaxial layer of the LTG GaAs material with submicron interdigitated electrodes fabricated on the top surface. Terahertz photocurrents are generated in the gaps between the electrodes and power is radiated by coupling these currents efficiently into a self-complementary spiral antenna. The experimental roll-off in photomixer output power is explained by two time constants - one for the electron-hole recombination time of 0.35 ps and the other for the photomixer-antenna RC time constant of 0.62 ps. The photomixer demonstrates the capability to generate continuous-wave radiation in a spectral region where tunable coherent radiation has been lacking.

Arsenides, such as GaAs and AlGaAs, contain a dispersion of metallic As clusters in a high-quality single crystal semiconductor matrix. These composite materials exhibit interesting and useful properties, including a large electro- optic effect, and a combination of properties that make it useful as a high-speed photoconductor.

Submillimeter source needs for the NASA astrophysics Submillimeter Intermediate Mission and the Earth Observing System Microwave Limb Sounder instrument are presented. Solid state local oscillators using planar devices are planned. State-of-the- art performance for these components is reviewed.

A discussion is given of current and projected needs for local oscillators for submillimeter and terahertz receivers used in astrophysical and astrochemical applications. Arguments are given for frequency range, tunability, power, and noise specifications for local oscillators in various applications. The receivers will generally employ SIS or Schottky diode mixers, and these may be used in single elements, receiver arrays or aperture synthesis interferometry, each of which has differing requirements. The platforms may be ground-based, probably on high mountain sites, balloons, air or space-based, again each demanding somewhat varying local oscillator capabilities.

Molecular spectroscopy was the earliest application in the terahertz spectral region and remains one of the most important. With the development of modern technology, spectroscopy has expanded beyond the laboratory and is the basis for a number of important remote sensing systems, especially in atmospheric science and studies of the interstellar medium. Concurrently, these spectroscopic applications have been one of the prime motivators for the development of terahertz technology. This paper will review these issues in the context of the requirements placed on future technology developments by spectroscopic applications.

Currently several weapons systems use millimeter wave, infrared or both for sensing. THz technology is spectrally located so as to be able to exploit the best features of MMW and infrared technology. The items for discussion are Army needs that could be addressed by THz technology. The emphasis is on active and passive sensing parameters for 'smart' munitions and combat vehicles.

The self-induced phase mismatches caused by nonuniform heating and heat dissipation and overall temperature rise in a nonlinear crystal reduce the efficiency and the beam qualities of nonlinear optical processes routinely used in uniaxial and biaxial crystals. As the crystal is heated, the phase-matching angle changes because of the dependence of refractive indices on temperature. A ray-optic measure of this effect is given by the acceptance temperature of the crystal. The induced thermal gradient leads to nonuniform, nonlinear conversion over the beams and to a nonuniform wavefront (phase) in the output beam. We have developed a model of the linear and nonlinear processes and a computational procedure for assessing the impact of these effects, including diffraction and depletion. The analytic method consists of the coupling of an optical beam propagation analysis with a themal analysis of the temperature gradients induced in the crystals. We examine three application examples: second- haromic generation in beta-barium-borate (BBO), three-wave mixing in lithium triborate (LBO), and SHG in LBO. Results have significance for experiment design in the development of high- average-power, frequency-agile laser systems. In laser beacon and illuminator applications, degradation in beam quality and conversion efficiency limit system scaling to increased average power. Our model can be used to quantify the design parameters for the input laser beam and the cooled crystal package for SHG or three-wave mixing.

Observed thermal lensing in AgGaSe_$2 and ZnGeP_$2 is compared with theoretical predictions of the thermal gradients and resulting laser beam aberrations. Accurate simulation of the thermo-optical behavior of these crystals is important for scaling high-average-power mid-IR 3-wave mixing applications.

We report the results of our efforts to increase the infrared output power from a cw difference frequency generation source based on the nonlinear material AgGaS₂ for use in infrared kinetic spectroscopy. Experimental and theoretical infrared conversion efficiencies are compared as a function of pump power, signal power, and focusing conditions. The predicted and observed IR yields agree well for the unfocused beam; however, the agreement is poor (experimental conversion a factor of 3 to 5 less than theory) for all focused conditions. The highest IR output powers obtained corresponded to experimental conditions that are slightly overfocused compared to the theoretical optimum. Thermal loading of the AgGaS₂ decreases the parametric conversion efficiency when the power in one of the input lasers beams exceeds approximately 800 mW. Infrared powers of 10 to 30 (mu) W have been achieved near 5.2 micrometers . Future improved crystals should yield powers at the AP 500-(mu) W level.

The suitability of III-V single-mode cw diode lasers for difference-frequency generation of tunable IR radiation has been explored by mixing a red single-mode diode laser with a tunable single-mode cw Ti:Al₂O₃ laser in AgGaS₂. More than 1 (mu) W of cw tunable, ((lambda) approximately equals 5 micrometers ), narrowband coherent radiation has been generated by using type I noncritical phasematching. The feasibility of a more compact, solid state cw laser source based on the mixing of two single-mode diode lasers (808 and 690 nm) as pump sources in AgGaS₂ has also been demonstrated (infrared power generated approximately equals 3 nW). Techniques to increase the infrared difference-frequency output power level such as the use of a high-power optical semiconductor amplifier or an external buildup cavity for the nonlinear mixing crystal have been investigated. As much as 47 (mu) W of cw infrared radiation and 89 (mu) W of pulsed infrared radiation, tunable around 4.3 micrometers have been generated by mixing the outputs of a high-power tapered semiconductor amplifier at 858 nm and a Ti:Al₂O_{3$ laser at 715 nm in AgGaS(subscript 2}. The GaAlAs tapered traveling-wave amplifier delivered up to 1.5 W of diffraction- limited cw power into the nonlinear crystal. Recent progress in generating cw infrared radiation near 3.2 micrometers by mixing the outputs of an extended cavity diode laser near 800 nm (pump wave) and a compact diode-pumped Nd:YAG laser at 1064 nm (signal wave) in AgGaS₂ with an external enhancement cavity to resonate the signal wave inside the nonlinear mixing crystal is also described.

We have developed and demonstrated a solid state wavelength converter that has potential for upgrading current military laser designator/rangefinders to include an eyesafe operating mode. To provide high repetition rate, compact design, and low weight in a unit that will perform under demanding environmental conditions, a KTP OPO was developed that can be pumped by the multi-axial and transverse-mode Nd:YAG lasers. Two resonator configurations were tested: forward and reverse coupling geometries. Neither configuration used 1.06-micrometers feedback, which eliminated any feedback problems into the pump resonator yet achieved GRT25% conversion. An increased margin of eyesafety was realized in the reverse coupled OPO by dumping the residual pump. Sensitivity to the pump's polarization was found to be 10% greater for hydrothermally grown crystals while flux grown crystals exhibited higher conversion efficiency. The beam quality of the converted signal tracked with the pump and scaled with the ratio of the wavelengths. OPO output coupling and resonator length were optimized for conversion efficiency. The final configuration using a 20 x 5 x 5 mm crystal was 3 to 4 inches in length, used a 50% signal reflector, and achieved greater than 35mJ output with a beam quality of 8.5 mm*mrads. Testing is currently underway to complete environmental characterization.

A number of applications, including long-range remote sensing and antisensor technology, require high-average-power tunable radiation in several distinct spectral regions. Of the many issues which determine the deployability of optical parametric oscillators and related systems, efficiency and simplicity are among the most important. It is only recently that the advent of compact diode laser pumped solid state lasers has produced pump sources for parametric oscillators which can make compact, efficient, high-average-power tunable sources possible. In this paper we outline several different issues in parametric oscillator and pump laser development which are currently under study at Lawrence Livermore National Laboratory.

We are developing the diffusion-bonded stacked (DBS) structure for quasi-phasematched interactions to meet the need for high- power nonlinear conversions in the infrared. In our preliminary investigations, we have compared optical and thermal properties of some potential DBS materials. Theoretical projections of device performance were compared for DBS GaAs and ZnSe and birefringent crystals ZnGeP₂ and AgGaSe₂ for both second- harmonic generation (SHG) of 10-micrometers radiation and 2-micrometers pumped optical parametric oscillators (OPOs). We are refining bonding processes for GaAs and have initial diffusion bonding results for ZnSe. We have fabricated and tested DBS GaAs structures for SHG, demonstrating that the crystal orientation is conserved during the bonding process, and that the nonlinear generation of the individual layers sums coherently. These studies indicate that DBS materials have potential for application in high-average-power OPOs.

Thermal dephasing due to ultraviolet pump absorption in beta- barium borate and lithium triborate crystals has been investigated. Interferometric measurement of optical path difference under thermal loading provides a method of estimating the average-power-scaling potential of these crystals.

For application of optical parametric oscillator (OPO) to investigation of nonlinear interaction of laser radiation with matter the factor of importance is stability of light spatial distribution and spotsize position on a target through a tuning range. Collinear temperature tunable schemes show an advantage for these purposes, in particular, for small pump beam diameters. A simple and efficient visible range parametric converter can be realized using two-pass configuration, where parametric luminescence is excited on the first pass through a nonlinear crystal and amplified on the second pass after spatial filtering. Lack of resonator simplifies high power UV pumping and getting of relatively narrow emission spectrum. Using such an approach, we have made the oscillator-amplifier system temperature tunable in the range of 440 to 670 nm employing 4-cm-length ADP crystal pumped by 266 nm radiation from the single-mode YAG:Nd laser. The output energy of 3 mJ in about 1-ns pulsewidth has been achieved with total conversion efficiency of 10%. A spatial profile of the output beam kept its shape within the branch of the tuning curve. This allowed us to use the device as a proper tool for investigation of two-photon excitation in undoped CsI and KI single crystals. The OPO signal output was used to record photoconductivity spectra in these materials.

The aim of present work is, first, a global investigation of fundamental nonlinear optical processes in optical fibers, determined not only by an imaginary, but the real component of cubic nonlinear susceptibility in an optical fiber core material. Secondly, a search of the preferential regimes of the most effective light energy transformations in optical fibers, including an investigation of the spectral, temporal, and spatial characteristics of the high intensity picosecond light pulses. Finally, the creation of a new effective optical device which uses the results of these investigations is discussed.

The optical mixing of sufficiently coherent radiations of frequency (omega) ₁ and (omega) ₂ inside a suitable molecular medium can result coherent radiation at new frequency (omega) ₃ equals [(omega) ₁ + ((omega) ₁ - (omega) ₂)]. If (omega) ₁ is kept fixed and (omega) ₂ is varied so that a condition (omega) ₁ - (omega) ₂ equals (omega) is reached where (omega) is a molecular frequency of the medium, then (omega) ₃ equals (omega) ₁ + (omega) equals 2(omega) ₁ - (omega) ₂. In this case (omega) ₃ coincides with the anti- Stokes Raman frequency associated with the molecular frequency (omega) . This process of coherent anti-Stokes Raman Scattering (CARS) is an important nonlinear optical process which provides intense collimated signal beams and is also an excellent spectroscopic tool. CARS being a four-wave mixing process, it is of interest to study the process in multi-level system to obtain a better insight into the problem. In the current paper we investigate the different quantum statistical properties of CARS in a inhomogeneously broadened three-level system based on density-matrix formalism where both the atomic system and the radiation fields are quantised. The photon-statistics, coherence characteristics and the occurrence of anti-bunching of the CARS field, in the steady state, are investigated.

In this paper, we report on the design and fabrication of a wavelength demultiplexer integrated with photodetector array in InGaAs/AlGaAs/GaAs structure. The demultiplexer is based on the Rowland circle grating. InGaAs MSM detectors grown on the top of an GaAs/AlGaAs waveguide structure are used for light detection. Only one step epitaxial growth is used. The cross-talk between adjacent channels measured from the detectors response is about -11 dB. The device can provide 38 output channels operating in the 1-micrometers wavelength region. The structure has the potential to be integrated with GaAs transistors without any complication in the fabrication.