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The seven papers making up this assessment are based on the Workshop on Nonlinear Optical Materials held in April 1986.

Measurement of optical absorption loss and photorefractive sensitivity (optically induced index changes) in Ti-diffused waveguides and bulk Ti-doped single crystals are reported. The absorption loss of strip guides is found to be 0.3 dB/cm at 0.633 microm, and this is expected to decrease at longer wavelengths. The results demonstrate that Ti impurities do not contribute to the absorption loss or to the photorefractive-index changes directly, rather these parameters are determined by impurities in the substrate crystal. A simple pyroelectric method for electrical adjustment of the optical input coupling is also reported.

Subnanosecond optical pulses (as short as 7 psec) have been generated over the 0.58-0.80-microm wavelength range with good beam quality, energies in excess of 1 microJ/pulse (up to 20 microJ/pulse), and repetition rates of at least 10 Hz using a flashlamp-pumped dye laser. The experimental details concerned with mode locking the dye laser and the selection of single pulses from the pulse train with an energy reproducibility of 10% are described.

We demonstrate a compact real-time optical image correlator using diode lasers and a semi-insulating GaAs/AlGaAs multiple-quantum-well (SI-MQW) device as the holographic element. With only 3 mW of power incident upon the SI-MQW device, the correlation is obtained in 1 microsec and can be erased in as fast as 2 microsec, which presents the possibility of a system capable of 3 x 10 exp 5 correlations/s. We also show that images can be stored for a controllable length of time (2-25 microsec), which presents possibilities for the use of the SI-MQW device as a data or image buffer memory.

Single-pulse and cw measurements of the response of a semi-insulating CdZnTe/ZnTe multiple-quantum-well photorefractive device are presented. In single-pulse experiments, photodiffractive (absorption) gratings have been written with less than 1.8-microJ/cm(2) incident fluence, and a diffraction efficiency of 1.1% is obtained from the 1.56-microm active layer of the device. With an optimized structure, the ultimate response time of the device can be below 100 ps. In cw measurements a maximum diffraction efficiency of 1.35% is obtained.

We have performed photodiffractive four-wave mixing in semi-insulating multiple GaAs/AlGaAs quantum wells at a wavelength of 0.83 microm. The quantum wells were made semi-insulating by proton implantation, which introduces defects that are available to trap and store charge during holographic recording. The experiments demonstrate how photodiffractive behavior using the large resonant nonlinearities of quantum-confined excitons yields highly sensitive material for optical image processing. When pump powers of 1 mW/cm(2) are used, the measured sensitivity is 2 orders of magnitude greater than that of bulk, nonresonant photorefractive semiconductors.

Order-of-magnitude improvement of the quantum efficiency of metal-oxide-metal detectors is achieved in the visible spectrum by fabricating the devices on silica substrates structured with a periodic array of bumps separated by about 0.25 microm. Structuring metallic films affords improved coupling of the detector to visible light by excitation of localized plasma resonances and surface-plasmon polariton modes. Voltage responsivities of 5 mV/W into a 50-Omega impedance has been demonstrated in preliminary devices. In suitable geometries, such detectors have the potential of subpicosecond response times.

The optical transmission spectrum of a discontinuous metal-island film is strongly affected by extremely thin layers (

Absorption, luminescence, and excitation luminescence studies of dye molecules adsorbed onto ultrathin, variable-thickness silver films show strong coupling of the particle plasma resonances to the molecules. Luminescence from the dye is increased by excitation transfer from the silver particles for certain film thickness. Increased luminescence has also been observed from dyes on Au and Cu films.

Not long ago photonics affected our lives only in the display interface between users and machines. Now photonics is spreading into areas that were the traditional domain of electronics: optical communications, optical memory, and optical interconnection. The trend is likely to continue, with photonics playing a major role in information switching and processing. This progress has been made possible by developing photonic materials with the required purity and physical and optical properties: glass fibers for transmission, semiconductors for lasers and detectors, and nonlinear optical materials for switching and processing. Future progress will continue to depend on improved materials and structures and, most of all, on high-yield, low-cost processing materials.

Current oscillations arising from voltage‐controlled negative differential resistivity are observed in optically pumped semi‐insulating InP at low temperature. The presence of high‐field domains sweeping through the sample are detected using photorefractive four‐wave mixing. Direct evidence of the role of defects in these current oscillations is presented using deep‐level photodiffractive spectroscopy. A deep hole trap with an energy Ev +85 meV is observed. The current oscillations occur only when this defect level is partially occupied.

Strong permanent gratings (index change Δn∼3×10−3) have been formed in bulk hydrogen treated germano‐silicate glass with 242 nm exposure. Diffraction efficiencies of 6% and initial sensitivity (Δn/incident energy density) of 2×10−5 cm2/J have been obtained. The observed sensitivity is comparable to the values achieved in other holographic storage materials such as LiNbO3. It is estimated that by more uniform hydrogen loading of the sample, diffraction efficiencies of 100% can be obtained in 3 mol % GeO2 samples of only 175 μm thickness. These materials are potentially useful as fast access time (sub‐μs), high capacity (∼0.8 Tbits), robust, inexpensive write‐one‐read‐many storage media, narrow (subangstrom) linewidth optical filters, and other optical components.

We report on the effect of carrier escape time on the performance of semi‐insulating photorefractive self‐electro‐optic effect devices by investigating three samples of Cr‐doped GaAs/AlxGa1−xAs multiple quantum wells of varying barrier thickness and height. Reduction of barrier thickness from 100 to 35 A and Al fraction from 0.42 to 0.29 results in a three orders of magnitude increase in diffraction efficiency at a given voltage. The effect of shorter carrier escape and sweep‐out times on the diffraction efficiency, resolution, and sensitivity of these devices is discussed.

Semi‐insulating multiple quantum well photorefractive devices using GaAs/Al0.29Ga0.71As with an electric field applied perpendicular to the layers are demonstrated. Semi‐insulating behavior is obtained by doping with Cr(1016/cm3) during epitaxial growth of the material. Diffraction efficiencies as high as 3% with an applied voltage of 20 V and microsecond response times are obtained in a 2 μm thick device. These devices are of importance for implementation of fast and sensitive two‐dimensional optical information processing systems at wavelengths compatible with current diode lasers without the spatial‐bandwidth limitations of thick photorefractive materials.

We present results on the operation of a high sensitivity semi‐insulating multiple quantum well device for optical image processing. This device operates in the visible spectrum using II‐VI CdZnTe/ZnTe multiple quantum well structures. Incident light creates charge carriers that screen an applied ac electric field modulating the absorption and refractive index of the structure through the quantum confined Stark effect. In this way, an incident intensity pattern is recorded as an absorption and refractive index variation. The semi‐insulating nature of the material eliminates the need for pixelation. In a wave‐mixing experiment, a peak diffraction efficiency of 0.25% was observed from 2.25 μm active layer of the device. Two‐beam‐coupling gain coefficients of ∼500 cm−1 at wavelengths longer than the exciton absorption peak should be possible.

Optically pumped lasing has been observed up to 100 K in a Cd 0.2 Zn 0.8 Te ZnTe superlattice grown on a GaAs Si substrate. At 100 K, rapid degradation of the lasing signal is seen. This result is inferior to that previously reported for a similar structure grown on a GaAs bulk substrate, for which room-temperature lasing was seen with no degradation. X-ray rocking curve and morphology measurements suggest that the II–VI epilayer has limited ability to heal the extra defects that are present when a GaAs Si , rather than a bulk GaAs, substrate is used.

We report the first study of the room‐temperature electroabsorption effects in CdZnTe/ZnTe multiple quantum well structures which exhibit sharp excitonic absorption peaks. The magnitude of the Franz Keldysh and quantum‐confined Stark Effects are found to be comparable to those of III‐V semiconductors. With optimized structures we expect II‐VI semiconductors to be important components for information processing in the visible spectrum.

We report photorefractive two beam coupling gain coefficients as high as Γ=19 cm−1 and net gains of Γ‐α=14 cm−1 in InP:Fe at around 970 nm. This enhancement was achieved by combining band‐edge resonant nonlinearities with a second resonant enhancement derived from the bipolar transport in this material. Measurements of gain as a function of pump intensity, applied field, grating spacing, wavelength, and temperature are presented. No moving gratings were required.

Defect levels in semi‐insulating GaAs and InP are investigated by deep level photodiffractive spectroscopy. Temperature‐dependent four‐wave mixing based both on photochromic and photorefractive effects permits a determination of the energy levels within the semiconductor band gap. This optical technique eliminates difficulties encountered with high‐resistivity material using conventional electrical measurements.

Semiconductors have been investigated for their photorefractive properties for some time [1]. Most of theseinvestigations have applied methods previously used in photorefractive oxides to the semiconductors. However,semiconductors possess additional properties that can be taken advantage of. It has been known for some time [2J, thatapplication of an electric field to a semiconductor will change the absorption coefficient and refractive index of thematerial near its band-edge due to the Franz-Keldysh effect. We have recently shown that strong photorefractivegratings can be formed in bulk semiconductors by taking advantage of these band-edge electro-optic nonlinearities [3].Semiconductor photorefractives are now capable of achieving nonlinearities comparable to photorefractive oxides such

Volume holographic gratings are written with ultraviolet light in high-optical-quality, commercially available Ge-doped silica films and in Ge-doped optical-fiber preform sections loaded with molecular hydrogen. In the film samples, peak refractive-index changes exceeding 10(-2) and a sensitivity (index change/absorbed energy density) of 0.4 × 10(-7) cm(3)/J are measured. Angular multiplexing of up to 51 gratings is demonstrated in the preform samples.

We use semi-insulating multiple quantum wells to combine the holographic properties of the photorefractive effect with the large resonant optical nonlinearities of quantum-confined excitons. GaAs–AlGaAs multiple-quantum-well structures are made semi-insulating by proton implantation. The implant damage produces defects that are available to trap and store charge during transient holographic recording by means of coherent excitation. The advantages of charge storage and resonant optical nonlinearity combine to produce new optical devices with large sensitivities. The potential use of these devices for image processing is demonstrated by using the Franz–Keldysh effect in four-wave mixing at wavelengths near 830 nm.

The nuclear positions in a single crystal of Ba(NO/sub 2/)/sub 2/xH /sub 2/O at 20, 102, and 298 K have been measured by neutron diffraction, and the change in spontaneous polarization between 20 and 300 K has been continuously determined. A polarization change of -30 x 10/sup -4/ Cm/sup -2/ between 102 and 298 K is calculated from the nuclear positions, based upon an electric dipole moment of 6.2 x 10/sup -30/ C m for H/sub 2/O and 6 x 10/sup -30/ C m for NO/sub 2/ /sup -/ and the charge on NO/sub 2/ /sup -/ as midway between the oxygen atoms. Integral ionic charges are assumed. The measured polarization change between 102 and 298 K is -26 x 10/sup -4/ C m/sup -2/. The calculated polarization change between 20 and 102 K is -5 x 10/sup -4/ C m/sup -2/; the experimental value is +3 x 10/sup -4/ C m/sup -2/. The error in the calculated spontaneous polarization is estimated at 10 x 10/sup -4/ C m/sup -2/.

GaAs hereroepitaxial films on Si contain a high number of misfit dislocations and large internal stresses due to the lattice and thermal mismatch of the two materials. These greatly affect the structural, optical, and electrical properties of the films. We report a study of these effects in films grown by metalorganic chemical vapor deposition using x‐ray diffraction, sample curvature, photoluminescence, and carrier concentration measurements. The x‐ray data indicate that the lattice misfit strain is almost entirely relieved by the generation of dislocations, but that the difference in thermal expansion between the film and substrate causes significant tetragonal distortion of the GaAs lattice which results in wafer bowing and, for thicker GaAs layers, film cracking. Wafer bowing was successfully eliminated by growth of GaAs films on both sides of the Si substrate. Photoluminescence spectra of crack‐free GaAs layers indicated that the thermally induced strain was distributed in a nonuniform but continuous...

Optical storage of information offers great potential for high capacity and speed. A very promising approach to the embodiment of an optical memory is based on volume storage in the form of phase holograms. Attractive storage materials for such a system are electrooptic crystals. The storage mechanism in these materials is based on light induced permanent changes of the refractive index-the photorefractive effect. In this article the physical processes underlying photorefractive hologram recording are outlined, and some of the advantages and limitations of this method are discussed.

Optical absorption in pyroelectric crystals is accompanied by electrical effects (in addition to the well-known pyroelectric effect) which are not present in other materials. Optical excitation between localized states gives rise to an instantaneous macroscopic polarization change due to the change of dipole moment at the absorbing center. Excitation of free carriers from localized states will in general result in a bulk photovoltaic effect due to asymmetric charge transfer. Thus spatially non-uniform illumination gives rise to internal fields, resulting in refractive index variations. Studies of these effects for fast detection, optical logic, memories, and microwave generation will be described.

The electronic transport mechanisms in pyroelectrics which give rise to light induced polarization changes are examined with a view to maximizing the sensitivity of materials for optical storage. Experiments demonstrating diffractive scattering due to localized excitation of Cr3 + ions in LiNbO3 are reported.

Liquid encapsulated Czochralski growth of low etch pit density, low optical absorption InP single crystals has been achieved by double doping with sulfur (donor) and zinc (acceptor) impurities. The etch pit density is controlled by the total impurity content NA + ND while the free carrier concentration, which determines the optical absorption, is controlled by the difference ND − NA of the active sulfur and zinc concentrations. Crystals having etch pit densities as low as 100 cm-2 and optical absorption as low as 1 cm-1 at 1.3 μm are reported.

Direct photoelectrochemical (PEC) etching of diffraction gratings on n‐InP and n‐GaInAsP in a 2‐M HF/0.5‐M KOH solution has been demonstrated using laser interference holography. Development of a maskless technique for producing gratings has potential application in the fabrication of distributed feedback lasers which are currently made by a multistep photoresist process. Submicron diffraction gratings having a period of 0.5 μm, corresponding to second‐order feedback in GaInAsP at λ=1.55 μm, have been achieved. Measurements were obtained on the exposure characteristics, diffraction efficiency, and PEC etching sensitivity of gratings produced in InP and GaInAsP as a function of the writing beam intensity, laser wavelength, material doping level, and grating spatial frequency. For grating frequencies greater than 100 mm−1 the sensitivity was observed to decrease approximately as the inverse square of the spatial frequency. In addition, undoped InP and GaInAsP exhibited significantly lower sensitivities than...

Multiphoton excitation allows reversible photorefractive recording without the disadvantage of erasure during readout. The light intensity required for two‐photon storage can be drastically reduced if two‐step excitation via long†lived real intermediate states is used. Experimental results for LiNbO3:Cr3+ and LiTaO3:Cr3+ demonstrate the advantages of the two‐step process. The energy and intensity requirements for optical two‐step recording are within reach of presently available repetitively pulsed lasers.

A wide glass-forming region has been observed in the Li 2 SO 4 -La 2 (SO 4 ) 3 system with essentially pure glass produced by roller quenching compositions of 20 to 90% Li. Even water or air quenching produced glasses. Many of these glasses showed double exotherms during crystallization and some showed glass transitions. Limited experiments also showed some glass-formation in other alkali-La sulfates, in compositions containing Y and Sc sulfates and in mixtures. Ionic conductivities and some infrared measurements are also reported.

Alkali niobate and tantalate glasses have been prepared by rapid quenching. These glasses exhibit room‐temperature ionic conductivities as high as 10−5 (Ω cm)−1 and electronic conductivities less than 10−11(Ω cm)−1. In the case of LiNbO3, the glass conductivity is many orders of magnitude greater than that of the single crystal. These conductivities are sufficiently high for rapidly quenched vitreous oxides to be considered for applications as solid electrolytes.

Amorphous silicon films have previously been utilized to make high speed photoconductive detectors with response times ranging from 4–200 psec in microstrip transmission line structures. However, in typical devices, the low carrier mobilities resulted in poor carrier collection efficiencies resulting in quantum efficiencies in the range 0.001–0.3%. We have demonstrated photogenerated carrier sweep-out in Schottky barrier photodetectors with internal quantum efficiencies of 36% (0.18 A/W) and sampling oscilloscope limited response times of 40 psec (FWHM) using 100 nm-thick films of rf-glow discharge deposited a-Si:H in a new transmission line structure. The estimated deconvolved response time was 27 psec. These a-Si:H detectors are compared in speed and sensitivity to high mobility crystalline detectors.

Ferroelectricity has been discovered in calcium vanadate. Large, transparent crystals have been grown by Czochralski pulling. Electrical 180° domains of millimeter dimensions are observed by etching as-grown crystals. The ferroelectric Curie temperature, Tc = 1383 K, has been determined from dielectric, thermal and pyroelectric studies.

The problem of hologram degradation during read-out that exists for photorefractive recording via single photon electronic excitations can be eliminated by using multiphoton absorption. Different two-photon excitation schemes are discussed, and the feasibility of this method is demonstrated experimentally in doped and pure LiNbO3 and KTN.

The nonequilibrium occupation of multiple defect states in inhomogeneously illuminated crystals is demonstrated to have dramatic and previously unexpected consequences for the photorefractive behavior of electro-optic semiconductors. By using a simple screening formalism, we show for the first time that the standard photorefractive effect can be partially or entirely quenched. This nonequilibrium screening is observed in the thermally stimulated relaxation of diffraction efficiencies in four-wave mixing experiments performed on semi-insulating InP

The temperature dependence of the lifetime of the excited 2T2 state of the Cu2+ ion in a LiTaO3 crystalline host has been measured with picosecond optical pulses. Its value ranges from 450 psec at 22°K to 10 psec at 423°K. Consistent with this result is the observation of weak fluorescence at 1.75 μm having a low temperature quantum efficiency of ∼ 10−6. The experimental results are in reasonably good agreement with recent theoretical models based on strong coupling of the Cu2+ impurities to the host lattice.

The first study of the photorefractive properties of doped CdTe has demonstrated high sensitivity for optical processing applications. Of the binary II‐VI and III‐V semiconductors, CdTe has the highest electro‐optic coefficient r41 in the infrared, some three times larger than that of GaAs and InP. Deep levels introduced into CdTe exhibit appropriate absorption and photoconductivity at 1.06 μm by doping with V and Ti impurities. Photorefractive beam coupling experiments in CdTe:V gave small signal gains of 0.7 cm−1, and diffraction efficiencies with no applied electrical field of 0.7%. Thus, CdTe appears to be superior to previously studied III‐V semiconductors, in the near‐infrared spectrum. Optimization of doping and trap densities is expected to result in gain which exceeds the absorption loss, thereby allowing phase conjugation with infrared injection lasers.

Birefringence changes are induced in a cubic potassium tantalate niobate crystal with an external electric field by two−photon absorption of picosecond optical pulses. The recording sensitivity of 100 μJ/cm2 for elementary holograms with a few percent reconstruction efficiency is comparable to the sensitivity of holographic silver halide emulsions. The high sensitivity of KTN is shown to be due to efficient charge transport of photoexcited carriers.

Making use of the photorefractive effect, we have developed a versatile method of imaging various crystal properties of semi‐insulating compound semiconductors. The magnitude and time evolution of refractive index gratings are monitored via diffraction. The observed diffraction is directly related to the electric fields present, and quantitative information concerning the spatial variation of dark conductivity, photoconductivity, and deep level absorption can be extracted. Wafers of undoped GaAs and InP:Fe have been characterized in this manner, and comparisons of images are made which demonstrate the capabilities of this technique.

Optically pumped lasers emitting near 600 nm at room temperature have been fabricated for the first time in Cd0.25Zn0.75Te/ZnTe superlattices grown on GaAs substrates. The threshold pump intensity using pulsed 0.53 μm radiation increased from ∼7 kW/cm at 10 K to ∼60 kW/cm2 at room temperature with a threshold temperature dependence described by T0∼111 K.

We report here for the first time the demonstration of self‐pumped phase conjugation in photorefractive semiconductors using 1.32 μm light. Using an ac field technique to enhance the gain coefficient in InP:Fe and a single input pump beam, phase conjugate reflectivities of 11% were measured using an input beam intensity of less than 1 mW/mm2. These results open up many possibilities for using photorefractive semiconductors in applications with low‐power infrared diode lasers.

An asymmetric δ‐doped GaAs structure is described which exhibits novel photovoltaic effects and low‐intensity nonlinear optics. Of particular interest in this kind of structure is the ability to design the electro‐optical and nonlinear optical properties and the material response time over a wide range by appropriate design of the doping profile.