We review the current status of the photorefractive tungsten bronze ferroelectric crystals in terms of their electro-optic character and applications, with special emphasis on the current results for doped SBN:60 crystals. New results pertaining to phase conjugation and double phase conjugation (Bridge Conjugator) and the effects of internal fields on beam fanning are discussed.

Cr-doped strontium barium niobate has shown significant reduction in the time of response compared to previously grown Ce-doped crystals, with room temperature response times as short as 0.2 sec. The experimental photorefractive two-beam coupling gain and response time of 1% and 1.6% Cr-doped SBN:60 and 1% Cr-doped SBN:75 will be presented and compared to results in Ce-doped SBN:60. The photorefractive effect in Cr-doped SBN:60 has also shown a strong temperature dependence, with gain increasing by a factor of two when the crystal was cooled from 40 to -20° C. Significant gain enhancement was also predicted and obtained by applying a DC electric field of up to 10 kV/cm.

We characterize the holographic storage characteristics of photorefractive SBN:60 (Sr_0.6Ba_0.4Nb₂O₆) under an externally applied electric field. The field dependence of the recording response time and sensitivity, gain coefficient, steady-state diffraction efficiency, and erasing response time were measured. Kukhtarev's band transport model is shown to predict the asymmetric erase time / write time behavior of SBN:60. Using these results, we estimate as a function of applied field the number of equal diffraction efficiency holograms which can be superimposed in the crystal.

We have demonstrated self-pumped phase conjugation of semiconductor lasers at 830 nanometers in barium titanate using a self-contained geometry. The reflectivities and response times of this geometry are compared to those reported for self-pumped ring passive phase conjugate mirrors.

The stabilization of volume phase holograms against erasure during read-out is of fundamental importance for the storage of volume phase holograms. There are several proposals to avoid the erasure of a hologram when reading it. In this paper we shall especially be concerned with the method of frequency-difference holograms. The basic idea is to record a hologram of the same object with two different frequencies and to read out with the difference of the frequencies. First we shall discuss light diffraction by frequency-difference holograms in terms of virtual holograms not physically present in the crystal. Since the efficiencies of the two-step processes involved turn out to be much too small compared to experimental data a refined mechanism which allows the formation of real frequency-difference holograms is outlined. The theory is based on off-Bragg terms in Maxwell's equations and is able to explain the order of magnitude of the experimental data.

We describe a sensitive technique for measuring nonlinear refraction in a variety of materials that offers simplicity, sensitivity and speed. The transmittance of a sample is measured through a finite aperture in the far-field as the sample is moved along the propagation path (z) of a focused Gaussian beam. The sign and magnitude of the nonlinearity is easily deduced from such a transmittance curve (Z-scan). Employing this technique a sensitivity of better than λ/300 wavefront distortion is achieved in n₂ measurements of BaF₂ using picosecond frequency doubled Nd:YAG laser pulses.

We report the growth of doped Potassium Tantalate Niobate (KTN) crystals, and the characterization of their photorefractive properties L. the paraelectric region. First the Top Seeded Solution Growth Method is reviewed and the growth process of a KTN:Cu,V crystal is described. Results of diffraction efficiency measurements of photorefractive gratings in these crystals at the paraelectric phase, are then presented. These experiments show high diffractio- efficiencies, and indicate the possibility of amplitude modulation of the gratings by an external field. Results showing fixation of the gratings when the sample is close to the phase transition temperature are also described.

The internal polarization characteristics of photoferroelectric barium titanate (BaTiO3) monocrystal have been investigated in detail. The polarization state in the crystal has been induced by the simultaneous application of a d.c. field and an illumination of an appropriate radiation for a known polarizing time. It has been found that the persistence and the magnitude of polarization depend on the polarization conditions. Therefore, the crystal has been polarized by varying the polarizing field, polarizing intensity, polarizing duration, and wavelengths of illumination. Soon after the polarization was terminated, the photodepolarization current decay characteristics of the polarized sample have been studied by reilluminating the crystal. The depolarization current is found to decay fast in the initial few minutes, but the decay rate becomes slower thereafter. This results in the retention of a volume polarization in the crystal, and depending on the polarization conditions, the current can persist for several hours to weeks. The photodielectric properties of the crystal have been also studied. The characteristics of the polarized BaTiO3 crystal show that they can be utilized successfully in volume holographic recording, beam coupling devices with added gain, and in image storing and retrieval devices.

A theoretical investigation of four-wave mixing in a photorefractive oscillator in the limit of one strong probe beam has been performed. The perturbational approach is compared with an exact numerical calculation.

The pulsed plasma deposition technique described in Part 1¹ and Part 2² of this series has been used to deposit layered structures of silicon dioxide ( SiO₂) and germanium sulphide (GeS_x), and layered structures of silicon dioxide and germanium selenide (GeSex). Layer thicknesses down to 25 Å have been deposited, and by analysing the layers using Auger sputter profiling and infrared absorption, it has been demonstrated that the interface between the deposited layers is sharp and distinct. The deposited films have a band gap that is dependent both on the composition of the chalcogenide deposited and the size of the chalcogenide layer. The band gap shifts to higher energy with higher selenium or sulphur concentration in the film and with reduced layer thickness. The shift of the band gap with layer thickness is consistent with a simple quantum-well model. Preliminary results are presented which show the films appear to have high non-linear optical coefficients near the band edge, which leads to induced absorption at high laser intensities.

HgTe, HgMnTe, and zero-gap HgCdTe are shown to have record, picosecond speed, optical nonlinearities at 10.6u. The largest occurs in Hg_0.84Cd _o.16T-e, whose X⁽³⁾=2x10^-3 esu at 80K. A theoretical model suggests that these nonlinearities are caused by laser-induced carrier temperature modulation, that produces large carrier density variations in zero-gap materials. The thermal processes have saturation power densities in the 100kW/cm² - 1MW/cm² range. At such intensities, the dielectric constant of HgTe is modulated by about 10%.

In recent years, with the advent of MBE and MOCVD techniques, there has been considerable interest in studying the physical properties of 2D electron gases in ultra-thin films and inversion layers of semiconductors due to their importance in devifle technology. In the presence of a quantizing magnetic field along z-direction, the free 2D electron motion is also quantized forming Landau levels an&l leading to diamagnetism. Moreover, the twining motion of the electroas generates paramagnetism due to spin-splitting of the Landau levels. We wish to note that considerable efforts have been made in the literature to study the magnetic susceptibility in degenerate semiconductors under different physical conditions. Keeping this in view, an attempt is made for the first time, to study the dia and para magnetic susceptibilities of the 2D electrons under strong. magnetic quantization in ternary chalcopyrite semiconductors which have been widely used as nonlinear optical materialg.

Detailed measurements of the free exciton dephasing time were performed in high quality CdS platelets at different detunings. A large asymmetry in the detuning dependance of the exciton damping is measured and is explained in terms of the intrinsic properties of exciton-polaritons. Below the transverse A exciton frequency, and at temperatures between 2 and 80 K, the damping is extrinsic in nature and is due to impurities. Above the transverse A exciton frequency, and for temperatures below 80 K, the damping is due to acoustical phonons. Above 80 K, LO-phonon scattering sets in. Nonlinear optical transmission measurements at low intensity (~5kW/cm²) below the A exciton resonance demonstrate the importance of self-broadening of exciton-polaritons.

All-optical modulation in a GaAs/AlGaAs multiple quantum well nonlinear directional coupler is observed at room temperature using femtosecond pulses. The ultrafast (< 1 ps) response and recovery of the device is attributed to the optical Stark effect. All-optical switching in various nonlinear optical devices is of interest because of its possible application in high-speed photonic switches. In semiconductor devices, which exploit the relatively large optical nonlinearities near a semiconductor bandedge, the source of the nonlinearity is typically the presence of free carriers photoexcited by a temporally short pump (i.e. control) beam. Previously we have demonstrated all-optical switching both in a GaAs/AlGaAs multiple quantum well (MQW) nonlinear etalon, as well as in a MQW nonlinear directional coupler (NLDC) using real carrier excitations to generate the nonlinearities. In both cases the device has a response which follows the pulse, and a recovery time which was limited to ≈10ns by the recombination and the diffusion of the photo-generated carriers. Recently, the operation of a nonlinear etalon with a subpicosecond recovery time was demonstrated. This was accomplished by using femtosecond pulses tuned in the semiconductor's transparency region to generate instantaneous nonlinearities which were present only while the pump travelled through the etalon. In this case the source of the optical nonlinearity was the now well known optical Stark effect. Here we report the first observation of ultrafast switching, including < 1ps recovery time, in a GaAs/AlGaAs NLDC. We believe the optically-induced temporally-short index change responsible for this fast switching is generated by the optical Stark effect.

We investigate the exciton ionization process in InGaAs quantum wells using 100 fs time resolved spectroscopy. The ionization time is studied as a function of temperature. We find a room temperature ionization time of ≈200 fs and longer times at lower temperatures. At room temperature the ionization is dominated by collisions with LO phonons, whereas at lower temperatures another, extrinsic mechanism becomes increasingly important.

This talk discussed application of the electron/hole model of the photorefractive effect to observations in GaAs:EL2, InP:Fe, and CdTe:V that were obtained with cw and picosecond lasers. In this model both electrons and holes are photoionized from a single defect level with two ionization states (e.g., EL2⁰ and EL2⁺) as discussed by Valley (1986) and Strohkendl et al. (1986). In photorefractive semicon-ductors where the diffusion length is much larger than typical grating periods, it can be shown that the gain coefficient for photorefractive beam coupling depends primarily on two properties of the deep level defect, an effective trap density N_E = N₁N₂/(N₁ + N₂) and a competition factor ξ = (a_e - a_h)/(a_e + a_h) where N₁ and N₂ are the densities of the two levels while ae and ah are the absorption coefficients for production of electrons and holes. Other properties of the deep level defect, such as the recombination cross sections, can be obtained from corrections to the gain formulas for large grating periods and for low irradiances (Valley et al., 1988).

Low power nonlinear optical materials are needed for devices such as all-optical spatial light modulators and optical wave mixers (devices which couple energy between two or more optical beams). In particular, operating intensities of a few milliwatts/cm² are desirable. It is not essential that the response times of the materials be extremely short. By taking advantage of the inherent parallelism of optics, significant information processing can be achieved even if response times are of the order of microseconds. With these requirements in mind, we are studying charge transport enhanced optical nonlinearities in semiconductors. A bonus of this approach is that these materials are compatible with existing semiconductor technologies.

Excitonic effects in multiquantum wells and the associated non-linearities can be exploited to realize a new class of devices for electro-optic and photonic switching applications. Methods to tailor the excitonic behavior and the operating characteristics of these devices are described.

Most analyses and simulations of all-optical guided-wave devices are based on Kerr nonlinearities. In practice, most media exhibit index saturation and reduced absorption with increasing intensity in wavelength regions where their nonlinearity is usefully large. Such non-Kerr-like behavior can deteriorate the all-optical response of guided-wave devices such as nonlinear Mach-Zehnder interferometers, directional and distributed couplers, and nonlinear gratings. Positive effects of saturation with regard to an integrated-optics beam scanner are mentioned.

Since the discovery by Osterberg and Margulis of efficient second harmonic generation in germanosilicate fibers, there have been several explanations put forth for this effect. Theoretical models which are based on either 1) structural changes in the glass⁽³⁾, 2) the orienting of "defects" by x⁽³⁾ generated dc fields, 3) the orientation of enclathrated GeO, and 4) core cladding interface effects have emerged. With the exception of the last proposal, all of the theoretical models assume that a macroscopic composite χ⁽²⁾(2ω) results due to optical excitation.

Photo-induced charge redistribution is used to model the efficient second-harmonic generation of light in optical glass fibers. In this theory, second-harmonic light redistributes charge through a hopping process, giving rise to an electric field that follows the gradient of the transverse distribution of light intensity. The electric field, in conjunction with the third-order nonlinear susceptibility of the material, induces an effective χ⁽²⁾. The theory predicts that the fundamental wave cannot directly convert into a second-harmonic mode that has inversion symmetry about the transverse dimensions of the fiber, it may only convert directly to an antisymmetric mode. A self-consistent treatment reveals that a single mode of the second-harmonic cannot self phase-match and therefore cannot grow in intensity to the levels experimentally observed. The presence of two modes can, however, self-consistently give rise to the growth of one of them via a spatial parametric process caused by the nonlinear nature of the charge redistribution mechanism. An intense second-harmonic field having either even or odd symmetry can ultimately arise through this modal nonlinear interaction. Apparently, however, the static electric fields predicted by the hopping model are insufficient to account for the upper range of the experimentally observed effective χ⁽²⁾'s.

Second harmonic generation can occur in optical fibres illuminated with both an intense pump and a seeding beam at the SH wavelength. The latter is essential but can be provided by an internal seed generated through a quadrupole interaction. This will be discussed below. A comparison of theoretical and experimental results shows that the main contribution is from quadrupole interactions at the core-cladding interface and that the magnitude of the susceptibility is 10^-18 esu.

A single crystal-cored fiber using 4-(N, N-dimethylamino)-3-acetamidonitrobenzene (DAN) was fabricated. Single mode-propagation was achieved for dense flint glass cladding. The single ring-pattern of the second-harmonic wave corresponding to the singly guided fundamental mode was obtained and its conversion efficiency was 0.42% at the fundamental power of 29.3W. After collimation using an axicon lens, the second-harmonic wave from the fiber could be focused into a smaller spot than the usual light beam with a circular aperture.

Phase-matching nonlinear interactions by periodic variations in the nonlinear susceptibility, quasi-phase-matching, offers advantages in accessible tuning range and in choice of nonlinear coefficients over the conventional birefringent technique. Periodically reversed ferroelectric domains can be used to create monolithic structures with the necessary high-spatial-frequency variations in the nonlinear susceptibility. We present two techniques for the fabrication of periodically-poled lithium niobate crystals, and results for bulk and guided-wave second harmonic generation of blue and green light.