During the past years, rapid progress has been made in the development of optically bistable semiconductor devices that may be the key elements in future photonic switching and information processing systems. This paper examines especially the potentialities of hybrid components where a self-electrooptic effect is achieved by combining optoelectronic and electrooptic mechanisms. The switching properties of thermooptical and electrooptical Fabry-Perot SEED structures are discussed in detail. The results are compared with experimental data of best reported switching devices that are capable of achieving a processing rate of more than 1012 bits per second and chip in a two-dimensional array.

Cross-phase modulation (XPM) occurs when copropagating ultrafast pulses interact in a nonlinear optical medium. XPM effects are being investigated as a new technique for controlling with femtosecond time response the spectral, temporal and spatial properties of ultrafast pulses. This paper describes theory and measurements of XPM effects bulk glasses, optical fibers and semiconductor ZnSe crystals.

New possibilities of governed temporal modulation, tunable focussing and fast deflection of far infrared radiation are discussed. Such possibilities are based on use of thin semiconductors films with free carriers as a mirror with the variable reflection coefficient.

Research directed at two-dimensional optical information processing requires large arrays of binary logic elements with moderate power demand. To minimise power consumption intermediate switch times of ≤ 1 μs would appear to be optimum. A short-term target is to develop a 104 element array of such switches with ≤ 1 watt power consumption. This puts specific demands on the nonlinear materials being exploited. For example, those devices based on optoelectronic nonlinearities need high quality material with long carrier lifetimes. A novel time-resolved photoluminescence microscope spectrometer has been developed for measuring such lifetimes. The alternative nonlinear interference filter (NLIF) devices, based on optothermal nonlinearities, also look promising in this context. Because of the need for high quality structurally stable thin-film material, a new UHV molecular beam deposition (MBD) facility has been set up to permit production of low damage threshold, stable dielectric layers. Good thickness uniformity has been demonstrated over large, 100 mm diameter, substrates.

Optimization of photothermal CdS SEED's for better performance depends to a great extent on the possibility of improving the optical, electrical and photoelectrical properties of thin layers deposited on various substrates. Some fundamental aspects of this problem are discussed. Three alternatives have been explored : evaporated, epitaxial and spray deposited layers. The physical properties (X-ray diffraction, optical absorption, electrical and photoelectrical conductivity) and their influence on the bistability effects were investigated. Films epitaxial grown on CaF2 by the close space vapour transport technique showed excellent properties. Nevertheless only a short-lived bistability could be observed. Vacuum evaporated layers showed poor physical characteristics. The optical properties could be significantly improved by air annealing, but it was possible to observe all-optical bistability only. The best results were obtained with spray deposited air-annealed films. The four SEED - characteristics could be observed. The opto-electrical and purely electrical hysterisis loops are comparable or better than for CdS platelets.

In order to study optical bistability in a non-linear distributed feedback structure (NLDFB), we have generalized previous work. A mono-mode rectangular waveguide is chosen as the basic configuration for the device, although other guides could be considered. The grating, providing the passive feedback mechanism, has a geometry that can be arbitrarily chosen. Linear absorption is taken into account and the non-linearity is taken to be of the Kerr type. The effect of these three perturbations is treated using the Coupled Mode Theory and the Slowly Varying Envelope Approximation, including the radiation modes. Expressions for the coupling constants due to the three fundamental perturbations are derived and we show that new terms arise due to the interplay between the latter. Our model shows that an extra term, causing a shift of the Braggfrequency occurs in the equations. The analytical expressions are integrated numerically with the proper boundary conditions. Overall reflection and transmission properties of the device are displayed as a function of the intensity, the period of the grating, its length and several other device parameters.

Non-linear waveguides formed of a saturable intensity dependent refractive index medium bounded by linear materials are studied by means of a simple and exact numerical method, based on the transfer matrix approach in addition of the self-consistent nature of the field. Some results in the TE waves are reported.

In this paper we present our investigations concerning thermally induced optical bistability (OB) in CdS and ZnSe. With CdS platelet type single crystals basic boolean functions are realized. The possibilities of cascadability and parallelity of the information processing and the switching dynamics of the system are examined. Further on the effect of optical noise upon the switching process and the instable branch of the hysteresis loop are measured. In ZnSe both absorptive and dispersive OB are observed. An efficient opto-electric modulator is built using the photothermal SEED-effect.

Picosecond wave-mixing experiments are performed on CuCl at low temperature near the two-photon absorption resonance of the biexciton. In a two-beam configuration, a degenerate four-wave mixing experiment, using a streak camera system, enables us to separate two different contributions to the generation of the signal : self-diffraction and induced recombination. Transverse relaxation times of the biexciton could thus be determined. In a three-beam configuration, the test pulse is diffracted on the grating induced by two interfering pump pulses. The time evolution of this grating is governed by coherent and incoherent scattering processes. Due to the incoherent part, a signal is observed on a much longer time scale than in a two-beam configuration where only coherent processes show-up. Its decay presents different time constants which characterize the time evolution of the different quasiparticle populations determining the optical properties of the material.

We study the third order nonlinearity associated with the biexciton resonance in CuCl. The magnitude of X3 is deduced from polarization rotation. A lower limit value X3 ≥ 9.10-6 esu is measured for a light intensity Io ≤ 103W/cm2 in high purity single crystals at T = 2K.

The coherent nonlinear optical properties of direct gap semiconductors have been investigated by picosecond time resolved degenerate four-wave mixing (DFWM). The experiments were performed at low temperatures and with low, or moderate, excitation densities, focusing on the strong excitonic resonance enhancement of the nonlinear optical susceptibility x(3). Free exciton transitions, including two-photon absorption to the biexciton state and induced exciton-biexciton transitions, contribute strongly to x(3)in CdSe, whereas the contribution from excitons, localized by fluctuating potentials, dominates in the mixed crystals CdSexS1-x,and in the layered GaSe. The magnitudes of the coherent and the incoherent contributions to the nonlinear signal are compared, and the response time (dephasing time) of the coherent nonlinear signal is investigated in detail. The observed dephasing times are dominated by exciton-exciton collisions in CdSe, by exciton-phonon interaction in CdSexS1-x, and by disorder and phonon induced intervalley scattering in GaSe.

The nonlinear optical properties of ZnO have been investigated with an excimer pumped ultraviolet dye-laser. The wavelength dependence of the first order diffracted intensity in a self-diffraction experiment has been performed for photon energies below but near the fundamental absorption edge. The measurements indicate a resonant behaviour of the third order nonlinear susceptibility just below the bandgap. Further we have measured the temporal behaviour of the diffracted light. This time dependence indicates that the non-linearity is due to electronic excitations in the crystal.

Electron-hole pairs have been created in a near infrared II-VI semiconductor, CdTe, by picosecond optical pumping. The plasma relaxation dynamics have been investigated using time resolved luminescence technique. The plasma temperature and the bandgap renormalization are studied during the relaxation regime following the excitation pulse. Luminescence data are analysed in the frame of the standard free-particle model and also including the carrier dampings calculated in the plasmon pole approximation. In both cases, we conclude that the plasma cooling is slowed down at high initial plasma density (typically for p ≥ (3-4) x 1017/cm3). Nevertheless, the temperatures deduced during the relaxation kinetics are significantly lower when damping is taken into account. Band gap renormalization is also studied. The experimental results are in reasonable agreement with the usual gap shrinkage theories.

Optically controllable gates are of increasing interest in prospective optical data processing, in optical transmission systems for external modulation, and, arranged in parallel, for all-optical interconnects. In optical data processing besides logical switching elements memory devices are of decisive importance for buffered input and output and for clocked processing cycles. In this contribution we investigate dynamic memory characteristics of InGaAsP photonic switching devices. We achieve a hold time greater than 10 ns which enables clocked signal processing.

The nearly-degenerate four-wave mixing technique is used in semiconductor lasers in order to demonstrate the essential role played by mixing processes in the period doubling route towards chaos. Using the two-level equivalence for the intracavity gain of semiconductor lasers, resonant four-wave mixing interactions are predicted. Interferences between these mixing processes are shown to be responsible for the dispersive like behavior of the conjugate intensity experimentally observed at half the relaxation oscillation frequency when the pump-probe detuning is varied. The same interferences provide the extra-gain which makes the route towards chaos much more probable via period doubling processes.

Here we report on some results of investigation of the physical mechanisms responsible for optical nonlinearity in cubic crystals with tunneling centers. The nonlinear susceptibility of the system becomes anisotropic under the resonant polarized photoexcitation of the intracenter electronic transition. This anisotropy manifests itself in the self-induced change of light polarization which may form the basis of the nonlinear polarization spectroscopy. We use this experimental technique for the investigation of reorienting FA and F2 centers in KCl.

Below-gap excitation of semiconductors induces a blue shift of the absorption spectrum, known as the optical Stark effect. Using femtosecond pump and probe spectroscopy, we monitor the temporal behavior of the optical Stark shift and observe an apparent non-instantaneous response. However we show that the actual response of the material is ultrafast, only its observation is perturbed by the exciton coherence. We also investigate the dependence of the Stark shift upon the band-structure, leading to a light-induced exciton splitting.

We investigate the linear and nonlinear optical properties of semiconductor doped glasses. In thin samples we observed quantizied electron levels in absorption measurements. In the luminescence spectra we see a blue shift of the high energy side with increasing excitation. For nonlinear optical investigations we used spectroscopy with laser-induced gratings. We have seen optical bistability due to bleaching of the electronic transitions and due to photothermally induced absorption.

The nonlinear optical properties of commercial semiconductor-doped glasses containing "large" particles being now reasonably well understood, attention is now focused on smaller particles clearly exhibiting quantum confinement. Nonlinear absorption measurements clearly demonstrate the important role of electron-phonon coupling in the broadening of the ls-ls transition. Spectral hole burning is observed at low temperature. Optical phase conjugation measurements confirm the expected increase in the nonlinearity due to confinement.

Metal-doped glasses, composed of very small metallic particles embedded in a glass are known to be very efficient, fast-responding media for optical Kerr effects as phase conjugation in the degenerate - four - wave - mixing configuration. The origin of the high non-linearity of these media lies in the intrinsic electronic nonlinearity of metallic spheres which is greatly amplified by the existence of the surface plasma resonance. Such a resonance effect in the nonlinear optical properties is well understood from a general calculation of the response of a metal-doped glass to an electromagnetic radiation. We perform both theoretical and experimental studies of optical phase conjugation in the case of gold-doped glasses. Various origins of the nonlinearity of a gold sphere are analysed : conduction electron intraband contribution, saturation of direct interband transitions and change in dielectric constant due to hot photoexcited electrons. The last two are shown to be the most important contributions. Experimental studies of the dependence of the conjugate signal with the size of the spheres, and with the wave polarization of the beams confirm these conclusions. Saturation effects for absorption and phase conjugation are also fully understood.

We present new experimental results concerning the optical nonlinearities of CuCl microcrystallites embedded in a glass matrix. Using a "pump and probe" technique, the induced absorption is measured in the spectral range of the Z3 exciton. We observe two phenomena: first, a bleaching of the excitonic resonance accompanied by the generation of a new absorption band at higher photon energies when the test probes the sample during the pump pulse; secondly, a weak shift of the excitonic resonance towards higher photon energies when probing the sample during and after the pump excitation. We vary the intensity and the photon energy of the pump beam as well as the delay between the pump and the probe pulses. Thus we can show that the pump beam populates excitonic states, which leads to a complete bleaching of the excitonic absorption. In addition, excitons can be created at higher energies, inducing the new absorption band. Its energy depends on the number of states already occupied in the microcrystallites. The shift of the excitonic resonance, on the contrary, can be explained by the heating of the microcrystallites through this high population of excitons and its non radiative decay during and after the excitation pulse.

A multiple quantum well layer on top of a dielectric reflector is used as an optical switching device. With a test beam at λt = 868 nm and a 1 mW control beam at λc = 790 nm a switching contrast of 10:1 is achievOd. This high modulation depth is enabled by the coincidence of an excitonic resonance and a Fabry-Perot resonance. Excess carrier lifetimes of a few ns allow for bit rates of ca. 100Mb/s.

Second Harmonic Generation of a new hemicyanine dye was measured. The LB-films were either pure dye-monolayers or mixed dye-arachidic acid films. An increase of SHG was observed with a decrease in dye concentration in the mixed films. For multilayer films a subquadratic increase in SH-intensity with the number of dye layers was observed. Aggregation and orientation of the dye molecule in the monolayers and films were studied by reflection and absorption spectroscopy.

The second-order nonlinear susceptibility X(2)xxx its two molecules, an azo dye and a polyene, is measured at various fundamental frequencies, so as to study its frequency dependence. The molecules are deposited on an hydrophilic glass substrate by use of the Langmuir-Blodgett technique. A resonant enhancement of X(2) is observed for both molecules, and the X(2) frequency dependence displays two well separated maxima. The origin of these peaks is discussed in terms of resonance with vibronic levels.

The temporal response and stability of all-optical switches at power levels down to 14µW are studied; cell design and optimisation are also considered for extension of this work to infrared wavelengths with experimental results presented for 1.3 μm.

A novel spatial light modulator (SLM) based on nonlinear absorption of light by excited molecules is suggested. A kinetic analysis is applied to calculate the relative populations of molecular levels in singlet and triplet manifolds. It is shown that for many actual cases a probe light can be modulated by propagating through a medium excited by another light source. The dependence of the molecular SLM on the photophysical parameters of the SLM medium is discussed. The same type of molecular nonlinearities can be utilized for optical bistability, as is demonstrated for some dyes.

The rotationnal diffusion of Disperse Red One dopant in PMMA is studied by electrooptical polarimetry. The temperature is varied from Tg to far below the secondary transition temperature Tβ. Above Tβ, disordering but also ordering of the dopant are possible. This rotationnal diffusion cannot be described by a diffusion coefficient and is strongly related to the free volume relaxation inside the polymer. The mobility is almost completly frozen under Tβ.

The second-order optical nonlinearity of a novel side-chain polymer is investigated in relation to the nonlinearity of its precursor molecules. The polyacrylate homopolymer contains 4-oxy-4'-nitrostilbene moieties as hyperpolarizable pendant side groups. The effective hyperpolarizability of the nonlinear optical (NLO) moiety is measured by electric-field-induced second harmonic generation in solutions. Measurements are reported on modifications of the NLO group, on the monomer and on the polymer. The experimental results reveal, that in this organic material the effective nonlinearity of the pendant group is little influenced by the attachment to the polymer backbone. It is found that the macroscopic second-order NLO coefficients, as measured by electro-optic methods in thin polymer films, are smaller than the theoretically expected coefficients based on microscopic parameters.

The polydiacetylenes are known for their large and fast third order optical nonlinearities, mainly due to the one dimensional delocalization of their π-electron system. Since they are available in several easily processable forms (Solutions,gels...) they can be considered as prototype materials for nonlinear optics. These nonlinearities are usually characterized by a third order nonlinear susceptibility (x(3)) but higher order effects take place when the pump intensity is increased. These higher order effects are usually observed under one photon resonant conditions, where they correspond to a saturation of the optical nonlinearity with pump intensity. Here we report on investigations of high order nonlinearities with a pump wavelength in the transparancy domain of the polydiacetylenes. Under our experimental conditions the nonlinear responses are dominated by higher order effects for pump intensities of the order of 1 GW/cm and, on the contrary to the one photon resonant situation, they correspond to an increase of the optical nonlinearity.

Envelope time reversal can be achieved in a phase conjugation experiment. In this work, we study the influence of the various microscopic parameters of the nonlinear medium as well as the effects of rotational diffusion of the dipole moments on the envelope reversal. To this end, we consider a backward degenerate four-wave mixing experiment with pulsed fields and assume that any characteristic times of the envelopes or of the rotational diffusion are larger than the transverse decay time. Propagation and field polarizations effects are introduced. From the propagation equations solved in the slowly varying envelope approximation, we calculate the amplitude of the conjugated wave. Its expression depends on the field polarization as well as on the various overlap integrals of the pump and probe envelopes. In order to obtain a complete time reversal between the amplitudes of the conjugated wave E4(0,t) and probe field E3(0,t), supplementary contributions to E4(0,t) which do not satisfy the proportionality relation between E4(0,t) and E3*(0,-t) must wash-out. Some of these contributions cancel by taking a convenient choice of beam polarizations. It is shown here, that if the dipole moments undergo a rapid rotational diffusion, then complete time reversal is observed.

Widespread application of photorefractive materials depends to a great extent on the possibility of optimizing the extrinsic optical, electrical and photo-electrical properties which are indeed closely related to crystalline defects and impurities. This paper recalls how deep levels are involved in the figures of merit which characterize the performance of the photorefractive materials. Some fundamental difficulties for characterizing and controlling defects in insulators are reviewed. New opportunities are provided by the recent development of specific thermal and optical spectroscopic methods. The techniques are based on transient photoconductivity and differential optical measurements. The principles of the methods are described shortly. Typical results obtained with Bi12 Ge 020 are presented.

Bismuth silicon oxide is one of the most sensitive photorefractive materials available today. It is therefore important to develop an understanding of how its photorefractive properties can be altered by adding impurities during growth. In this paper results from analyses of doped bismuth silicon oxide crystals grown by the addition of impurities to the melt during Czochralski growth are presented, and the influence of these impurities and their concentrations on the photorefractive effect are determined. Doped crystals are used in a holographic arrangement, and the effects of dopants and temperature on grating formation (erasure time and diffraction efficiency) are reported. Data on the effect of dopants on long-term retention time are also presented. The apparent effects of certain impurities on traps are reviewed. The results are discussed and compared with current models.

In the recent year, the photorefractive effect has become the nonlinear mechanism of choice for highly parallel information processing with low power lasers. The use of these nonlinear interactions has resulted in numerous new applications such as information processing, phase conjugation, image amplifiers and resonators. Under optimized recording conditions, most of the photorefractive crystals now exhibit large gain coefficients in the visible and near infrared wavelengths. Some of these applications based on two wave and four wave mixing interactions in different electro-optic crystals (BaTiO3, KNb03, BSO, BGO, GaAs...) are detailed in the following of this paper. Moreover, the specific properties and the basic mechanisms of the photoinduced index change are also presented.

Based on the results of two wave-mixing experiments in the nanosecond regime conducted in the semi-insulating crystal InP:Fe we present an analysis of the build-up of the photoinduced space charge field that governs the photorefractive effect. A numerical integration of the material's differential equations is carried out taking into account charge transport by holes and electrons. Competition between drift and diffusion, and between hole and electron charge transport limits the coupling gain. The time evolution and the role of the important quantities are explained by the use of appropriate approximations.

We review the conditions of efficient interactions at 1.06µm wavelength with photorefractive GaAs. These interactions are based on the recording of moving interference patterns in the volume of the crystal (drift recording mode with an externally applied electric field) This technique results in large values of the exponential gain coefficient Γ ~ 6-7 cm -1 for an optimized grating velocity and grating spacing. Applications to image amplification, amplified four wave-mixing (R = 5) and self-induced optical oscillations are presented.

Using the coupled wave theory, we derive an analytical expression for the amplified beam. We demonstrate, both theoretically and experimentally, that the amplified beam can be made time independent.

We present measurements of the steady-state two-wave mixing gain in photorefractive InP:Fe crystals cooled with a Peltier cell, as a function of fringe spacing, external d.c. field and pump intensity. High gain values (up to 11.4 cm-1 at 10 kV/cm) can be obtained with an accurate control of pump intensity and sample temperature. Such behaviour can be explained with a new model which predicts a new resonance mechanism yielding a strong en-hancement of the gain for a critical intensity.

Lithium niobate is a well-known electrooptic crystal but its application is limited by the photorefractive effect. Magnesium doping was reported to reduce the value of the photorefraction /1-5/. It was shown that for mg concentrations higher than some threshold the decrease of the photorefraction by a factor of 102 as compared to the pure LiNbO3 is due to the corresponding increase of the photoconductivity /2,3/. Similarly in LiNb03:Mg:Fe2 with Fe concentration up to 0.03 wt% the photorefraction is reduced by a factor 102 as compared to LiNb03:Fe /5/.

Theoretical and experimental results are presented for simultaneous multibeam coupling in photorefractive BaTiO3. Within a single crystal, multiple signals are amplified through a coupling processs that employs a single pump beam. The amplification of each signal results from both coupling between signal and pump beam and coupling between different signals. The amount of gain that each signal receives is dependent on the angle relative to the c-axis of the crystal and on the intensities of the incident beams. Thus a competitive interaction for the amplification of the signals takes place: For a specific range of coupling coefficients and intensities the strongest incident signal receives the largest energy increments. In a resonator that employs photorefractive gain such a process can be used to keep the strongest beam oscillating and to suppress weaker ones. Thus it is possible to apply that competitive process to an associative holographic memory.

We show how photorefractive two wave mixing allows a nonlinear input-output response providing compational abilities of decision. Particularly, we investigate the realization of a photo-refractive "optical neuron". Signal amplification with saturation is obtained and could compensate for losses due to holograhic connections. Moreover, the treatment of bipolar signals is allowed by preserving the phase of the wave in a coherent situation. Experimental evidence, using a crystal of BaTiO3 is presented.