We review the theory of the nonresonant optical Stark effect in semiconductors. The solution of the stationary equations allow to see the origin of the energy shifts and changes in the oscillator strengths. The solutions for pulsed pump and probe beams are discussed. We emphazise the importance of a dynamical (frequency-dependent) damping for the induced interband polarization for the understanding of the pulsed nonresonant optical Stark effect.

A variational wave function well suited to describe exciton dynamics in different confined geometries is used to compute the lowest exciton state in quantum well wires. The exciton quantization is studied as a function of lateral dimensions. The oscillator strength increases with the wire dimensions. In spite of this, the third-order nonlinear optical susceptibility, computed taking into account the exciton-exciton interaction and the phase-space filling mechanism, shows an opposite trend. Numerical results for CdS and GaAs samples are discussed.

We study the coherent nonlinear optical response of two-dimensional excitons in a perpevthcular magnetic field. The transition from two-dimensional behaviour in the low-field limit to quasi-zerodimensional behaviour in high fields is investigated and pronounced manybody effects which are highly field dependent are found. Differential pump-probe spectra are calculated for various excitation conditions. It is shown that cyclotron motion can be time resolved with ultrashort optical pulces.

The relationship between the three main one electron mechanisms that lead to an intensity dependence of the refractive index of a semiconductor namely the band filling effect, virtual interband transitions and the motion of free carriers in a nonparabolic band, is investigated theoretically. The semiconductor is modelled as an ensemble of independent two level electron systems and its nonlinear response to a light pulse is evaluated algebraically. In particular, it is shown (i) how the dependence of the dephasing time on optical frequency allows a smooth transition from the band filling regime to the virtual transitions regime and (ii) how the nonlinear motion of free carriers in a nonparabolic band is related to the nonresonant optical Stark effect.

We report carrier lifetime reduction in GaInAs/InP multiple quantum wells from 3Ons to 95ps using mesas with ion implantation of the sidewalls.

A model is presented for the dynamical evolution of optical bistable devices based on Multiple Quantum Well structures, in which the electric field is near resonance with an excitonic line, and the parameters which appear in the equations are discussed. For room ternperature operation, examples of steady state response curves are presented. In conditions of adiabatic elimination of the exciton density and within the assumption of small carrier density, it is shown that the set of equations is equivalent to the well known model for optical bistability in two level systems.

An attempt is made to study the photoemission :eron 3D quantum well boxes (QWBs) of nonlinear optical materials in the presence of crossed electric and magnetic fields, taking ternary chalcopyrite semiconductors as an example. Consi3ering the anisotropic crystal potential in the Harniltonian, we have formulated the generalized electron energy spectrum taking into account the anisotropies of the 0ther energy band par arne ter s, within the fr sine work of theory. We have then derlved.the photoernission from 3D QWBs of ternary chalCopyrite compounds by using the modified dispersion law under cross field configuration in the said material. It is found, taking 3D QWBs of n-CdGeAs2 as an example, that the photoernission exhibits ladder like dependence with incident photon energy as found in quanturn Hall effect and the corresponding results for three and two-band Kane models together with that of parabolic energy bands have been obtained from the present generalized exjressions as special cases. The photoeinission decreases with increasing magnetic field and decreasing electron concentration respectively. The oscillations in accordance with the present generalized model show up much more significantly and are in agreement with the experimental results as given elsewhere.

For many applications such as spatial light modulators, optical nonlinearities with high sensitivity are desirable. Carrier transport, coupled with the use of internal fields, resonant enhancement near the band-edge and selective carrier confinement by use of hetero-structures, offer ways to enhance the sensitivity of optical nonlinearities. This is particularly true in semiconductors, which show the largest known optical nonlinearities, defined in terms of the saturation value of the optically-induced index change per absorption length. In addition, semiconductors exist with a high level of fabrication technology, making complex device structures practical. Three examples of enhanced optical nonlinearities due to carrier transport which have been demonstrated in our laboratory will be discussed [ 1 , 2 J : 1) Lengthening of carrier lifetimes due to separation of optically-induced charges within internal fields. As an example, ultra-sensitive band-filling has been observed in hetero- Schottky barriers; 2) Using optically induced modulation of the Quantum Confined Stark Effect (QCSE) within a n-i-p-i structure to increase the index change per induced carrier density over that measured from band-filling; 3) Enhanced photo-refractive effect observed using electro-refraction and operating near the bandedge in semi-insulating semiconductors. These concepts have lead to some very promising experimental results.

Single period nipis (i.e. pnp-structures) turn out to be interesting elements for the design of arbitrary nipi-potentials. In addition, the possibility of applying selective contacts to n-type layers allows the use of p-type buffer layers on either side of the pnp-structure in order to screen band bending effects both at the buffer interface and at the surface. From transient photoconductivity experiments typical lifetimes result up to the millisecond time regime (i.e. about iO to iO times the bulk lifetimes). The amplitude of the photoconductive response cannot be attributed to simply an equivalent change of the electron density in the selectively contacted n-layer. This fact is confirmed by Photo Hall experiments. The enhancement of the response can be explained in terms of the special characteristics of the narrow gap material PbTe. Additional tuning of the electron densities via background illumination is performed in order to control the lifetimes. The resulting trends are well understood if the modification of the nipi-potential caused by excess carriers is taken into account. In a simple model the role of the background radiation is explained.

We show how suitable band structure engineering can result in an enhancement of the nonlinear optical response in a semiconductor superlattice. This enhancement occurs at excitation energies below the absorption edge and is a result of many photon virtual excitation of electrons into higher conduction subbands. The criterion which results in this band structure enhancement of nonlinear response is described and examples of such superlattices with band gaps spanning the near infra-red region of the spectrum are given. We present the results of the first full band structure calculation of the third order nonlinear susceptibility in a semiconductor microstructure. For example, for the CdTe-HgCdTe superlattice our calculations predict a third order susceptibility of 1O_6e.s.u. at frequencies 10meV below the band edge. The importance of performing such a full scale band structure calculation to provide an accurate assessment of the nonlinear susceptibility is demonstrated and the effects of varying the band structure investigated.

We use a Ginzburg-Landau Phenomenological model to calculate the nonlinear optical response from superconducting microparticles embedded in a transparent dielectric material. Specifically, we predict a strong frequency dependence of the nonlinear optical susceptibility and a nonlinear change in the transmission coefficient in the JR to microwave region. Also, we predict a field-intensity--dependent renormalization of the transition temperature and third harmonic generation in the JR to microwave regime below the transition temperature.

Modulators and detectors integrated in two-dimensional arrays are key elements for parallel optical signal processing. Self electrooptic effect devices (SEEDs) have been suggested as switching elements for these systems. SEEDs are employed as gates, memories or optical flip-flops {1,2,3]. Various logic functions can be implemented using suitable optical feedback. The switching characteristics can be controlled by the internal structure of the epitaxial layers of the device. In the present study we investigate inverted bistable switching and optimized optical modulation in SEEDs with resonant optical feedback.

In the present work, we report the systematic observation of the optical blue shift induced by electric field in a GaAs/GaAIAs double-well system as far as 18 meV at room temperature. To this end, a detailed electroreflectance study of a GaAs-Alo.3 Gao.7 As double quantum well structure was carried out at room temperature. This effect occurs at moderate values ofthe applied field. In addition, a computer modeling of the electric field dependent optical properties is presented, which is based upon a variational calculation, in the frame of the effective mass approximation. The calculated spectra agree with the expefimental results. The model was used to engineer a coupled QW structure, which optimises the achievable modulation depth, due to application of an electric field.

GaSe is a Ill-VI semiconductor, which exciton at 2 eV is observable at room temperature. In the experiments reported herein, a 15 im thick sample was excited by a 2 eV, 150 fs, pump pulse and the transmission was measured with a delayed "white light" probe pulse. At early times (t < 100 fs) the exciton undergoes a fast shift towards high energy which has been interpreted in terms of optical Stark effect. Because of the resonant excitation, the optical Stark shift of the exciton depends on the electromagnetic field amplitude rather than on the field intensity, as in the non-resonant case. Despite this less favourable situation, excitonic shifts of 10 meV are observed for the highest pump intensities (of the order of 16 GW/cm2). Taking advantage of these large shifts, we demonstrate the realization, at the exciton energy, of an optical gate having a rise time of 150 fs, associated with an "off-on" transmission ratio of 2.

We review a number of nonlinear guided-wave effects using gratings in indium antimonide planar structures. In particular, we report on the experimental demonstrations of guided-wave optical bistability and limiting in an input distributed coupler; butterfly bistability of an input-output grating-coupler combination; all-optical tuning of a distributed feedback reflector; and enhancement of bistability effects by the use of a counter-propagating feedback beam.

All optical filtering and switching can in principle be realized by the nonlinear distributed feedback device (NLDFB), combining an intensity dependent refractive index and a grating on an integrated optical waveguide. In previous work 1,2,3 the influence of different physical parameters was studied using Coupled Mode Theory and a first order approximation to calculate the coupling coefficients. In this paper we present a more accurate method to calculate these coefficients. This allows us to compare the filter and nonlinear transfer characteristics of the NLDFB device for different grating profiles, including a-periodic gratings such as tapered or shirped gratings. Furthermore we can compare the behaviour of NLDFB devices based on different rectangular waveguides, such as embedded strip guides or raised stripguides with the grating etched at the substrate-waveguide interface. Qualitative results were obtained for InSb on sapphire waveguides.

A silicon film sandwiched between a sapphire substrate and a silver layer is used as a nonlinear optical waveguide. The geometrical characteristics of the film and grating coupler have been optimized. This device has been studied in the nonlinear regime using nanosecond and picosecond exciting pulses. Very fast switchings are obtained which can be explained theoretically.

The transfer matrix formalism, well known to describe the linear optical response of multilayered structures, is extended to the plane wave response of multilayered structures with an arbitrary number of Ken-type nonlinear layers, and including linear absorption too. It is shown that the dummy variable technique, well known in the description of the nonlinear Fabry-Perot, can be extended for multilayered structures. Several effects are highlighted for both superlattices and nonlinear interference filters.

We report here intrinsic optical bistability calculations for structured nanoparticle composites, ie. particles consisting of one or more coatings of different materials. The bistability is based on the nonlinear field dependence of the plasmon resonance introduced through the nonlinear component. The single shell structured nanosphere model consists of a nonlinear organic or semiconductor core with a metallic shell; and the two shell model has a metallic core, nonlinear dielectric shell and metallic second shell. The metals examined are Ag or Osmium, the nonlinear dielectrics are PDA, Si, GaAs, InSb and the host medium is water or a glass. Calculations show that the switching power level for intrinsic bistability can be reduced in the one spherical shell nanosphere composite to less than 1 MW/cm2 with no loss in the speed of response. In the two shell model, power levels for onset of bistability can be 1 kW/cm2. CdS with a silver coating has a threshold of 12 W/cm2, when an ellipsoidal shaped particle is used.

Optical processor architectures are discussed for operation with subnanosecond optical gates without memory capability. Switching is discussed for GaAs gates with and without Fabry-Perot resonator. Recent demonstrations of picosecond absorptive inverter gates are reported and discussed in II-VI semiconductors in relation with the requirement for cascadability and stability of operation.

The reflectivity of surface oriented distributed feedback (DFB) structures is studied, both experimentally and theoretically. The devices consist of periodic multilayers of MOVPE- or MBE- grown AlGaAs/GaAs on top of a GaAs substrate. Electrical contacts are integrated to provide an electric field perpendicular to the layers. A Ti:Sapphire Laser pumped by an Argon Ion Laser is used to study the optical and electrooptical properties at wavelengths between 840 nm and 930 nm. As an experimental result, electrooptical modulation by more than 60 % has been found at a wavelength of 878 nm, and the measured switching time is smaller than about 100 ns. Theoretically the modulation is traced back to electroabsorption and electrorefraction properties in the GaAs layers due to the Franz-Keldysh effect (FKE). From the measured electrooptical effects we conclude that A1GaAs/GaAs DFB structures can be used as interesting artificial materials for photonic switching. The modulator characteristics can further be combined with photodetector properties to achieve hybrid optical bistability and to realize self-electrooptic effect devices (SEED).

GaAs/A1GaAs epitaxial structures allow the fabrication of monolithic bistable devices with very appealing characteristics for all-optical switching applications, such aslow threshold power and good thermal stability. They rely upon the excitonic and band-gap resonant nonlinearities in bulk GaAs or MQW. Very compact devices of a few micron size with A1As/A1GaAs integrated Bragg reflectors can be grown during a single epitaxial process. We report linear and nonlinear reflectivity measurements on such a monolithic étalon with a MQW active layer grown by MOVPE. A bistability power threshold lower than 3 mW at 836 nm was observed with a reflective contrast ratio as high as 30: 1 .We show evidence that the refractive index dependence on optical intensity is strongly sublinear, indicating that a substantial degree of saturation occurs at intensity levels of iO W/cm2. We discuss the origin of this saturation and its implications on the design of future devices.

We discuss that hybrid concepts are fundamentally more suited to meet simultaneously the goal of low optical and electrical switching energies and high speed operation. Recently, we have developped an optical threshold switch with gain which can be monolitically integrated and which is suitable for frequencies up into the GHz range and has a gain-bandwidth product of up to 30 GHz. By combining this switch with n-i-p-i or hetero n-i-p-i based electro-optical modulators to form "smart pixels" we obtain optical gates which fulfill all the requirements for applications in optical processors such as low optical and electrical switching energy, high speed, high fan-out, and large tolerances in signal energy.

Anall-opticalprocessing ioop circuit, pumped entirely by semiconductor diode lasers, has been constructed and operated. Functional features include optically programmable logic, thresholding, and synchronization; these areachieved using threebistable interference filterdevices. Thecircuitispresently single-channel, however 15 x 15 capability of the devices has been demonstrated using Dammann holograms and array-to-array coupling of a pair of bistable plates; potential parallelism is in excess of i04. Circuit tolerancing is also described.

The new type of seif-electrooptic--effect--device is presented. Optical nonlinearity and bistability due to the internal electric field redistribution in nonuniform i-layer of double GaAs/A1GaAs PIN heterostructure are achieved.

In this work, we have demonstrated experimentally the electro-optical switch, logic and memory operations at the speed of as fast as nanosecond region, utilizing the mode-hopping phenomenon of a laser diode. We have also confirmed the hysteresis (wavelength bistability) region being still effective under even such a high speed. The sample laser diode used in the experiments was a O.8m GaAlAs-type and 1.3m InGaAsP-type one. The measured speed of switching was within nanosecond region, which was limited by the driving capabilities and the light detection response, not by the phenomenon itself. In order to confirm the reproducibility of the hopped wavelength in the experiments, the fluctuations in the ambient temperature of the laser diode are precisely controlled to be within 1.0 mK through a thermoelectric cooler attached to the heat sink of the laser diode by which both cooling and heating are possible. As an example of the fast logic operation, we have achieved the actual N X N channel optical wavelength-division-multiplexing system using a O.8m GaAlAstypevvisible injection laser diode and also a 1.3m InGaAsP-type long-wavelength laser diode of which the wavelength is rather suitable for fiber optic routing systems. Moreover we carried out the simulation to explain the behavior of the wavelength bistability seen in these laser diode samples and also estimated how fast these switches or memories can operate, based on some well-known analyses for the mode-hopping phenomenon. In addition, the gain-suppression mechanism in semiconductor lasers was included in this analysis. As a consequence, the result of estimation suggests that the speed of wavelength-switching may become about an order of magnitude longer than the carrier lifetime of the device. Therefore, we can conclude that the dominating physics of the wavelength bistability based on the mode-hopping phenomenon is a very fast thermal processes caused by modulating the injection current.

A novel operating technique has been developed for a differential optical switch based on a set of parallel-connected pnpn structures. The differential function at O.2nW input power and 400fJ input energy was realized with A1GaAs/GaAs pnpn devices. Its application to an optical S-R flip-flop was also demonstrated by introducing a storage operation of pnpn structures.

These photoconductive devices operate with c.w. Nd-YAG laser and use Joule effect enhanced nonlinearity, associated with the sharp angular resonance corresponding to the TEo guided mode excitation in a SOS (Silicon On Sapphire) film. Transfer characteristics and spatial uniformity of the transmitted beam for devices of various geometries have been measured and contrasted characteristics have been observed locally. Potential interest of these devices is discussed.

The status and potential of a new type of device, the surface plasmon spatial light modulator (SPSLM) is reported. The attractive features of surface plasmon resonance (SPR) for use in SLM's are explained and results from prototype devices reported. These are of the liquid crystal (LC) light valve configuration, using nematic LC with a silicon photodiode backplane. Demonstrated advantages include process simplification and increased response speed. These are obtained due to the thin, single surface nature of the plasmon active region, whilst high sensitivity is retained due to the resonant enhancement of the optical field in this region. The theoretical principle of the liquid crystal SPSLM is described, in terms of the propagation of plasmons on anisotropic materials. Various alignment configurations are considered to show how both nematic and smectic materials could provide high sensitivity and speed in future devices. The need for a grating coupled SPR technology is explained, and the design and fabrication of holographic gratings for SPSLM's is discussed. Finally, the present and ultimate performance limitations of these new SLM devices are assessed, and related to their potential use in optical information processors such as image correlator and neural network systems.

Optical bistability in a Fabry-Perot cavity filled with nonlinear riedium possessing reversible bleaching under illumination by two light beams having different wavelengthes is considered theoretically. The erIiancement of optical feedback efficiency is shown to take place when simultaneous intensity changes of light waves have opposite signes. Analytical solution is obtained in a low absorption limit showing that the optical bistability can be realized with four times greater level of background losses as compared with the conventional case.

In polarization-based logic gates the binary logic values are represented by two orthogonal linear polarizations of light which can be interconverted by means of the programmable half-wave devices. We present a device consisted of a SSFLC-cell, a lithium niobate birefringent crystal and polarization-maintaining highly birefringent (HB) optical fibers. Due to the birefringent Li-Ni crystal attached to the SSFLC cell and two output HB fibers with proper oriented axes both orthogonal polarizations were spatially separated. In this way we obtained a non-absorptive binary logic gate which did not loose any information and enabled signal retrieving in any state. Power losses of the FLC logic gate were estimated in the order of 20 dB and contrast ratio between two stable logic states achieved a mean value 5 : 1.

A new type of GaAs/GaA1A5 bridge-contacted ridge-waveguide (BCRW)twin-stripe laser with two asymmetrical waveguide channels is reported for realizing high speed optically induced near-field intensity switching, which can be used for digital optical logic devices as optical inverters, (N)OR- or (N)AND-gates. Optimum performance is achieved for equal wavelengths of the injected optical input signal and the switching twin-stripe laser. The total response time for one switching cycle is 70 p5.

In the field of real-time optical processing and computing based on the principles of nonlinear optics and dynamic holography a key role belongs to recording materials. There are certain properties of bacteriorhodopsin (BR) that make it attractive for these applications. Materials based on BR have a very wide range of photoreply time from 100 s down to 10 ps depending on the preparation procedure, and an axtremely high spatial resolution limited by the dimensions of the molecules. Moreover, they are sensitive to the state of polarization of the recording light, due to molecular dichroism. In this report we analyse the mian optical properties of BR films and aqueous suspensions, discuss the mechanisms of the photoinduced anisotropy in these materials and dèscribe the results of nonlinear optics and dynamic holography experiments: nonlinear filtration, control of polarization, polarization-preserving phase conjugation based on vectoral four-wave mixing.

We discuss the competition between oscillating waves in two ring cavities coupled by a common amplifier on a photorefractive crystal. We determine the threshold conditions and steady-state intensity for each wave. Different steady-state regimes of oscillations were revealed by analytical treatment. Conditions for single wave oscillation and two waves simultaneous oscillation were found. Oscillation with a bistable switching is also shown to be possible. Numerical simulation of transient effects in the system has shown the possibility of controlling the oscillation regimes by external optical signals The results obtained may be of interest in the systems with optical information processing.

Miniature semiconductor lasers with resonant ring cavities in both heterostructure and quantum well material have been developed at Glasgow. Two laser structures, rib ring laser and polyimide-embedded ring laser, have been investigated. The resonators produced are between 10 and 1 00im in diameter and because of their small size, the longitudial ,mode spacing is increased over that of conventional cleaved-cavity lasers, reducing the numbers of lasing modes in the mode spectrum. But the small structure will cause thermal problems because of the difficulty of heat dissipation. Detailed thermal measurements have been made on the surface of the laser by an infrared thermal imaging technique. The results give a fair qualitative assessment of the thermal behaviour of a ring laser compared to theoretical modelling results, and show that the temperature rise of the lasing ring is excessive at lasing threshold current for the very small ring and narrow rib structure. Increasing the ring diameter, with a shallower or wider rib structure or even a pill-box structure, will improve heat dissipation and reduce threshold current.

The effect of electric fields on the optical properties of GaAs-A1GaAs ultiquantu well (HQW) structures are attracting much attention due to potential applications in high-speed modulation and switching of optical signals4 The existing technology of growing GaAs-A1GaAs MQW's allows to manufacture devices which have responce required for for implementation of neutral networks and associative meffiories.