Nonlinear optical processes have received international attention because of their importance in optical signal processing and computing. Organic molecules and polymers form an important class of nonlinear optical material. This paper discusses the basic relevant concepts in the newly emerging area of nonlinear optical processes in organic molecules and polymers. It provides the results of a comprehensive study in this area being conducted in my laboratory. This study includes theoretical calculation of microscopic optical nonlinearity; design, molecular engineering and synthesis of novel organic structures; experimental investigation of nonlinear processes using picosecond and femtosecond laser pulses; and the study of device phenomena. At the end, the current status of this field and future directions of research are discussed.

This paper summarizes recent results on nonlinear absorption and refraction, related bistability and ultrafast optical switching at visible wavelengths using wide-gap II-VI semiconducting compounds.

We first recall the results of our extensive studies of the nonlinear properties of commercially available semiconductor-doped glasses which led us to a fairly complete understanding of the nonlinearity in these materials. We also observed experimentally that at high fluences, Auger recombination of free carriers is an efficient recombination process in these small crystallites. We then present recent results obtained on experimental semiconductor-doped glasses clearly exhibiting the quantum confinement effect. We observed that phonon broadening is very important in these very small Cd S Se particles. At low temperature however, spectral hole burning was observed. These results are in agreement with conventional models of electron-phonon coupling.

The technology for the fabrication of semiconductor-doped glass film by co-sputtering of Coming 7059 and a CdS powder or pellet is described. The presence of semiconductor microcrystallites was verified by using Raman spectroscopy. XPS measurements show that CdS content of the films varies from 3 to 20% by weight depending on the kind of target used and the RF power supplied to the sputtering system. Nonlinear coefficient n2 of the films is about 10^-10 m²/W. The nonlinearity is probably of thermal origin. All-optical beam deflection experiment based on nonlinear properties of the deposited films is described.

We report here on photo-electronic nonlinearities in direct gap seminconductors, especially those connected with the transition from a low-density gas of excitons to an electron-hole plasma. We consider both three-dimensional and quasi two-dimensional semiconductors and compare both systms. Finally we show that these photo-electronic optical non-linearities can be used to obtain various types of optical bistability.

We report on two applications of nonlinear optical effects of semiconductor-laser amplifiers in optical communications and optical signal processing. We describe an all-optical frequency converter with a 4000 GHz conversion range and with a fibre to fibre efficiency of -10 dB, and an all-optical switch operating as logic AND gate up to frequencies of at least 100 MHz. An advantage of both devices is their internal gain.

We have measured the intensity dependence of the room temperature optical transmission of an In_.53Ga_.47As/InP multiple quantum well (MQW1) and an In _.53Ga_.47As/In_.52A1_.48As multiple quantum well (MQW2) from 1.5μm to 1.7μm. The absorption is calculated from the transmission taking into account the wavelength dependence of the reflection coefficients. At the edge of tlAe 1H-1C transition absorption band the absorption is found to fall by 1/2 if the intensity is 16kWcm^-2 and 15kWcm^-2 at the surface of MQW1 and MQW2 respectively and is completely saturated at intensities exceeding 10⁷ Wcm^-2 , showing no nonsaturable absorption. A theoretical model is described, which fits the intensity dependence of the absorption right up to saturation at two wavelengths with an assumed carrier lifetime of 0.66ns for MQW1 and 0.75ns for MQW2.

The exact theory of degenerate four-wave mixing in a Kerr-like medium in the Bragg scattering regime is used to study the reflectivity of phase-conjugate mirrors pumped by intense laser beams. The analysis takes into account pump depletion, nonlinear phase modulation and the relaxation time of the nonlinearity. The steady-state response exhibits multivalued reflectivity, hysteresis and potential bistability; however, the transient response shows that at most one value of the phase-conjugate reflectivity is stable. Periodic or chaotic pulsations are predicted beyond threshold values of pump intensity. The onset of a natural oscillation in the Kerr-like medium is also predicted in the absence of a continuous probe beam; the resulting field amplitude is stable and weakly sensitive upon the presence of external noise.

We demonstrate all-optical intensity modulation in GaAs/AlAs multiple quantum well (MQW) devices with integrated dielectric reflector grown by molecular beam epitaxy. A 4 mW control beam at λ_c = 790 nm wavelength produces a reflection change of 1:3 by dynamic band filling in a twenty-period GaAs MQW structure of well width d_OW = 20 nm for a test beam at λ_t = 866 nm which is close to the lowest energy transition ifl the quantum wells ( λ= 868 nm). The integration of a multilayered dielectric reflector of 92% reflectivity consisting of 20 pairs of A1_0.08Ga_0.92 As/AlAs quarterwave layers allows for operation in reflection and thus removal of tWe opaque GaAs substrate is not required. Dynamics are shown to be limited by excess carrier lifetimes of τ = 4 ns corresponding to a 3 dB frequency of 70 MHz. Surface recombination in microresonator structures may be used to improve the dynamics. For a theoretical analysis the carrier-induced absorption change by dynamic band filling of the quantized energy states in the conduction and valence bands of the quantum wells is considered.

Picosecond NOR gates using II-VI and III-V bulk semiconductors are described, based on the induced picosecond absorption by the electron-hole plasma. The realization of cascadable devices is discussed.

We have observed self-limiting and optical switching of near bandgap 1.06 micrometer light using MBE grown silicon waveguides on an n-type doped silicon substrate. The guided light intensity lowers the local index of refraction of the waveguide coupling region due to the photogeneration of electron-hole pairs. This brings the guide closer to its cutoff condition thus limiting the peak intensity that can be guided. Short 20 ns pulses gave rise to optical limiting with less than 160 nJ total integrated energy, while longer 150 ns pulses yielded envelope self-switching. Fiber and lens edge coupling to the silicon was used in all cases in contrast with the usual prism coupling geometry. Four guides, each with a different cutoff condition, were used to prove that the limiting is not caused simply by free carrier absorption. A theoretical self-consistent model was developed taking into account the guide's integrating Kerr-like self-defocussing nonlinearities. Since the initiation times are subpicosecond, such a nonlinear waveguide can be used as a fast optical limiter and logic gate.

Optically bistable interference filters and single bistable absorption layers were prepared by thermal and electron beam evaporation of CdSe, CdS, ZnSe, ZnS, MgF₂, Ta₂0₅ and Si0₂ on glass substrates. The bistable interference filters consist of a low absorption spacer, e.g. CdS, with multilaver mirrors on both sides and an absorption layer, e.g. CdSe, on the beam exit side. Absorption leads to heating and thermooptic tuning of the spacer resulting in optical bistability. Using low absorption spacers and an external absorber the temperature rise necessary for switching can be reduced. Therefore bistable elements allowing more than 10 mill. switching cycles have been achieved. Optical bistability is observed also in single ZnSe layers due to increasing absorption with an incident photon energy just below the band gap. The switching elements were operated between 500 and 600 nm wavelength and needed switching powers of several mW. Switching times in the microsecond range were measured.

Observation of the intrinsic optical bistability in a titanium-diffused lithium niobate optical waveguide with liquid crystalline nonlinear prism coupling is reported and a comparison with the optical bistable effect in a glass thin-film waveguide is also presented.

Here we present experimental observations of amplitude, phase and frequency distortions exhibited by picosecond optical pulses after transmission through a single FABRY-PEROT interferometer with a fast Kerr material (CS₂) between the plates. Strong self-phase modulations are demonstrated and measured both in the spectral and temporal domains.

A new way to approach the characterization of an all optical regenerator is presented in this paper. The device consists of a semiconductor amplifier laser containing an homogeneous mixture of saturable media (gain and absorption) with different saturation parameters. A modification introduced in the photon density matrix equation is the clue idea of the proposed characterization. Using this characterization it will be shown in the present paper that a semiconductor with such characteristics, and a proper design, may improve the profile of the pulses transmitted through an all optical link. It is also shown that the device may reduce the relative statistical fluctuations of the incident optical power.

Large (7x4x4 cm³) crystals of the organic, nonlinear optical material MBA-NP have been prepared by growth from solution, in an optically and structurally highly perfect state. Oriented, cut and polished specimens were examined by the Maker fringe technique. Analysis of the data yielded a coherence length, 1₂₂, of 1.8 μm (cf MNA 1₁₁, = 0.7μm and quartz 1₁₁ =20μm). Type I, phase-matched second narmonic generation (etficiency 50% at 117 MWcm^-2 was observed for 1.06μm radiation incident at 28.8° to the y-axis on 1 the y-z incident plane. There was no evidence of type II phase matching. d₂₂ = 30x10^-12 m.V and d₁₄ is between 1 and 10x10^-12 m.V^-1.

2-(N-prolinol)-5-nitropyridine (PNP) is a nonlinear optical organic material. Its linear and nonlinear optical properties have been investigated. The refractive indices and the orientation of the index ellipsoid were measured in the spectral range from 488 to 1064 nm. At 632 nm the main refractive indices are n_x = 1.990, n_y = 1.788, n_z = 1.467. The two major nonlinear optical susceptibilities were determined (d_yxx = 48 pm/V, d_yyy = 17 pm/V). Angle tuned phase-matched second-harmonic generation was demonstrated at 1064 nm. The influence of beam walk-oft for the efficiency of this process is discussed. Type I noncritical phase matching using the nonlinear optical susceptibility d_yxx is predicted for a fundamental wavelength λ = 1020 nm.

A molecular design method of second-order nonlinear optical materials for SHG device applications of diode lasers is proposed. The method consists of a semi-empirical MO calculation (CNDO/S3-CI) to evaluate molecular nonlinear optical properties, i.e. hyperpolarizability and the lowest excited energy, and an intermolecular interaction energy calculation to evaluate its crystallization property, in particular whether it crystallizes in a centrosymmetric structure or not. In the latter calculation, the empirical atom-atom pairwise potential functions, for example Lennard-Jones type, are used to evaluate intermolecular interaction energy of two molecules or the crystal energy. Using this method we design and synthesized novel second-order nonlinear optical materials for our target applications: xanthone and benzophenone derivatives which have relatively large optical nonlinearity and very short absorption cut-off wavelength.

The growth and characterisation of fibres containing a crystalline core of the nonlinear organic compound DAN in silica and higher refractive index capillaries is described. In addition to measuring the optical properties in transmission a method is described of measuring the sideways scatter from such fibres in order that a fuller understanding be made of factors which limit the achievement of very high SHG efficiencies.

Measurements of optical second-harmonic generation from well ordered Y-type Langmuir-Blodgett (LB) multilayers of 2-docosylamino-5-nitropyridine (DCANP) are reported. The layers have a good optical quality over the whole film area. They are transparent in the visible and the near infrared. The birefringence and nonlinear optical susceptibilities have been investigated. The anisotropy of the second-harmonic efficiency indicates that the chromophores are aligned along the dipping direction.

By frequency-doubling of a pulsed dye laser iii beta-bariumborate, ultraviolet radiation down to 205 nm can be generated with high efficiency. A simple method of sum frequency generation (ω + 2ω + 3ω) extends the wavelength range close to 197 nm, in a cooled crystal even to 195 nm. The most interesting excimer laser wavelength 193 nm can be obtained by sum frequency generation using an infrared dye laser and an excimer laser operating at 248 nm. The lowest wavelength obtained in this way is 189 nm.

Guided-wave parametric interactions require the matching of phase-velocities of the interacting waves. Often this is possible only by using the inherent birefringence in the waveguide to offset material dispersion, as in the case of LiNbO₃. This places very severe tolerances on the guide dimensions and tends to reduce the effective length of a practical device. An alternative technique is to use waveguides with spatially periodic properties to allow cancellation of phase-mismatch. Periodic structures are commonplace in conjunction with waveguides, either as passive wavelength filters or as linear electro-optic mode-converters. In this paper, recent results on the use of a periodic structure for phase matched harmonic generation in isotropic single-mode fibres are presented. We have observed second-harmonic generation by applying a static-electric field across silica optical fibres to induce a centre of asymmetry. Phase-matching between all propagating modes has also been demonstrated by using a periodic structure. Experimentally, a metal overlay periodic-electrode is rotated to select a particular mode-interaction for frequency doubling. This powerful technique can allow phase-matching in non-linear waveguides, even with a very large phase-mismatch, and is applicable to many types of non-linear inorganic, organic-crystalline/polymeric or glassy materials.

In this paper we present first results on intracavity frequency doubling of a diode-pumped Nd:YAG laser at λ=1.064 gm with single domain KNbO₃ crystals. Potassium niobate, with its large nonlinear optical coefficients, is well suited for efficient frequency doubling of light from various near infrared lasers. Using the coefficient d₃₁=-15.8 pm/V the second harmonic of the Nd:YAG wavelength (= 1064 nm) can be generated. Non critical 90° type I phase matching can be achieved by heating the crystal to a temperature of 188°C. The width at half maximum of the temperature tuning curve was measured to be 0.5°C. For the frequency doubling experiments reported here we mounted the 6.2 mm long KNbO₃ crystal in a small, temperature stabilized oven at the center of a 10cm long confocal resonator. The mirrors had a reflectivity at 1064 nm of 99.5%. One mirror was used as an output coupler for the frequency doubled radiation (Transmission factor of 80% at λ=532 nm). An end pumped configuration was used. The pump radiation, with a wavelength of 810 nm was focused through the other mirror in a 5 mm long Nd:YAG rod. An output power of 2 mW at λ=532 nm has been measured for a laser diode power of 200 mW.

The cw second-harmonic generation (SHG) properties of three nonlinear crystals were in-vestigated: KTP, BBO and LiI0₃. Therefore we built up a special Nd:YAG laser resonator producing a beam waist at the crystal location by an intracavity focusing lens and so increasing the intensity at the SHG crystal. Another advantage of the used resonator design is the possibility of coupling out the whole generated second-harmonic power. The obtained power outputs at 532 nm were greater than 10 W for KTP and about 1 W for BBO and LiI0₃.

Two beam coupling experiments with 29-ps (FWHM) pulses at a wavelength of 1.06 μm have been performed in GaAs and Si. By optimization of pump-probe energy ratio, time delay and fluence we obtained optical gain up to 25 in GaAs at a fluence of 25 mJ/cm compared to roughly 100 mJ/cm2 required to obtain similar gain in Si. Optical amplifiers with high gain for cw radiation have been demonstrated over the past ten years using the photorefractive effect in BaTiO₃, KNb0₃, Bi₁₂Si0₂₀ and GaAs. Amplification in these materials, which occurs even when the probe (or signal) and pump are of equal intensity, is produced by the shifted grating found in photorefractive materials when diffusion is the dominant transport mechanism or obtained by a frequency offset between the beams when drift is the dominant transport.

Several methods are investigated to avoid undesired erasure during read-out of volume phase holograms stored in photorefractive crystals. We present results on thermal fixing and on the utilization of two-step excitations. Furthermore we propose two new methods to read volume holograms nondestructively using light of low photon energy not sufficient to excite charge carriers.

Interferometers containing phase-conjugating elements are providing some interesting properties. Due to the phase front correcting abilities of the phase-conjugating mirror they can compensate for several types of disturbances. In our experiment, a photorefractive BaTiO3 crystal served as self-pumped phase-conjugating mirror and a phase-conjugating interferometer was used to detect phasefront aberrations introduced by a transmissive object.

A general expression for the photorefractive gain under step like electric field is derived. It shows the gain dependence on the applied field frequency which is confirmed experimentally. We show how optimum gain enhancement is achieved by adjusting the illumination and the field frequency to the characteristic times of the crystal. Simultaneous contribution of holes and electrons is also considered to explain reduced values of the gain observed in some cases.

The holographic recording mechanism as well as nonstationary energy transfer between the interferense beams and selfamplification of holograms were investigated for the multy-component polymerizable material FPK-488.

Energy exchange between counter propagating light waves of equal frequency is possible in media with instantaneous and local nonlinear optical response which, however, provides a special form of nonlinearity tensor. The major requirement is that there has to be no Lagrangian describing the light-medium interaction. There is a limited region of the pump wave intensity where the signal wave amplification takes place. The thermal mechanisms of interaction between light waves and liquid crystals provide examples of "non-Lagrangian" nonlinearity.

We determined switching times vs. excess power of a thermooptical bistable self-electro-optic effect device (SEED), operating as a logical AND gate. The transient behaviour is mainly determined by heat diffusion in the sample. Observed switching times are compared with a numerical solution of the heat diffusion equation. It is shown, that the frequently used relaxation-time-approximation, with its assumption of a constant relaxation time, is in good agreement with the experimental data only in the "ON-switching" mode of the SEED-element. For the "OFF-switching" of the device, the change of the shape of the transversal temperature profile has to be taken into account.

The optical and nonlinear optical properties of 2-cyclooctylamino-5-nitropyridine (COANP) have been investigated. The refractive indices exhibit strong dispersion and a high birefringence (0.15 - 0.3) in the visible. The three main refractive indices at λ = 550nm are n_a = 1.670, n_b= 1.875 and n_c= 1.683. Calculations carried out show interesting critical (frequency-doubling of λ = 1064 nm (Nd:YAG laser)) and noncritical phase-matching properties. The nonlinear optical susceptibilities d₃₂ = (32±16) pm/V and d33 = (13.7±2) pm/V were measured.

Large crystals of DAN, (2-3 cm diameter, 2 cm long) with a very low defect concentration have been grown by the Bridgman technique. Oriented, cut and polished specimens examined by the Maker fringe method showed both types I and II phase matching. Tentative refractive index values at 1064 nm were deduced from previously reported indices at 532 nm by a combination of Maker fringe analysis and interpretation of the angles of incidence for phase matching. From these results it is concluded that the phase matching locus is probably of the Hobden type VII.

We have studied optical bistability in Ti0₂/Si0₂ interference filters ("hard coatings"). These systems compare favourably with the more conventional ZnSe filters in important characteristics, particularly in durability, switching contrast and long term stability. Unfortunately, switching is very slow. Our analysis reveals a unique mechanism: water molecu)es in pores of the coating are reversibly desorbed from well below the outside surface as the spot temperature is driven up and down by the irradiated light.

Generally barium titanate is used in holographic measurements (two-wave mixing, light diffraction, phase conjugating mirror...) with no external device. The large values of the electrooptic coefficients lead to good results . In 1980 few authors (Feinberg, Kratzig) showed briefly the effect of an applied electric field on photorefractive barium titanate crystals, in diffraction efficiency experiments. In this paper, the diffraction efficiency of coherent light (514 nm) on single domain BaTiO₃ sample is studied as a function of the applied electric field. The enhancement of the effi-ciency with the applied d.c. field and the behaviour of the phaseshift between the photoin-duced refractive index grating and the interference light pattern are studied. Many differences are observed between Fe-doped samples and nominally pure ones. From these experiments, the limiting space charge field Eq, the density of trapped centers N, and an experimental value of the diffusion field Ed are determined.

Optical bistability has been observed in highly concentrated fluorescein dye solution and in thin (~1 μm) doped polymeric films. For fluorescein, at concentrations larger than 10^-5 mole/1 dye dimers are formed. The dimer-monomer equilibrium constant is 10⁵1/mole so that most of the dye species are in the dimer form. At 480 nm the dimer absorption cross section is 10^-18cm²/molecule, while that for the dye monomer molecule is 7.6x10^-17cm²/molecule. Upon laser excitation dimers dissociate to form monomers thus providing a highly nonlinear laser induced absorption. The high nonlinear absorption coefficient can be utilized for optically bistable response of the dye system. Optical bistability was observed by placing dye solutions or dye thin films inside a Fabry-Perot resonator and exciting it with 480 nm dye laser pulses of 10 ns duration. The effect is more pronounced in 10^-4; mole/1 fluorescein than in 10^-6mole/1 fluorescein in which dimer formation is not that efficient. In disodium fluorescein, eosin and erythrosin dye samples no significant dimer formation is observed even at 10^-3 mole/1 dye concentration. The observed bistability in both solution and in thin films can be explained in terms of recent models for optical bistability in nonlinearly absorbing molecular systems.

Nonlinear propagation effects in birefringent optical fibers are investigated and their implications for generating ultrashort light pulses in the IR are discussed. Stable compression of intense Nd:YAG laser pulses is obtained due to combined action of self-phase modulation, stimulated Raman scattering, and group-velocity dispersion. First operation of a fiber-grating Raman laser with subpicosecond output is demonstrated in the regime of positive group-velocity dispersion.

Using 25 ps pulses of Nd:YAG laser with tunable wavelength we studied relaxation processes, diffusion and fieldstrength dependent carrier mobility in semiconductors by different ps techniques. The sensitivity in reflectivity and transmittance experiments was improved by optimization of polarization, angle of incidence and wavelength of a probe beam. Using different carrier distributions we were able to distinguish between nonlinear and linear bulk and surface recombination and ambipolar diffusion, for example in crystalline silicon. At ps excitation the photoconductivity was measured using a correlation technique. The first optoelectronic switch of the correlator was a high mobility semi conductor to achieve extremely short electrical pulses (down to 6 ps) a part of the gap was covered by a light-tight layer. We measured the linear and nonlinear recombination rates in amorphous silicon, SOS, GaAs, GaSe etc. Furthermore, we have estimated the mobility-recombination time product for various semiconductors. Furthermore, we measured the dependence of carrier mobility on the applied field strength in GaAs.

In addition to results of earlier experiments where Bragg scattering from a distributed feedback dye laser is observed, in the experiment described here the occurrence of a phase conjugated reflected beam is demonstrated. It is shown that a distributed feedback laser can be used as a frequency selective reflector for a seperate dye laser. Some time ago Golub and Shuker investigated the feasibility of optical switching by Bragg scattering of a light beam from a spatially modulated gain grating. This grating formed the basis of a distributed feedback dye laser (DFDL) and the reflection was only visible if the DFDL was pumped above threshold. In that case reflectivities of more than 100% for small input signals could be obtained. Golub and Shuker emphasized that in their experiment phase conjugation could not occur. Their main argument was, that the probe beam and the pump beams had completely different origins. Consequently these beams had no constant phase relation, and hence degenerated four-wave mixing leading to phase conjugation was excluded.

In previous works we have demonstrated that the MQW-coupled directional coupler is a promising design for all-optical switching elements. Experimental analysis has revealed that an efficient intensity-dependent coupling can be achieved between the two waveguides for a sample length as short as 127 μm. However, due to the presence of two competing nonlinearities, namely excitonic and thermal, there is some concern over the nature of the switching. W e report here on results which support our claim that the dominant mechanism in this case, where we are operating at low powers near the heavy-hole resonance peak, is the faster excitonic nonlinearity.