Dopants may influence light-induced refractive index changes - so-called photo-refractive or optical damage effects - in LiNbO3 by different means. In this contribution we investigate consequences of transition metal ions acting as filled and empty traps, of Ti4+ centers and protons which are necessary for the formation of waveguiding layers and of Mg2+ ions reducing optical damage effects.

Experiments were performed to investigate the photorefractive effect at large light intensities. Different LiNbO3 crystals were exposed to a focused argon ion laser beam and the refractive index changes generated were measured with a Mach-Zehnder Interference microscope. We Investigated nominally pure L1NbO3 with different Li/Nb ratio and Fe:LINbO3 with different iron concentration. For all crystals we find an increase of the index change with increasing intensity up to values larger than iO. According to the usual model of the photorefractive effect an intensity independent refractive index change is expected. The iron doped samples show the expected intensity independence up to intensities of 50 W/cm2. Below this value the spatial index profile is compatible with the model. At larger intensities, the additional Index rise is unexpected and exhibits a different spatial Index profile.

Peculiarities of crystal growth and wave-mixing in pure and doped photorefractive strontium barium niobate crystals are 1nves tigated. Some problems o their application in dynamic holographic interferometry and vibrometry are considered.

optical wave mixing in nonlinear media such as photorefractive crystals, semiconductors and liquid crystals have been vigorously studied in the past few years (1-4). Such researches are motived either by potential applications like amplification and phase conjugation, or by investigation on the physics of such effects. Here we report on the generation of conjugate wavefronts by degenerate forward and backward four wave mixing in some GaAs samples. We perform our experiment with a Nd-YAG laser emitting pulses of eight nanosecond duration. The semi-insulating GaAs sample used exhibits a large concentration of EL2 defects which present a good photoionisation cross section at 1.06 micron. The nonlinearity we observed is due to a hole and electron grating photogenerated from the EL2 defects. This is quite different from direct band to band absorption. Indeed, the hole and electron distributions are coupled through the concentration of ionized EL2+ and neutral EL2° defect. Nevertheless, as we will show,at high energy the densities of free electrons and holes tend to be equal due to an evolution of the respective concentration of EL2+ and EL20. This plasma of holes and electrons modifies the plasma frequency. The modulation of the index is then derived from the DrUde Lorentz model. We first present our experimental results .We show that most of them can be explained by the kinetics of the carriers photogenerated from deep levels. Analytical and numerical results are presented. From the numerical simulation, with these numerical results and semi analytical solutions of the wave propagation equations, one then predicts the reflectivity of our multiwave mixing experiment.

optical wave mixing in nonlinear media such as photorefractive crystals, semiconductors and liquid crystals have been vigorously studied in the past few years (1-4). Such researches are motived either by potential applications like amplification and phase conjugation, or by investigation on the physics of such effects. Here we report on the generation of conjugate wavefronts by degenerate forward and backward four wave mixing in some GaAs samples. We perform our experiment with a Nd-YAG laser emitting pulses of eight nanosecond duration. The semi-insulating GaAs sample used exhibits a large concentration of EL2 defects which present a good photoionisation cross section at 1.06 micron. The nonlinearity we observed is due to a hole and electron grating photogenerated from the EL2 defects. This is quite different from direct band to band absorption. Indeed, the hole and electron distributions are coupled through the concentration of ionized EL2+ and neutral EL2° defect. Nevertheless, as we will show,at high energy the densities of free electrons and holes tend to be equal due to an evolution of the respective concentration of EL2+ and EL20. This plasma of holes and electrons modifies the plasma frequency. The modulation of the index is then derived from the DrUde Lorentz model. We first present our experimental results .We show that most of them can be explained by the kinetics of the carriers photogenerated from deep levels. Analytical and numerical results are presented. From the numerical simulation, with these numerical results and semi analytical solutions of the wave propagation equations, one then predicts the reflectivity of our multiwave mixing experiment.

Coherent beam coupling in photorefractive crystals leads to an energy transfer. An analytical expression of the c.w. gain can be calculated from the charge transport equations for a crystal under illumination. The experimental results of gain versus grating wavenumber is fitted with this expression and lead to a determination of the material parameters. Such an analysis has been performed on semi insulatin GaAs and InP. For this last crystal this study reveals the important role of the excited state of Fe (5T,).

Oscillat io n intensit y kinet ics in act ive system containing nonlinear holographic mirror on lithium niobate photorefractive crystal are investigated both theoretically and experimentally. Self-starting oscillation and also regime with external control of oscillation spatial structure are realised experimentally using copper vapor laser tube as an optical amplifier. The existence of local response in photorefractive crystal is shown to be essential for efficient optical control of oscillation spatial structure in the active system.

We investigate the phase-conjugate ring-resonator (PCR) consisting of a Sagnac interferometer and a phase-conjugating mirror, realized by four-wave mixing in a BaTiO3-crystal. Under certain conditions the effect of self-oscillation occurs with spatial and temporal fluctuations, but a stable integral intensity. The self-oscillation pattern is only determined by properties of the system and contains important information of the PCR. The Grassberger-Procaccia analysis and different qualitative tests indicate the self-oscillation pattern as a spatial-chaotic structure with a regular temporal behaviour.

Read-out of volume phase holograms in photorefractive crystals with a wavelength that is different from the wavelength of hologram formation imposes the difficulty to match the Bragg condition for all spatial frequencies of the object to be stored. Therefore, the quality of the reconstructed image decreases and resolution and image field losses are observed. Experimental and theoretical examinations of these effects reveal that the geometrical configuration of the writing experiment determines the reconstructed image field. Depending on the wavelength of read-out an appropriate reconstructing wave can be formed with the help of lenses diminishing image field and resolution losses considerably.

Linear and nonlinear optical properties of well ordered Y-type Langmuir-Blodgett (LB) multilayers of 2- docosylamino-5-nitropyridine (DCANP) have been investigated. The nonlinear optical susceptibilities have been determined for the wavelengths = 1064 nm (d33 = (7.8 1) pm/V. d31 = (2.0 0.5) pm/V) and ? = 1318 nm (d33 = (5.6 1) pm/V). The optical homogeneity was investigated using secondharmonic generation. Waveguiding experiments allowed the determination of the dispersion of the refractive index fl3 (n3632'8"'= 1.598). 1'E and TM modes propagating over more than 20 mm could be excited. Guided wave attenuation coefficients as low as 12 dB/cm (at the wavelength = 632.8 nm) could be measured.

This paper describes a joint study of the structure and nonlinear optical properties of vacuum evaporated thin films of copper phthalocyanine (CuPc for brevity). Film thickness ranes from 50 to 500 nm. The anisotropic paramagnetic resonance of Cu ions reveals that the Pc rings lie almost parallel to the substrate plane with however a large angular distribution (300 FWHM). Third harmonic optical generation measurements performed at 1. 064 im and 1. 907 .tmfundamenta1 velengths give respectivel2 an average value of the cubic susceptibility = (4 0. 4) • 10 e. s. u. and (2. 1± O.2)• 1O e.s.u. These values, although significantly higher than for a common ionic crystal, are about one order of magnitude lower than in conjugated 1-D systems, which shows that the 2-D u-electron delocalization is less profitable than the l-D one. Besides third harmonic, we have also observed second harmonic generation. Its polarization dependence is characteristic of a quadratic susceptibility enhanced in one direction, almost pependicular to the substrate, with d 2 1O e.s.u. The possible origins eff of d are discussed. eff

In the present work we analyse the nonlinear, nonlocal response tensor describing optical second-harmonic processes in centrosymmetric free-electron-like bulk metals with a flat surface. On the basis of the classical infinite barrier (CIB)-model and the Boltzmann equation in the relaxation time approximation we present new analytic results for the fully nonlocal nonlinear response tensor. Via numerical calculations the nonlinear, fully nonlocal response tensor's dependence on the fundamental frequency is discussed and compared with that of the near-local (hydrodynamic) response tensor. Finally, the significance of the contribution to the nonlinear, nonlocal optical response stemming for single-particle excitations, i.e. Landau interactions, is considered.

Non-linear waves in dielectric Kerr-type waveguides depend strongly on the initial excitation conditions. The well-known z-homogeneous, stationary modes, which are similar to the linear ones, are not immediately obtained in practice, because highest order modes appear too in the excitation process. In this work some results about light coupling into Kerr-type guides, obtained by means of a numerical procedure, are presented.

Methods of creation of wide angular converters of laser radiation frequency (with lowered sensibility to radiation divergence) are discussed.

Transitions between the subbands of quantum wells have extremely large oscillator strengths. Since second-order nonlinear optical coefficients are proportional to the square of oscillator strength, strong nonlinear effects are expected in such systems, provided inversion symmetry is broken. This paper reviews the main results (optical rectification, second harmonic generation) obtained in structures in which the symmetry breaking has been realized by growing AlGaAs multiquantum wells with asymmetrical Al gradients. We show that, in these structures, the dipole matrix elements have magnitudes comparable to the well thicknesses, i.e. in the few nanometers range instead of the few picometers usually found in molecules. These Compositionally Asymmetrical multiquantum wells may thus be viewed as giant "quasi-molecules" optimized for optical nonlinearities in the mid infared.

A phase-conjugate (PC) mirror on the basis o InAs in a cavity o a TEA CO2 laser was studied. Efficiency B o such a PC-mirror is 1O% ror the signal coherent to the pump waves and 3-5% ror the incoherent signal. The PC fidelity or a signal wave in the scheme studied is shown to be as high as 1 00%. The insertion of Inks plate into the laser cavity is round to result in : i) the stretching o the FWIIM pulse duration, ii) the stabilization of the mean intracavity radiation intensity, iii) the Lonnation o the hypergaussian intracavity beam profile due to strong nonl inear absorp t ion in Inks ; and due to the nonl inear change of index in Inks iv) the diffraction of the laser radiation on intracavity apertures and v) the transition from periodic to chaotic pulsations of the output power.

Two kinds of efficient acousto-optically phase-matched non collinear optical SHG are predicted in Te for the incident optical wave at lO,6p in. The first one is calculated with the shear acoustic wave propagating along the X localized by the three Euler angles cp=° , 0=1 500, 1600 , 4 450, and is interpreted as being due to mixing of the incident optical wave and the acousto-optically scattered wave. The other SHG is observed with the same slow acoustic shear wave and is due to the mixing ot the two acousto-optically scattered optical waves. It is shown from absolute SHG calculations and corresponding SF10 measurements previously reported in Te, that the non linear optical coefficient which governs these high efficiency SHG interactions is d1 2 = 920 x 10- '2mfV The corresponding SHG efficiencies are two orders of magnitude larger than the previously reported phase-matched non-collinear SHG in Te for the same input optic power and acoustic conditions. Another highly efficient SHG configuration in Te is discussed as well as the possible application of these techniques to a parametric oscillator and to a fast infrared two dimensional optical inverter array.

In connection with a recently obtained quantum mechanical expression for the optical second-harmonic response tensor (OSHRT) for a BCS-like superconductor, we present a Taylor series expansion of the OSHRT to lowest order in the wave vector of the fundamental electromagnetic wave. It is shown that in the limit T > Tc , the Taylor series expansion of the OSHRT is identical to that obtained in the collisionless limit from the Boltzmann equation approach in the near-local regime. The dispersion relation for waves at the fundamental frequency is obtained using a Taylor series expansion of the linear conductivity tensor. Numerical results are presented showing the frequency dependence for parts of the obtained near-local OSHRT involving transversely polarized fundamental waves. In the superconducting state an increase in the OSHRT in comparison to that in the normal state is found for frequencies above the gap frequency. Below the gap the OSHRT is smaller in the superconducting phase than in the normal phase.

We report the observation of off-resonance electronic energy transfer from an excited rhodamine 6G molecules to an azulene molecule in solution. The process requires the coupling of the radiation field in the transfer process. We demonstrate that the relation YAcCI1"2 between the acceptor's induced fluorescence intensity and the laser intensity, predicted by Jortner et al. is observed.

A pump and probe experiment is described which allows to study input coupling efficiency of a CdS grating on top of a linear glass waveguide as a function of input power for wavelengths shorter than CdS band gap wavelength. A monotonous decrease of coupling efficiency is found by increasing the pump power. Reference is made also to results obtained for wavelengths larger than the band gap wavelength.