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.

Electro-optic half-wave voltages have been calculated for a longitudinal geometry in crystals of point group symmetry mm2 (orthorhombic) and 4mm (tetragonal). The predicted minimal longitudinal half-wave voltages range from 194 V in KNbO3, 100 V in Sr0,75Ba0,25Nb2O6 and 40 V in BaTiO3 to 9 V in KTa1-xNbxO3. The angular, wavelength and temperature dependences of the half-wave voltage have been determined. In KNbO3 measurements of the minimal longitudinal half-wave voltage at room temperature reached (195±8) V and (118±7 V) with red (λ=633 nm) and blue light (λ=458 nm) respectively. At -40°C, close to the orthorhombic-rombohedral phase transition, the half-wave voltage for red light reduces to (135±6) V. From the measurements we determine a negative sign of the electro-optic coefficient r42=-380 pm/V. The possibility of spatial light modulation by the longitudinal electro-optic effect is discussed.

We shall report on the energy levels of several 3d impurities in the gap of LiNb03. The sequence of their relative level Rositions has been established showing that Mn3+/2+ is lowest, followed by Cu2+/+, Fe+/2+ and Ti4+/3+. Cr3+/2+ and Co2+/+ are probably lying close to or within the conduction band. The ionisation energy of Ti3+ has been determined to be 0.25 eV. Furthermore we shall report on the EPR of Ti3+, including a new signal, which is due to Ti3+ in rutile-like environment caused by a not finished indiffusion process.

Le increased photoconductivity in highly Mg-doped LiNb03, suppressing optical damage is attributed partly to a decrease of the electron-phonon coupling, leading to enhanced carrier mobility. This is concluded from investigations of the dark conductivity of reduced LiNbO3, X-ray-induced luminescence, fundamental and below bandgap absorption and of the frequencies of the lattice modes.

Strongly nonlinear optical and fast responding materials are required for real-time optical information processing. KNbO3 has been shown to have very fast photorefractive response times and good photosensitivity. With the electro-chemical reduction method developed recently we were able to improve the photorefractive properties (photoconductivity, beam coupling gain, response time) of KNbO3 by changing the valencies of iron-impurities. The preparation of large monodomain crystals with good optical and photorefractive homogeneity for photorefractive incoherent to coherent optical conversion (PICOC) is described. The image conversion was achieved by the anisotropic self-diffraction technique. The optimal crystal configuration for this application has been experimentally verified. A resolution of up to 10 linepairs per mm has been achieved for converting a white light image onto a laser beam.

Electron spin resonance spectra, which can consistently be explained by the models Fe4+ - Vo and Fe5+ - vBa, have been observed with BaTiO3 containing Fe. Also Fe3+ and - Fe3+ - Vo have been identified. All these centers as well as several unidentified ones are observed to change their charge states under illumination. They thus are possibly in involved in photorefractive processes in BaTiO3 containing Fe.

The essential features of the photorefractive phenomena in electrooptic oxides are reviewed. Emphasis is laid on the active point defects and their role in determining physical behaviour. The situation is illustrated in some detail for LiNb03, where a great deal of information is available.

The discovery of ferroelectricity in smectic C liquid crystals, in 1975, has introduced a new electrooptic effect, driven by linear coupling between the permanent electric polarization of chiral smectic C molecules and the addressing electric field. Ferroelectric liquid crystals allow a reduction of at least 3 orders of magnitude of the switching time with respect to conventional twisted nematics, and yield permanent stable states used for memory effects (surface stabilization for small thickness cells and AC stabilization for small and medium thickness cells). All this has open the way to new and faster devices ranging from highly multiplexed displays to optical shutters. The authors describe the present trends in material synthesis, molecular anchoring and device fabrication. New mixtures have been prepared at Thomson-CSF and other laboratories which yield low viscosity materials by using high polarity chiral molecules as dopants. Response times of less than 40 μs have already been obtained and lower ones are forseable. Small pitch (less than 50 μm) optical shutters will be described and applications such like light modulators and line printers will be discussed.

A set of dynamic optical interconnects and spatial light modulators and controls which are based on photorefractive wave mixing will be discussed. These devices are bi-directional, self adjusting and controllable in real time.

We have fabricated multi-layer thin film structures using Poly 4-Vinyl Pyridine (P4VP) as one of the layers. A "vertical" directional coupler has been constructed in which light is coupled between two layers by an intermediate layer of lower refractive index. In single P4VP layer devices we have measured Pockels and Kerr effects, we have also observed an enhanced Kerr response which passes through a null when a square wave of about 120 V r.m.s. is applied. A single-polarisation thin-film interferometer has been made which shows interference between two discrete modes, one confined mostly in the polymer and the other in a thin Si02 film beneath the polymer.

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.

Thin viscoelastic layers with active-matrix addressing are proposed as high-resolution spatial light modulators (SLM's) for eventual use in a reflective schlieren light valve. Light modulation is achieved by diffraction from the periodically deformed mirror electrode covering the SLM layer. A two-dimensional array of Si-MOS transistors will be employed for addressing the viscoelastic SLM. Orthogonal and diagonally offset arrangements of pixels with two grating periods each are suggested for this active matrix. For measuring the deformation behavior of viscoelastic layers with a reflective top electrode, a standard microscope interferometer was modified so that the phase-shift technique could be employed. The deformation profile is recorded by means of a CCD image sensor, while the temporal development of the deformation is detected with a photomultiplier tube. Qualitatively, the experimental results agree with previously obtained predictions from an extensive theoretical analysis of the SLM's time behavior and spatial-frequency response.

We investigate all-optical transmission control of InGaAsP epilayers on InP substrates by optical generation of excess carriers. A modulation depth of more than 40% is achieved at the near-band-gap wavelength of λt = 1.3μm of the 2μm thick epilayer at a power of a few mW in the control beam of λc = 790nm wavelength. The maximum bit rate is limited by excess carrier lifetimes to ca. 100 Mbit/s. Several channels can be operated in parallel. At a distance of 25pm between adjacent spots crosstalk between channels is less than -202 dB. Considering the low power dissipation a density in the order of several 100 channels/mm can be processed in parallel making this device attractive for all-optical data processing. We have used the capabilities of spatial light modulation to demonstrate dynamic holographic beam deflection. A theoretical analysis of the optical transmission control based on dynamic band filling in parabolic bands and direct electronic transitions provides a good description of the experiments.

A technique issued from the Senarmont method is developped in order to determine the electro-optic (E/0) coefficients of solid compounds. The transfer function of the transmitted beam is given by : I/Io=(1/2)[1±sin(Γ-2B], where Γ is the phase shift induced by the natural birefringence and the applied (AC +DC) electric field in the sample I and Io are, respectively, the output and input beam intensities, while B represents the analyser azimuthal angle. We show the various methods derived from the Senarmont arrangement which can be used to determine the E/0 coefficients. The method which provides both good accuracy and correctness is based on a direct determination of the phase shift when the DC and AC fundamental components of I are equal to zero and the frequency of the demodulated output transmitted beam is double of the AC frequency. The measurements take into account of the optical absorption of the set-up and the temperature dependence of the natural birefringence. This method is applied to the determination of the rc coefficient in LiNbO3 around room temperature.

With the exception of paper and photographic film no storage media have been as thoroughly studied and improved as magnetic tape. The result is a large industry providing low cost, high quality media. In recent years the density allowed by high coercivity magnetic media crossed the micron mark thus challenging optical media for their high capacity. Optical recording on a tape offers great advantages as far as readout is concerned for the availability of highly integrated sensors such as CCD. On the other hand, writing is not easy since no individually modulated laser array with sufficient integration is available. Nor the deflection process is fast enough for high data rate recording. Therefore we studied an hybrid solution where recording is insured by a multiplexed array of magnetic heads and readout is optical through a magnetooptic converter. This converter is made of a rare earth iron garnet with perpendicular anisotropy. Domains in this garnet are coupled to the recorded pattern through the strayfield and reproduce it in a given range of spatial frequency. We shall discuss the optimization of various parameters of the garnet, as well as the performances of such a recording technique for high data rate digital recording applications.

A single-mode asymmetric tri-layer weakly guiding film waveguide made of bismuth-iron garnet has been grown on a (111)(Gd,Ca)3(Ga,Mg,Zr)5O12 substrate by rf magnetron sputtering. The stack consists of an epitaxial core, an epitaxial cladding, and an amorphous absorber layer. The thickness and the refractive index of the three layers, and the absorption coefficient of the absorber layer are optimized in terms of low insertion loss of the fundamental mode but strong dissipation of higher modes. Light propagation measurements at 1.3 µm wavelength show that the fundamental mode is well confined to the core layer while the higher modes, extending over the whole stack, are suppressed by at least 29.5 dB. These data are confirmed by theoretical predictions.

Magnetic garnet films can be used as optical waveguides in the near infrared region. Light propagates in the waveguides as TE or TM modes. Due to the linear and circular magnetic birefringences these modes are coupled depending on the direction of the magnetization with respect to the direction of mode propagation. The efficiency of the mode conversion caused by the coupling is calculated. If the ferrimagnetic resonance is excited in the waveguide, a dynamical TE-TM mode conversion and thus a modulation of the light is induced. The spectrum of the modulated light contains besides the fundamental frequency higher harmonics. For a gallium-substituted film of yttrium iron garnet the dynamical mode conversion is measured for different orientations of the magnetization. A modulation efficiency of 6% at a frequency of 1 GHz is achieved. The experimental results are in qualitative agreement with calculations.

Buried channel waveguides in magneto-optic yttrium iron garnet have been prepared for application to waveguide isolators. The channel waveguides are fabricated in a three-step procedure: planar single-mode waveguides are grown, then rib waveguides are wet-etched into the planar waveguides and successively overgrown with top cladding. Light propagation and mode conversion characteristics of such waveguides have been investigated.

An integrated nonreciprocal mode converter to be used in an optical isolator has been fabricated and tested. This device is realized by sputter etching a microchannel waveguide in a bilayer gadolinium gallium substituted Yttrium Iron Garnet film. Bonded to a heater, in order to control the mode phase matching, this component is packaged with single mode polarization maintaining optical fibers stuck at the input and output of the magnetooptical waveguide. Using bulk high quality polarizers, a 23 dB isolation ratio was obtained at λ = 1.55 μm.

It is shown that light, initially linearly polarized, becomes endowed with elliptical polarisation on traversal of an antiferromagnetic crystal acted on by a static magnetic field (induction B). The ellipticity can be linear in B, thus being the Magnetic Analog of Pockels' Effect (MAPE).

Analysis of light intensity modulation problem related to various degrees and states of polarization is discussed in the present paper. The emphasis is put on investigation of modulation depth in elliptically polarized light beam, because of this problem being important in design of light amplitude modulators. In result, the simple corelations between modulation depth and polarization ellipse parameters as well as modulation depth and polarization degree are found. Next, some problems of magneto-optical modulation of partially polarized light are discussed. In result, a procedure to obtain of optimal polarization arrangement for magneto-optical amplitude modulation is presented.