Journal of Nonlinear Optical Physics & Materials

New cumulative effects in laser-induced birefringence have been observed under 10-Hz-pulse-repetition-rate, nanosecond-duration laser irradiation of azo-dye-doped planar-nematic liquid crystal layers at incident intensities I ~ 1–10 MW/cm². An irradiation geometry with the incident polarization parallel to the nematic director was used. his geometry does not permit a first-order electric field induced reorientation of the nematic molecules, allowing us to exclude its contribution to the nonlinear response. New laser-induced birefringence effects with a buildup time of several seconds to minutes manifest themselves in:

• the appearance of a polarization component perpendicular to the nematic director;

• two different modes of spatial pattern formation with different patterns for parallel and perpendicular polarization:

(1) At I ~ 1–5 MW/cm², the perpendicular polarization component forms a four-leaf-clover (a Maltese-like cross) spatial pattern in the far-field from the initial Gaussian spatial intensity distribution. The incident, parallel polarization component forms a round spot with a single ring spatial pattern.

(2) At higher incident intensities (I ~ 5–10 MW/cm²), a second regime of pattern formation is observed in the form of high definition patterns and only for the polarization component parallel to the nematic director.

Vortex quadrupoles and a nonlinear optical cusp diffraction catastrophe were observed at the output face of a self-defocusing nonlinear medium. The initial beam had a smooth cross-sectional intensity profile, but was elongated to an aspect ratio of 2:1. The power-dependent evolution of the beam is described.

Various poly(amic acid)s with azobenzene-chromophore sidegroups have been synthesized and structurally and functionally characterized. Their amphiphilic properties allowed us to prepare stable monomolecular layers at the water/air interface of a Langmuir trough, and to transfer these highly organized monolayers to solid supports via the Langmuir–Blodgett–Kuhn deposition protocol. The resulting multilayer assemblies were investigated by surface plasmon- and waveguide-optical techniques, by X-ray reflectometry, and by UV-vis and IR spectroscopies. Thermal imidization of the assemblies resulted in functional poly(imide) multilayers that still could undergo photoisomerization reactions in their azobenzene sidegroups. The kinetic parameters of this trans-cis and cis-trans isomerization, respectively, as well as, the resulting control of the alignment of a liquid crystal in contact to these "command layers" were evaluated.

We present an overview of the physics of self-written waveguides created in photosensitive optical materials, including the experimental observations and the corresponding theoretical models for describing the growth of both bright and dark self-written beams. We discuss in more details the properties of self-written waveguides created in photosensitive polymers, which have been discovered in the recent experimental and theoretical studies. The self-writing process is essentially a nonlinear phenomenon, since the temporal dynamics depends on the optical exposure. Under appropriate conditions, permanent large changes in the refractive index are induced along the propagation direction of an optical beam, so that optical channels, or "filaments", appear as waveguides becoming "frozen" in a photosensitive material. We describe the growth of individual filaments as well as the interaction of several filaments, also making a comparison between the physics of self-written waveguides and the concept of spatial optical solitons in self-focusing nonlinear media.

The propagation of ultrashort laser pulses into a microstructured fiber having a large nonlinear coefficient is found to give rise to various nonlinear phenomena. By using an input wavelength in the range 1300–1550 nm, the observed output spectrum presents, besides supercontinuum generation, sharp lines due to third-harmonic generation and four-wave mixing. When the input wavelength is around 800 nm, close to the zero-dispersion wavelength, the solitonic propagation is accompanied by a very intense and narrow-band emission in the blue spectral region.

The optical properties of nanoscopic arrays of metal particles are dominated by plasmon resonances and electromagnetic interaction between the particles. We use electron-beam lithography to prepare arrays of noncentrosymmetric gold particles and study their linear and second-order nonlinear optical properties. By varying the orientation of the particles in a fixed lattice, we observe shifts in the polarized linear extinction spectra. The second-harmonic generation efficiencies of the two types of samples differ by up to 60%. The results show that the properties of the samples are sensitive to the smallest details of their structure.

Nonlinear processes in quantum well infrared photodetectors (QWIP) are reviewed. Being an intersubband dipole transition based photoconductor, the nonlinear behaviors in QWIPs are caused by both the (extrinsic) photoconductive transport mechanism and (intrinsic) nonlinear optical processes. Extrinsic nonlinearity leads to a degradation of QWIP performance at high incident power or low operating temperatures. Some intrinsic nonlinear QWIP properties are useful in applications, such as in autocorrelation of short pulses by two-photon absorption. The general area of QWIP nonlinear properties has not been extensively investigated. We point out some directions for further studies and hope to stimulate more research activities.

We review our results on conversion of CW laser Ar-ion beam power into pulsating multi-channel outputs: optical, electric and piezoelectric with simultaneous dynamic pattern formation. We show, that electrical pulsations generate high-voltage signals that may be used as a driving voltage for the conventional electro-optical modulator. We have also demonstrated the possibility of synchronization of two optical pulsators, through regulated optical coupling in a photorefractive LiNbO₃ crystal. Spatial distribution of scattered light is self-organized in different patterns (hexagonal and cross-type).