Tigran Galstyan

Novel Azo Dye-Doped Poly(Methyl Methacrylate) Films as Optical Data Storage Media.

Azo dye-doped polymer (ADP) systems have been shown to be remarkable photosensitive media for holographic recording and optical computing. In this work we have studied the dynamic holographic capabilities of poly(methyl methacrylate) (PMMA) fims doped with five new azo dyes named AZD1-5. The totally reversible trans \rightleftharpoons cis photoisomerization induced by exposure of the ADP samples to actinic light allows recording of holograms in the dye-doped polymer matrices, and diffraction efficiencies up to 10% have been attained. These ADP systems can also be used to perform polarization holographic recordings as they present a photoinduced anisotropy called the Weigert effect. Depending on the intensity of the reading beam, the recorded information can be rapidly erased or stored for a posteriori use. The storage time can be as long as 800 s depending on the azo dye employed. These photosensitive materials can be classified as promising holographic media for optical computing and optical mass storage.

Photoexcited azo-dye induced torque in nematic liquid crystals

We study molecule reorientation initiated by absorption of resonant light, in D2 azo dye doped nematic liquid crystals. The photoexcitation of the dye subsystem creates a torque on the host, that is not collinear with the traditional dielectric torque. The temporal and geometrical characteristics of these torques are also different. Transient three dimensional molecular reorientation is detected through different paths for excitation and relaxation regimes. The complex coupling of the photoisomerizing dye with the nematic host is probably the origin of the observed phenomena.

Diffusion of photoexcited azo dye in a liquid-crystal host

Dynamic holograms are created and studied in a D2 azo-dye-doped nematic liquid crystal. The dependence of diffraction on the excitation and probe polarizations is observed. The efficiency of excitation and the corresponding relaxation time depend also on the period and orientation of the light-interference pattern with respect to the initial molecular orientation. The excitation and diffusion of dye molecules are suggested to be the origin of these phenomena.

Polarization recording and reconstruction in Disperse Red 1 -doped cellulose acetate.

Cellulose acetate films doped with the azo dye Disperse Red 1 were prepared by dip coating. This matrix provides holograms of comparable efficiency for gratings recorded by intensity and polarization modulations. We used this matrix to store and successfully reconstruct the polarization state of an elliptically polarized object beam. A single plane-polarized reference beam was used for both recording and reconstruction.

NONRECIPROCAL DIFFRACTION BY SPATIAL MODULATION OF ABSORPTION AND REFRACTION

Linear diffraction gratings that provide strongly asymmetric diffraction without surface modulation are created and studied. The spatial phase shift of the refractive-index grating relative to the absorption grating is the origin of nonreciprocal behavior.

Error-Diffusion Binarization for Neural Networks

In optical implementation of neural networks, binarization is often a necessity. Error-diffusion (ED) is presented has a more reliable binarization technic over hardclipping. We use a simple self-organizing learning algorithm for its demonstration. It is showed in particular that ED keeps more of the original image characteristics upon variable lighting, thus enabling a better extraction of the common features in a set of processed images, and a faster learning convergence of the neural net.

Nonlinear Polarization Shaping in a Nematic Liquid Crystal

High efficiency nonlinear polarization conversion in nematic liquid crystal is demonstrated in reflection geometry. The polarization shaping is realized via coupling of four orthogonal polarized beams on orientational transmission phase gratings. The vectorial character of excitations allows a collinear phase matched interaction with a best overlapping of signal and pump beams, thus enhancing the coupling efficiency. The schema allows the continuous control of polarization conversion by the light exposition.