Saswati Ghosal

New copolymers for applications as organic LEDs

During the past five years, (pi) -conjugated organic polymers, such as poly[p-phenylene vinylene] (PPV) and polythiophenes, have become attractive alternatives to semiconducting materials as light-emitting diodes. Various approaches to tuning emission wavelength have been proposed, such as controlling the conjugation length by employing polymer oligomers or via steric effects in the fully conjugated polymers, or by using substituent effects to fine-tune the band gap. In the present study, we demonstrate that these features can be designed into copolymeric structures in which (pi) -conjugated emitters of carefully controlled length alternate with various non-emitting flexible spacers which improve solubility and thus processibility. These copolymers display predictable emission characteristics which can also be fine-tuned by incorporation of electronic substituent effects.

Bifunctional chromophore for photorefractive applications

A bifunctional chromophore has been developed which serves as a charge transporting agent as well as a second‐order nonlinear optical compound. The chromophore has been incorporated into an inert polymer to form a photorefractive material when doped with a photocharge generation sensitizer. The dependence of the four‐wave mixing diffraction efficiency, holographic grating writing rate, and two‐beam coupling gain on the chromophore concentration has been studied. The results confirm the photorefractive character of the composite.

Novel optical composites: second-order nonlinear optical and polymeric photorefractive materials for optical information storage and processing applications

Recent progress in sol-gel-processed second-order nonlinear optical and photorefractive materials for photonics applications is de- scribed. A number of composites have been prepared in an effort to obtain superior and cost-effective replacements for inorganic crystals. These composites are described followed by a series of measurements indicating the usefulness of these composites in the area of second- order nonlinear optical applications and photorefractive optical informa- tion storage and processing.

Photorefractive composite materials with bi‐functional charge transporting second‐order nonlinear optical chromophores

Two bi‐functional chromophores have been developed which possess second‐order nonlinear optical and charge transporting properties. The second‐order nonlinearity arises from conjugated structures terminated by donor and acceptor groups and the charge transporting property results from a triphenylamine moiety. The bi‐functionality of the chromophores is confirmed by the large electro‐optic coefficients and high photoconductivities of composites containing these chromophores. These chromophores have been incorporated into an inert polymer to form photorefractive composite materials when doped with a photocharge generation sensitizer. The electro‐optic coefficients and photoconductivity of these composites have been measured as a function of the applied electric field as well as of the chromophore concentration. The dependence of the four‐wave mixing diffraction efficiency, holographic grating writing rate, and two‐beam coupling gain on the applied electric field and on the chromophore concentration have bee...

New photopolymer for holographic optical storage technology

We present the design, development and characterization of MEMPLEX - a new photoreactive polymer that has widespread applications in holographic information storage and processing technology. The new polymer is characterized by its large M/# and, thus, large capacity for data storage (>Thit/5 inch diameter disk), good sensitivity in the visible, negligible shrinkage during recording, no wet chemical processing, and virtually unlimited storage time of information. MEMPLEX is inexpensive, completely self-supporting, can be fabricated to any size as required by the application, has excellent optical quality, is resistant to most chemicals, and has long shelf-life.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Photorefractive properties of a thiapyrylium-dye-sensitized polymer composite: the dynamics of holographic grating formation and erasure

We present the photorefractive properties of a new organic composite operating at near IR wavelengths. The composite contains an ionic thiapyrylium dye, 4-(4-dimethylaminophenyl)- 2,6-diphenyl thiapyrylium perchlorate as a photocharge generation sensitizer, 4-(N,N- diethylamino)-((beta) )-nitrostyrene as a second order nonlinear optical chromophore, and poly(9-vinylcarbazole) as a host polymer which also serves as a charge transporting agent. Both degenerate and nondegenerate four-wave mixing experiments have been carried out and holographic diffraction efficiencies as high as 2% have been achieved. Asymmetric energy transfer has been observed with a gain of 7 cm-1 in a two-beam coupling experiment. This paper focuses on the dynamics of the formation and erasure processes of the holographic gratings. Both the writing and erasure times exhibit sublinear dependencies on the illumination power intensity, which is a result of the sublinear photoconductivity of the material. The dependence of the rise time on the applied electric field is attributed to the field dependence of both the photocharge generation quantum efficiency and the charge mobility in the polymer matrix.

Polymeric composite photorefractive materials for nonlinear optical applications

Polymeric composite materials constitute a new and very promising class of photorefractive materials. In the design of polymeric photorefractive materials we use multicomponent composites in which necessary functionalities can be independently optimized. The investigated composites consisted of charge transporting polymeric matrix, and optically second-order active molecules. Two different photosensitizers were used to vary the wavelength response. Photorefractive properties of these materials were investigated using erasable volume holography in a non-degenerate four-wave mixing geometry and two-beam coupling techniques. A previously developed model of space-charge field grating formation in photoconductive polymers was used to explain the field dependence of four-wave mixing diffraction efficiency. The model takes into account the field dependence of charge photogeneration quantum yield, carrier field mobility, and electro-optic coefficient. Necessary information about these parameters was obtained from the results of photoconductivity and electro-optic modulation experiments. Special attention was focused on the kinetics of photorefractive response in the composite for optical signal processing and optical storage. It was found that a very effective switching of diffraction efficiency induced by dc electric field occurs in this system. Also, the results of kinetic studies of the index grating writing and its subsequent light-induced erasure, as well as a demonstration of the holographic image recording and retrieval, are reported.

New PVK-based photorefractive polymers

This work describes the development, characterization and optimization of materials for optical memory storage applications. The photorefractive polymers are designed to contain second-order nonlinear optical (NLO) chromophores and charge transporting (CT) molecules embedded in a polymer matrix. The necessary electrooptic response is achieved by subjecting samples to a strong dc electric fields in order to induce a non-centrosymmetric arrangement of the NLO chromophores. This paper presents work performed on the design of a new multifunctional NLO-CT polymer.

Crosslinkable polyimides for electro-optic applications

The development of second order nonlinear optical (NLO) polymers involves several important steps. The first step is the design, synthesis and characterization of second-order NLO chromophores followed by the identification of a suitable polymer matrix into which the chromophore can be incorporated either as a guest, as a side-chain or a main- chain group. Thin films of candidate polymers are then fabricated and studied by a number of spectroscopic and thermal analyses techniques in order to characterize their NLO properties such as EO coefficient (or (chi) (2)), optical losses, and temporal stabilities of poling-induced chromophore alignment. This work describes the development, characterization and optimization of materials for second- order nonlinear optical applications. The second-order nonlinear optical materials are designed to possess large second-order nonlinearity by embedding 2nd order NLO active chromophores in high glass transition temperature polyimide structures.

Novel Sol-Gel Processed Photorefractive Materials

The authors report the development and characterization of a new photorefractive multifunctional ormosil consisting of a second-order nonlinear optical chromophore and a charge transporting group covalently bound to a silicon atom. The sol-gel technique is used to process this ormosil into a homogeneous, single-phase material which exhibits electrooptic and charge transporting properties. When doped with a photocharge generation sensitizer, the material shows photorefractivity as evidenced by the electric field dependence of the four-wave mixing diffraction efficiency and that of the two-beam coupling gain.