Photorefractivity and Photoisomerization in Azodye-Doped Polymers

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B. KIPPELEN*, SANDALPHON, K. MEERHOLZ, B. VOLODIN, S.R. LYON A.B. PADIAS**, H.K. HALL JR.**, AND N. PEYGHAMBARIAN Optical Sciences Center, University of Arizona, Tucson, AZ 85721 * I.P.C.M.S. Groupe d'Optique Nonlineaire et d'Optoelectronique - 5, rue de l'Universite 67084 Strasbourg Cedex, France ** Chemistry Department, University of Arizona, Tucson, AZ 85721

ABSTRACT The photorefractive properties of photoconducting polymers doped with nonlinear azo dyes are investigated by means of polarization and field-dependent four-wave mixing experiments. The nonlocal response of the photorefractive effect is shown by two-beam coupling experiments. High diffraction efficiencies (> 5%) and net coupling gain (6 cm-1) are demonstrated at low power with a semiconductor laser diode in 105 ltm films. Under some conditions, gratings due to photoisomerization of the azo dye molecules are also present and their properties are discussed.

INTRODUCTION

Polymers are emerging as efficient materials for the development of future photonic devices. Their rich structural flexibility enables the tailoring of their optical properties. In addition, polymers can be processed easily and integrated in optoelectronic devices. As has been shown in recent years, polymers with second-order nonlinear optical properties are promising materials because in addition to processing capabilities, they have a high nonlinear optical coefficient and simultaneously a low dielectric constant. These properties are very attractive for high-speed electro-optic modulation and also for photorefractivity. In a photorefractive material, reversible refractive index gratings can be written with a low power laser. This index modulation is induced through the electro-optic effect and by the space-charge field that results from the redistribution of photogenerated carriers by migration and trapping in the material. This nonlinear effect is reversible, very sensitive and highly efficient. Therefore, new photorefractive materials are important for the development of future photonic devices especially in the field of reconfigurable optical interconnects, high density optical storage, real-time image processing, or holographic testing. In the last two decades, this effect has been studied mainly in inorganic materials, but recently it has been demonstrated in polymers and is the subject of active research [1-7]. The performance of newly developed photorefractive polymers 577 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society

are improving rapidly to a level where they compete with some of the best inorganic photorefractive crystals [7].

EXPERIMENTS AND RESULTS

To be photorefractive, a polymer must show simultaneously electro-optic properties, photocarrier generation, transport and trapping. Several approaches can be followed to design a photorefractive polymer. In the guest/host approach [1,3,6,7] the multifunctionality can be obtained by mixing components with specific functionalities into a polymer blend, while in the side-chain or main-chain approach