Abstract

Inorganic photorefractive materials are widely used for applications in the visible spectral range. The lack of sensitive photorefractive materials at telecommunication wavelengths (λ=800- l500nm) where low cost semi-conductor laser diodes are commercially available opens a new field of interest. Due to the shift of the absorption cut-off towards longer wavelengths with increasing nonlinearities, the newest organic materials fulfill the sensitivity requirements at near infrared wavelengths. Among the organic crystals, 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST) is a promising candidate to investigate the photorefractive properties of organic materials. Combining very large nonlinearities (r₁₁₁ of 92 pm/V at λ=720nm) with an absorption cut-off at around 700nm along the dielectric axis x₁, DAST shows promising results in the standard two-beam coupling experiments. Using the light interference pattern of two incoming coherent laser beams (ratio of pump to signal beam > 100), an index grating could be generated at a wavelength of λ =720nm. Due to the 90° phase shift of this index grating with respect of the light intensity pattern, an asymmetric energy exchange could be observed and qualitatively analyzed (Fig. 1). Using polished b-plates with dimensions up to axbxc= 12x1x4 mm³, a gain of up to 1.4cm^-1, corresponding to a net gain of (Ί−α)≥0.2cm^_l, was measured for light polarized along the dielectric axis x₁ at a pump intensity of 85mW/cm². Writing times of the order of one second have been measured. These first results point out the promising properties of the organic salt DAST for photorefractive applications at near infrared wavelengths. Further experiments of the dependence of the photorefractive gain on the applied electric field, the wavelength, and the grating period will also be reported.

© 1996 IEEE