Thin-Film Nonlinear Optical Crystals for Intracavity Electro-Optic Modulation
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CAL MATERIALS
Thin-Film Nonlinear Optical Crystals for Intracavity Electro-Optic Modulation O. D. Vol’pyana, I. Yu. Denisyukb, *, Yu. A. Obodc, M. I. Fokinab, Yu. A. Ignat’evab, and E. B. Sheklanovab a OOO
NPK Fotron-Avto, Moscow, 115191 Russia ITMO University, St. Petersburg, 197101 Russia c Scientific and Technological Center of Unique Instrumentation, Russian Academy of Sciences, Moscow, 117342 Russia *e-mail: [email protected] b
Received June 8, 2020; revised July 8, 2020; accepted August 28, 2020
Abstract—Methods of preparation of single-crystal 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST) films with a thickness of several micrometers, designed for electro-optic modulation, have been analyzed. Thin-film single crystals have been obtained, the shape and absorption spectrum of which confirm their crystal structure of the “red” DAST crystal form. Homogeneous luminescence in the polarization-contrast regime indicates their single crystallinity and uniform orientation. These materials are designed for intracavity modulation of laser radiation. Keywords: molecular crystal, DAST, electrooptics, crystal growth, single-crystal film DOI: 10.1134/S0030400X20110284
INTRODUCTION The use of intracavity electro-optic modulators is an efficient way of controlling and monitoring parameters of the generated radiation [1, 2]. Classical electro-optic materials [3] and thin graphene films [1, 4] are most often used as active materials for such modulators. The main requirements to these materials comprise high electro-optic coefficient and minimum insertion loss. Over the last decade, a new class of electro-optic materials (organic electro-optic crystals with ultrahigh electro-optic parameters) has been actively developed [5]. Among these materials, 4-N,N-dimethylamino-4'-N'-methyl stilbazolium tosylate (DAST) crystals have the highest parameters and can be used for intracavity modulation. DAST crystals are stable at temperatures up 250°C and characterized by low permittivity (ε = 5.2) and electro-optic coefficient of 98 pm/V (up to 700 pm/V for thin films) [6]. The main method of growing DAST crystals is based on the preparation of saturated DAST solution in anhydrous methanol at a temperature of 50°C with subsequent slow (about 0.5° per day) decrease in temperature. At a certain instant, supersaturation of the solution reaches a point of spontaneous crystallization, and the crystal growth begins. Since molecules in a crystal are ordered in-plane, the plane growth rate exceeds that for thickness by a factor of about 20. A further increase in the 2D growth rate requires provi-
sion of axis-different growth conditions (for example, using active substrates). The method for growing 4-μm-thick thin-film DAST single crystals and formation of electro-optic polarization modulator operating at a frequency of 1.5 GHz was considered in [7]. A single-crystal film was formed using a modified shift method. The substrate was comprised of two glass plates spaced by a small gap and slowly moving with respect to each other
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