As it is known, potassium dihydrogen phosphate KH₂PO₄ (KDP) crystal is a unique material, which attracts attention of specialists in various fields of science such as non-linear optics, quantum electronics, solid state physics. It is one of the best materials for transformation of laser radiation frequency (due to its high non-linear susceptibility). Moreover, KDP is widely used in electrooptics and is promising for bulk record of information as well as for other applications.

Recently it has been shown [1] that KDP is a remarkable general host for a wide variety of organic dyes. Therefore investigation of physical processes in dye inclusion crystals (DIC) with potassium dihydrogen phosphate as a host is a necessary step for creating DIC solid-state dye lasers on the base of nonlinear optical (NLO) media and thus is of considerable scientific and practical interest.

The aim of this work is to describe and discuss the experimental results concerning the influence of organic dye such as xylenol orange (XO) on the linear and nonlinear optical properties of the KDP crystals.

KDP single crystals were grown in the supersaturated solution containing an organic dye (xylenol orange). The potassium dihydrogen phosphate reagents were used as solutes, the concentration of impurities (Fe, Cr, Al, Sb, Bi, Cu, Hg, Ag, Pb) did not exceed 5·10^-5 wt. %. The content of XO dye in the crystals was determined by comparing the absorption spectra of the samples and the solutions with known dye contents. The absorption spectra were measured by UV-V-NIR spectrophotometer SF-56 (LOMO) and the luminescence spectra anisotropy was studied on "Hitachi F4010" fluorimeter with two linear polarizers.

It was shown that the triphenylmethane compound xylenol orange dyes the prism {100}, the pyramid {110} of the crystal remaining practically colorless. The performed study of anisotropy of the optical absorption and luminescence spectra shows that KDP/XO single crystals are intrasectoral zoned. In the prismatic part of the crystals the dye is distributed more uniformly in the crystal bulk in comparison with the pyramidal part. The dependences of the transmission coefficient on the angle between the direction of the electric vector of the incident light wave E and the directions of the crystallographic axes were measured for determination the spatial orientation of the transition dipole moment. The angle value was varied in the interval from 0º to 360º with a step of 10º. The angular dependences were measured along all the crystal axes. It was shown that the dipole transition moments of the dye in the neighboring growth sectors of the prismatic (and pyramidal) crystal parts are oriented in two mutually perpendicular directions.

The NLO experiments were performed with the picosecond (~ 40 ps FWHM) mode-locked Nd:YAG laser at 1064 nm with far field spatial profile analysis technique. It was found that dye doping caused the photoinduced absorption of the crystals with saturation threshold at about 1 MW/cm². The darkening of the samples is accompanied with selffocusing phenomenon of the laser beam. The highest nonlinear refractive index response was achieved for the annealed KDP:XO crystal (up to 150^oC). The effective cubic nonlinear susceptibility c⁽³⁾_eff ~ 2.7 10^-8 esu was 1.7 times higher in comparison with a pure KDP crystal.

1. M. Kurimoto, L.D.Bastin, D.Fredrickson, P.N.Gustavson, S.-H.Jang, W.Kaminsky, S.Lovel, C.A.Mitchell, J. Chmielevski, B.Kahr, Mat. Res. Soc. Symp. 620, M9.8.1- M9.8.10 (2000).

2. Borshch A.A., Brodyn M.S., Gayvoronsky V.Ya. “Diagnostics of Optical Nonlinearities: Spatial Beam Distortion Technique and Its Application to Semiconductors and Novel Materials”, Proc. SPIE 5024, 128-136 (2003).

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