Growth and properties of wide-aperture optical elements for high-power laser systems

The work presents the results of growing large-sized profiled KDP and DKDP monocrystals method of high-speed growing of monosectoral profiled crystals. High-speed growing method includes the following steps. The wafer seed cut parallel to one of the natural faces of the crystal is put to the special crystallization form bottom, usually rectangular with vertical side walls. The cross-sectional size and orientation of the working face of the seed relative to the walls of the chamber are selected such that the resulting crystal size and orientation to be the most consistent with the shape and orientation of the optical element which is planned to be made of it. In this work we research samples of KDP and DKDP monocrystals (fig.1a) (the percentage of deuterating is no less than 97, 6%) (fig. 2) of Z-cut grown in the 320×320×70 mm form in 150 L crystallizer. The growth was carried out in isothermal conditions at absolute over saturation not exceeding 3 °C and the pH of the solution equal to ~2.7. Crystals' grew by the brink of the bipyramid. To prepare the solution we used the salt KDP Prochem inc. with the following impurities: Fe 50 ppb, Mg 50 ppb, Si 100 ppb, Al 50 ppb, Cr 89 ppb. The monocrystals growth speed was up to 15 mm per 24 hours. The grown crystal boule was 307×320×70 mm³ (KDP), 303×320×70 mm³ (DKDP) were oriented and cut to elements with a thickness of 10 mm. The samples' resistance measurements were held via teraohmmeter Metrel TeraOhmXA 10kV MI 3210 (the instrument error does not exceed 5%), which allows to apply a DC voltage up to 10 kV, and measure resistance, polarization index, absorption index and coefficient of dispersion. Resistance measurements were carried out on three electrode scheme (primary, measuring and guard electrodes) with a voltage up to 10 kV. In the course of work we obtained maps of resistivity distribution in the studied samples. According to the results of the measurements current-voltage characteristics of the samples close to linear were obtained, which corresponds to the high quality of the grown samples. Also we measured the distribution maps of the magnitude of the resistivity for sample 325×330×13 mm³. Heterogeneity on the resistivity distribution maps were caused by the presence of growth defects which in turn are the consequence of high growth rates and morphological instability of the growing face of the crystal. (authors)