Institute of Electronic Materials Technology, Wolczynska 133, 01-919, Warsaw, Poland.
University of Rome "La Sapienza", 16 A. Scarpa, Rome, 00161, Italy.
Sci Rep. 2017 Mar 24;7:45247. doi: 10.1038/srep45247.
Second-order nonlinear optical materials are used to generate new frequencies by exploiting second-harmonic generation (SHG), a phenomenon where a nonlinear material generates light at double the optical frequency of the input beam. Maximum SHG is achieved when the pump and the generated waves are in phase, for example through birefringence in uniaxial crystals. However, applying these materials usually requires a complicated cutting procedure to yield a crystal with a particular orientation. Here we demonstrate the first example of phase matching under the normal incidence of SHG in a biaxial monoclinic single crystal of zinc tungstate. The crystal was grown by the micro-pulling-down method with the (102) plane perpendicular to the growth direction. Additionally, at the same time white light was generated as a result of stimulated Raman scattering and multiphoton luminescence induced by higher-order effects such as three-photon luminescence enhanced by cascaded third-harmonic generation. The annealed crystal offers SHG intensities approximately four times larger than the as grown one; optimized growth and annealing conditions may lead to much higher SHG intensities.
二阶非线性光学材料可通过倍频（SHG）产生新频率，这是一种非线性材料在输入光束的光频的两倍处产生光的现象。例如，通过单轴晶体的双折射，可以在泵浦光和产生的波同相时实现最大的倍频。然而，应用这些材料通常需要复杂的切割过程来获得具有特定取向的晶体。在这里，我们展示了在锌钨酸盐双轴单斜晶体中，在 SHG 的正入射下实现相位匹配的第一个例子。该晶体是通过微下拉法生长的，（102）面垂直于生长方向。此外，同时由于受三阶效应（如通过级联三波混频增强的三光子发光）的刺激拉曼散射和多光子发光，产生了白光。退火后的晶体的倍频强度大约比未退火的晶体大四倍；优化的生长和退火条件可能会导致更高的倍频强度。
