Structural Control of Ceramics with a Femtosecond Pulse Laser

本研究以飞秒激光诱导的多光子过程等非线性光学效应对玻璃进行内部改性和开发三维光路为目标，开展了基础研究。特别是对制作具有波长转换功能的实用玻璃进行了实验研究。本研究的主要结果如下。用重复频率为200 kH（800 nm）的飞秒激光辐照Li_2 O-Nb_2 O_5-SiO_2系统玻璃，随着辐照时间的延长，玻璃在焦点处的折射率发生变化，玻璃内部出现沉淀物。相反，当用重复频率为1 kHz的飞秒激光照射时，尽管照射时间长于1 h，但仅形成裂纹。X射线衍射分析表明，沉淀物为非线性光学晶体LiNbO_3。此外，在晶体沉淀后立即观察到蓝色发光。确认发射集中在400 nm处，对应于入射光波长的一半。结果表明，飞秒激光辐照使LiNbO_3晶体的非线性光学系数增大成为可能。实验结果表明，利用超快脉冲激光实现非线性光学晶体的空间选择性沉淀是可能的，并有望在波长转换器件中实现定向晶体的沉淀和波导的形成。

In this research, we carried out the basic research aiming at the internal modification of glasses and the development of three-dimensional optical circuit through nonlinear optical effect such as multiphoton process induced by femtosecond laser. In particular, we performed the experiments on making useful glasses with the function of wavelength conversion. The main results of this research are as follows. When Li_2O-Nb_2O_5-SiO_2 glass system were irradiated with femtosecond laser with a repetition frequency of 200kH (800nm), the refractive-index was changed at the focal point and then some precipitations appeared within the glass, as the irradiation time became long. In contrast, when they were irradiated with femtosecond laser with a repetition frequency of 1 kHz, only cracks were formed in spite of the irradiation time longer than 1h. X-ray diffraction pattern indicate that the precipitated compounds are nonlinear optical crystal, LiNbO_3. Also, blue emission was observed immediately after the precipitation of the crystal. It was confirmed that the emission was centered at 400nm, corresponding to half the wavelength of incident light. As a result, we have found that the irradiation with femtosecond laser makes possible the precipitation of LiNbO_3 crystals with large nonlinear optical coefficient. The overall results demonstrate the possibility or space-selective precipitation of nonlinear optical crystals using ultra-fast pulsed laser, and it is expected that the precipitation of oriented crystals and the formation of waveguides find their application to wavelength-conversion devise.