Structural Control of Ceramics with a Femtosecond Pulse Laser

在本研究中，我们针对飞秒激光诱导的多光子过程等非线性光学效应，对玻璃的内部改性和三维光路的发展进行了基础性研究。特别是，我们进行了制作具有波长转换功能的有用玻璃的实验。本研究的主要成果如下。用重复频率为200kH(800 Nm)的飞秒激光照射Li_2O-Nb_2O_5-SiO_2系统玻璃时，随着辐照时间的延长，焦点处的折射率发生变化，玻璃内部出现一些析出物。反之，在重复频率为1 kHz的飞秒激光照射下，即使照射时间超过1h，也只能形成裂纹。X射线衍射图表明，析出的化合物为非线性光学晶体LiNbO_3，并且在晶体析出后立即观察到蓝光发射。确定了发射中心在400 nm处，相当于入射光波长的一半。结果发现，飞秒激光的辐照使LiNbO_3晶体的大非线性光学系数的析出成为可能。结果表明，利用超快脉冲激光可以实现非线性光学晶体的空间选择性析出，定向晶体的析出和光波导的形成有望在波长转换装置中得到应用。

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.