Nanostructure formation inside glasses and their three-dimensional integration using ultrafast pulsed laser

强烈的飞秒激光脉冲的电场强度接近甚至超过将透明材料中的价电子束缚在其离子核上的电场强度。在这种情况下，激光脉冲与材料之间的相互作用变得高度非线性。这种非线性现象引起了透明材料内部结构的变化。本研究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进行内部修饰和三维光学电路的开发进行了基础研究。特别地，我们在玻璃内部进行了纳米制造的实验。本研究的主要成果如下：当用重复频率为200 kH （800 nm）的飞秒激光照射SiO_2玻璃时，我们观察到SiO_2玻璃体内部有自组织的周期结构。在垂直于光波电场的方向上排列了更多的20 nm尺寸的结构，周期小到140 nm。这是迄今为止由光创造的最小的嵌入式结构。从入射光场与体电子等离子体波电场的相互干扰，从而导致电子等离子体浓度的周期性调制和玻璃结构变化的角度解释了这种自组织纳米结构的形成机制。在TeO_2单晶中，焦点体积内的电离物质爆炸膨胀到被致密基质包围的区域，即微爆炸，形成周期性的纳米空洞。在微爆炸中，压力的增加是由恒定体积下温度的升高引起的。我们推测周期性无空洞形成机制如下。在光场与电子等离子体波相互作用的早期阶段，SiO_2玻璃中普遍存在周期性的电子密度模式。由于Te-O的结合能较弱，熔点较低（730°），在电子等离子体密度分布的最大值处形成空洞。利用飞秒激光系统在二氧化硅玻璃和TeO_2单晶内部直接刻写了亚微米光子结构。观察到纳米结构在蓝色光谱区域强烈反射，但仅沿原始书写光束的偏振轴反射。我们认为所观察到的现象可能是由自组织的周期性折射率调制引起的。这种效应可以系统地解释用这种脉冲激光图案化报道的其他观察到的各向异性行为的起源。这些与偏振相关的纳米结构应该在许多单片光子器件中很有用，并且可以用于信息存储，微机电系统（MEMS）应用或准相位匹配，其中需要纳米级周期性结构。纳米光栅形成的结构变化的详细机制正在研究中。少

An intense femtosecond laser pulses have an electric field strength which approaches or even exceeds the strength of the electric field that holds valence electrons in transparent materials to their ionic cores. In this regime, the interaction between the laser pulse and the material become highly nonlinear. This nonlinear phenomena cause the change in the structure inside transparent materials. 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 nanofabrication inside glasses. The main results of this research are as follows. When SiO_2 glasses are irradiated with femtosecond laser with a repetition frequency of 200 kH (800 nm), we have observed self-organized periodic structures within the bulk. The structures consists of oxygen depleted regions … More of 20 nm size aligned in a direction perpendicular to the electric field of light wave and with periods as small as 140 nm. These are the smallest embedded structures ever created by light. The mechanism of this self-organized nanostructure is interpreted in terms of interference between the incident light field and the electric field of bulk electron plasma wave, and then resulting in the periodic modulation of electron plasma concentration and the structural changes in glass. In the case of TeO_2 single crystal, periodic nanovoids are formed by an explosive expansion of the ionized material in the focal volume into the surrounded by densified matrix, i.e., microexposion. In the microexposion, the increase in the pressure is caused by a rise in temperature at constant volume. We speculate the periodic nao-void formation mechanism as follows. In the early stages of the interaction between light field and electron plasma wave, periodic pattern of electron density arises in common with SiO_2 glass. Due to the weaker binding energy of Te-O and lower melting point of 730 $degree$, a void formed in the maximum of the electron plasma density distribution. We have directly written submicron photonic structures into the internal bulk of silica glass and TeO_2 single crystal with a femtosecond laser system. The nanostructures are observed to strongly reflect in the blue spectral region but only along the polarization axis of the original writing beam. We think that the observed phenomenon can arise from a self-organized periodic refractive index modulation. The effect can explain systematically the origin of other observed anisotropic behavior reported with such pulsed laser patterning. These polarization-dependent nanostructures should be useful in many monolithic photonic devices and can be harnessed for information storage, micro-electromechanical systems(MEMS) applications or quasi-phase matching where nanoscale periodic structuring is required. A detailed mechanism of the structural changes responsible for the nanograting formation is under investigation. Less