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保持在其离子核心上的电场强度。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。这种非线性现象导致透明材料内部结构的变化。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，我们在玻璃内部进行了纳米加工的实验。本研究的主要成果如下。用重复频率为200 kH(800 Nm)的飞秒激光照射SiO_2玻璃，观察到块体内的自组织周期结构。结构由氧耗区…组成20 nm以上的尺寸沿垂直于光波电场的方向排列，周期小至140 nm。这些是有史以来由光创造的最小的嵌入式结构。这种自组织纳米结构的机理是入射光场与体电子等离子体波的电场发生干涉，从而导致电子等离子体浓度的周期性调制和玻璃中结构的变化。对于TeO_2单晶，焦域中的电离物质在致密的基质中爆炸膨胀，形成周期性的纳米空洞，即微爆炸。在微爆炸中，压力的增加是由恒定体积的温度上升引起的。我们推测NAO-空穴的周期性形成机制如下。在光场与电子等离子体波相互作用的早期阶段，出现了与SiO_2玻璃相同的电子密度周期图案。由于Te-O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。我们利用飞秒激光系统将亚微米光子结构直接写入石英玻璃和TeO_2单晶的内部。观察到纳米结构在蓝色光谱区域强烈反射，但仅沿原始写入光束的偏振轴。我们认为，这种观察到的现象可能是由自组织周期性折射率调制引起的。这一效应可以系统地解释脉冲激光图案化中观察到的其他各向异性行为的起源。这些偏振相关的纳米结构应该在许多单片光子器件中有用，并可用于信息存储、微电子机械系统(MEMS)应用或需要纳米级周期性结构的准相位匹配。导致纳米级形成的结构变化的详细机制正在调查中。较少

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