透明介质中laser脉冲击穿形成的亚波长周期性等离子体结构
Subwavelength periodic plasma structures formed during the laser-pulse-induced breakdown within the transparent dielectric
摘要：
在等离子体共振诱导电离不稳定性条件下，研究了飞秒激光脉冲在透明介质（熔融石英）中引起的光学击穿中场和等离子体的时空演化，这种不稳定性导致击穿等离子体参数在激光偏振方向上的深度小尺度周期调制.在所使用的模型的框架中，计算的光学电场与津贴的影响，其长尺度的结构（占在给定的射线管近似，相位和群延迟，和背反射的光束聚焦）和小尺度的一个（准静态增强等离子体共振区）。等离子体密度的演化由速率方程描述，该方程考虑了光电离、雪崩电离和双极扩散。在数值计算的基础上，我们描述了起源于聚焦区的击穿波的主要类型，并找到了这样一个激光脉冲强度范围，在该范围内，由很小的种子扰动引起的不稳定性导致形成对比度亚波长周期性结构，该结构包含许多具有超临界等离子体密度和增强的能量沉积的窄区。后者使我们能够考虑这种结构的基础上的nanograting形成实验观察到的熔融石英一系列重复的fs脉冲照射。
英文摘要：
The spatiotemporal evolution of the field and plasma in the optical breakdown induced in the volume of transparent dielectric (fused silica) by the focused fs laser pulse is studied under condition of the so-called plasma-resonance-induced ionization instability that results in the deep small-scale periodic modulation of the breakdown plasma parameters in the direction of the laser polarization. In the framework of the model used, the optical electric field was calculated with allowance for the effects influencing both its long-scale structure (the beam focusing accounted for in the given-ray-tube approximation, phase and group delays, and back reflection) and the small-scale one (quasi-static enhancement in the plasma resonance regions). The plasma density evolution is described by the rate equation taking into account the photoionization, avalanche ionization, and ambipolar diffusion. Based on the fulfilled numerical calculations, we have described the main types of the breakdown wave originating in the focal region and have found the laser pulse intensity range where the instability evolving from very small seed perturbations leads to the formation of the contrast subwavelength periodic structure containing the number of the narrow zones with overcritical plasma density and enhanced energy deposition. The latter allows us to consider this structure as underlying the nanograting formation observed experimentally in the fused silica irradiated by series of repeated fs pulses.
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