Subwavelength periodic plasma structures formed during the laser-pulse-induced breakdown within the transparent dielectric

TL;DR

This paper investigates the formation of subwavelength periodic plasma structures during laser-induced breakdown in fused silica, revealing how plasma-resonance instability leads to nanograting formation under specific laser conditions.

Contribution

It introduces a detailed model of plasma and electric field evolution that explains the formation of subwavelength plasma structures during femtosecond laser breakdown in dielectrics.

Findings

Identification of laser intensity ranges for instability development

Description of small-scale periodic plasma modulation

Correlation with experimentally observed nanograting formation

Abstract

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.