Pulse duration dependence of material response in ultrafast laser-induced surface-penetrating nanovoids in fused silica

TL;DR

This study investigates how ultrafast laser pulse duration affects nanovoid formation inside fused silica, revealing that pulse duration influences ablation and void size, enabling advanced microfabrication techniques.

Contribution

It demonstrates the control of nanovoid formation in fused silica via laser pulse duration and surface modification, advancing ultrafast laser processing capabilities.

Findings

Ablation surface intensity decreases with longer pulses.

Deep void diameter first increases then decreases with pulse duration.

Nanovoids with aspect ratios over 13000:1 were fabricated.

Abstract

The focused ultrafast laser, with its ability to initiate nonlinear absorption in transparent materials, has emerged as one of the most effective approaches for micro-nano processing. In this study, we carried out research on the processing of high-aspect-ratio nanovoids on fused silica by using the single-pulse ultrafast Bessel beam. The thermodynamic response behaviors of the materials on surface and deep inside are found to exhibit pronounced disparities with the variation in laser pulse duration. As the pulse duration increases from 0.2 ps to 9.0 ps, the intensity of material ablation on silica surface exhibits a gradually decreasing trend, while for the void formation deep inside silica, the void diameter exhibits a trend of initial increase followed by decrease. In particular, no nanovoids are even induced deep inside when the pulse duration is 0.2 ps. The mechanism causing such differences is discussed and considered to be related to the peak intensity, group velocity dispersion, and plasma defocusing. By covering a polymer film on silica surface to influence the energy deposition, the thermomechanical response behaviors of the materials to laser pulse duration are modulated, and the material sputtering on nanovoid opening is suppressed. On this basis, surface-penetrating nanovoid arrays are fabricated on a 2-mm-thick silica sample using 2 ps Bessel beam. Given the nanovoid diameter of approximately 150 nm, the aspect ratio of the nanovoids on fused silica sample exceeds 13000:1. This outcome creates significant possibilities for the stealth dicing and processing of 3D photonic crystals, optical integrated devices, and nanofluidics.