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

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 material on the 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 the nanovoid opening shows a gradual decrease, while for the nanovoid formation deep inside silica, the nanovoid 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. This difference has a significant impact on the processing of surface-penetrating nanovoids, and its mechanism is discussed in terms of nonlinear propagation effects of the Bessel beam. By coating a polymer film on the 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 in the fused silica sample reaches ∼ 13000:1. This outcome creates significant possibilities for the stealth dicing and processing of 3D photonic crystals, optical integrated devices, and nanofluidics.