Modeling thermal responses and evaluation of femtosecond laser damage in multi-layer gold-coated fused silica gratings

Abstract

Increasing the damage threshold of gold-coated pulse compression gratings is crucial for advancing petawatt-class laser systems considering the current low threshold of gold-coated photoresist gratings (GPGs). Various factors that influence the threshold of GPGs have been explored, but scant attention has been paid to the melting dynamics of the layers beneath the gold film. This study presents multi-layer gold-coated fused silica gratings (MGFSGs) that consist of a gold film, a chromium interlayer, and fused silica grating, wherein the interlayer functions as a heat sink. Ablation experiments were conducted using a femtosecond laser (800 nm, $30\pm 5$ fs) at varying laser fluences, followed by the resultant laser damage morphology analysis. Dynamic descriptions of the laser absorption and temperature evolution in the lattice at various layers were facilitated using a two-temperature model that considered the boundary conditions of heat transfer at different material interfaces. Theoretical analysis and experimental observations revealed that a chromium interlayer can enhance the grating’s threshold. MGFSGs exhibit the capability to enhance the beam-normal damage threshold of the grating to 0.41 $\mathrm{J/cm}$², i.e., approximately 40% higher than the baseline of 0.29 $\mathrm{J/cm}$² for GPGs. These results offer pivotal insights for improving the damage resistance of gold-coated gratings.