In situ ablation of thin aluminum films for determination of targeted beam properties in picosecond laser-induced damage testing

Accurate measurement of the targeted beam diameter is essential for a reliable quantification of the laser-induced damage threshold (LIDT), particularly for those tests requiring low-fluence levels. The targeted beam diameter is commonly obtained using a beam profiler located on the conjugate plane of the sample plane. Under special conditions such as vacuum conditions and oblique incidence in picosecond (ps) laser damage tests, the single dependence on the conjugate-plane beam profiler for beam diameter measurement demonstrates limitations. In this paper, the effects of aluminum (Al) film thickness, laser incidence angle, and vacuum conditions on the sample-plane beam diameter determination were first investigated for in situ metallic film ablation. Al film thickness (60-350 nm) and ablation depth showed negligible impact on beam diameter determination. The ablation evolves from a single outer circle to concentric inner and outer circles as the energy increases. The inner circle of 100-200 nm Al films was recommended for beam diameter determination due to its superior resistance to pedestal interference, and more stable ablation features. The experiment results demonstrate its applicability in both vacuum environment (5×10⁻⁴ Pa) and under oblique incidence (0°-70°). Furthermore, it enables identification of focal spot variations and can be used to ensure the targeted beam properties to maintain consistent beam properties in air and vacuum prior to each laser damage test. This parameter optimization approach provides a methodological framework for accurate beam diameter determination in ps-laser damage tests, which serves as a practical calibration reference for cross-laboratory and cross-instrument comparisons.