Prediction of ablation crater profiles in sapphire processed by single-pulse dual-wavelength femtosecond lasers

Sapphire is widely used in high-transmittance optical devices due to its excellent optical properties and chemical stability. Femtosecond (fs) laser processing is one of the most precise machining techniques and offers exceptional precision for fabricating functional microstructures on sapphire surfaces. However, the fs-laser ablation of sapphire involves complex nonequilibrium electron dynamics and phase transition processes which make precise control of the ablation profile a significant challenge. In this work, a plasma-based numerical model is developed to investigate femtosecond laser ablation of sapphire. The model combines the Fokker–Planck equation with the Drude model. It explicitly accounts for the transient optical response and plasma evolution during laser–material interaction. A time-dependent electron collision time is introduced to overcome the limitations of conventional models that rely on a constant value. This constant assumption fails to capture the dynamic evolution of plasma properties and leads to inaccurate estimations of laser energy absorption. To quantitatively validate the model, we performed systematic single-pulse fs-laser ablation experiments on sapphire at wavelengths of 1030 nm and 515 nm. We characterized the three-dimensional ablation crater morphologies using confocal laser scanning microscopy. The results show good agreement between the simulated and experimental ablation profiles. Under 1030 nm fs-laser excitation, the simulated ablation craters exhibit average relative errors of 4.7% and 2.3% in diameter and depth. Correspondingly, under 515 nm fs-laser excitation, the errors are 6.7% and 4.6%. This study provides valuable theoretical guidance and a reliable predictive tool for the precision fs-laser micromachining of sapphire.