Enhancement of ablation efficiency by a femto/nano-second dual-beam micromachining system

In this paper, a dual-beam laser micromachining system consisting of a femtosecond (fs) laser and a nanosecond (ns) laser has been developed to enhance the ablation efficiency. Experiments were conducted in different materials including dielectric (fused silica), semiconductor (silicon wafer), and metal (aluminum alloys). The amount of material being removed was determined for fs pulses alone, ns pulses alone, and pairs of fs and ns pulses with different time lags in between. It was found that the material removal efficiency increases in the dual-beam process for all materials being studied as compared to the fs alone or ns alone, particularly for dielectrics. The highest ablation efficiency for fused silica occurs when the fs pulse is shot near the peak of the ns pulse envelope. A corresponding numerical model for dual beam ablation of dielectrics was also developed by integrating the plasma model, the improved two-temperature model, and Fourier's law to understand the laser-material interaction. It was found that the fs laser pulse can significantly increase the free electron density and change the optical properties of the dielectric, leading to the increase of absorption for the subsequent ns pulse energy. This study provides a fundamental understanding for the enhancement of material ablation efficiency, particularly for wide-bandgap dielectrics.