Dual-wavelength femtosecond laser-induced low-fluence single-shot damage and ablation of silicon

A study of damage and ablation of silicon induced by two individual femtosecond laser pulses of different wavelengths, 1030 and 515 nm, is performed to address the physical mechanisms of dual-wavelength ablation and reveal possibilities for increasing the ablation efficiency. The produced ablation craters and damaged areas are analyzed as a function of time separation between the pulses and are compared with monochromatic pulses of the same total energy. Particular attention is given to low-fluence irradiation regimes when the energy densities in each pulse are below the ablation threshold and thus no shielding of the subsequent pulse by the ablation products occurs. The sequence order of pulses is demonstrated to be essential in bi-color ablation with higher material removal rates when a shorter-wavelength pulse arrives first at the surface. At long delays of 30-100 ps, the dual-wavelength ablation is found to be particularly strong with the formation of deep smooth craters. This is explained by the expansion of a hot liquid layer produced by the first pulse with a drastic decrease in the surface reflectivity at this timescale. The results provide insight into the processes of dual-wavelength laser ablation offering a better control of the energy deposition into material.