  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.
