Laser wavelength effect on nanosecond laser light reflection in ablation of metals

Reflection of nanosecond laser pulses with different wavelengths (1.06 and 0.69 µ m) in ablation of titanium in air is studied experimentally. The laser wavelength effect on reflection is essential at low laser fluence values. However, it becomes negligible for laser fluence values by about an order of magnitude higher than the plasma ignition threshold. We speculate that the disappearance of the wavelength effect is explained by counter-acting processes of the laser light absorption in plasma, which increases with laser wavelength, and absorption in the surface layer, which decreases with increasing laser wavelength. (paper)