Subsurface boiling during pulsed laser ablation of Ge

The role of subsurface boiling for micrometer-sized droplets emission during pulsed laser ablation of Ge was critically examined. Single-crystal Ge targets were irradiated by 1064, 532, and 266 nm wavelength nanosecond pulses from a Nd:YAG laser at intensities of around 270 MW/cm(2), which remove several nm of material per pulse. Scanning electron microscopy investigations of the targets surface after the laser ablation process with the 1064 nm wavelength showed the formation of a rough morphology, exhibiting circular, micron-deep cavities, whereas the surface remained rather flat after 532 and 266 nm irradiation. It is suggested here that these cavities were formed by the release of gas caused by subsurface boiling. Numerical,simulations of the temperature profile inside the targets during the action of the different wavelengths laser pulses support the occurrence of the subsurface boiling effect during laser ablation by the 1064 MI wavelength, which is poorly absorbed by the target material.