Effects of plasma confinement on the femtosecond laser ablation of silicon

We investigated the femtosecond laser ablation of silicon in a confined condition by covering the surface of a silicon wafer with a glass slide. The ablation was carried out by either irradiating the focused spot with different numbers of pulses or by scanning the laser beam on the surface of the silicon wafer. The morphology of the ablated surface was characterized by scanning electron microscope. For laser fluences much larger than the ablation threshold of silicon, cylindrical holes were generated in the confined ablation, in sharp contrast to the conical holes observed in the ablation performed in open air. Accordingly, grooves with U-shaped and V-shaped cross sections were achieved in the ablations carried out in the confined condition and in open air, respectively. For laser fluences close to the ablation threshold of silicon, the difference in the morphology of micro- and nanostructures obtained by these two ablation methods became not pronounced and two-dimensional nanohole arrays were created on the surface of silicon wafer. While the period of the nanohole arrays in the direction of the laser polarization was found to be close to the laser wavelength, the period in the direction perpendicular to the laser polarization was observed to be more than two times of the laser wavelength. In addition, the distribution of erupted nanoparticles was also found to be different in the two ablation processes. A ring-shaped distribution of nanoparticles was observed in the open air ablation while a monotonic decrease of nanoparticle density along the radial direction was found in the confined ablation.