Polarization dependent micro-structuring of silicon with a femtosecond laser

We experimentally demonstrate formation of a sub-micron rim around femtosecond laser ablated crater on silicon whose height and width were sensitive to laser polarization. Except for circularly polarized light we show that the rim height and width were asymmetric - larger along the direction of the laser polarization for n-type and intrinsic silicon, while in p-type silicon the asymmetry was perpendicular. Polarization dependent rim formation is attributed to the transient light-plasma interaction that gives rise to local-field enhancements resulting in an asymmetric electron density and energy deposition. Picoseconds later when the electron energy is transferred to the lattice, the asymmetry is retained in the temperature distribution within the interaction region. The temperature distribution eventually leads to non-symmetric radial outward fluid motion of a thin layer of molten material from the centre of the ablation crater that subsequently re-solidifies on a nanosecond timescale.