Ultrafast in-situ null-ellipsometry for studying pulsed laser - Silicon surface interactions

The measurement of transient optical properties due to pulsed laser excitation allows better understanding of the nature of laser induced processes. Conventional ellipsometry is not capable of following changes in the femto-, pico- or nanosecond timescale. In this work, the pump and probe technique is combined with a single wavelength null-ellipsometry. This enabled us to follow the optical changes of silicon due to sub-ps laser pulse irradiation with ps time resolution. The combination of the 496 nm probe pulses with a Polarizer - Compensator - Sample - Analyzer (PCSA) configuration imaging null-ellipsometer provided Ψ and Δ ellipsometric angles of silicon irradiated with 248 nm pump pulses. Different laser intensities and delay times between the probe and pump pulses are used in the experiments. It is shown that besides thermal effects, the in depth free charge carrier distribution and their electron-phonon relaxation time has to be taken into account in the frame of the two-temperature model for satisfactory interpretation of the experimental results.