Ultrafast imaging interferometry at femtosecond-laser-excited surfaces

A simple and robust setup for femtosecond time-resolved imaging interferometry of surfaces is described. The apparatus is capable of measuring both very small phase shifts (∼3 × 10^-2 rad) and amplitude changes (∼1%) with micrometer spatial resolution (∼1 μm). Interferograms are processed using a 2D-Fourier transform algorithm. We discuss the image formation and the physical interpretation of the measured interferograms. The technique is applied to measure transient changes of a GaAs surface irradiated with intense femtosecond laser pulses with fluences near the ablation threshold.