Ultrafast pump-probe ellipsometry setup for the measurement of transient optical properties during laser ablation

Ultrashort pulsed lasers offer a high potential in precise and efficient material processing and deep understanding of the fundamental laser-material interaction aspects is of great importance. The transient pulse reflectivity in conjunction with the transient absorption influences decisively the laser-material interaction. Direct measurements of the absorption properties by ultrafast time-resolved ellipsometry are missing to date. In this work, a unique pump-probe ellipsometry microscope is presented allowing the determination of the transient complex refractive index with a sub-ps temporal resolution. Measurements on molybdenum show ultrafast optical penetration depth changes of -6% to + 77% already within the first 10 ps after the laser pulse impact. This indicates a significant absorption variation of the pump pulse or subsequent pulses irradiating the sample on this timescale and paves the road towards a better understanding of pulse duration dependent laser ablation efficiency, double or burst mode laser ablation and lattice modifications in the first ps after the laser pulse impact.