Ultrafast pump-probe microscopic imaging of femtosecond laser-induced melting and ablation in single-crystalline silicon carbide

The temporal and spatial evolution of femtosecond laser-induced phase transitions and ablation on single-crystalline silicon carbide (SiC) were investigated via pump-probe microscopy in air on a time scale from approximately 100 fs to 1 ns. The largest reflectivity change was observed between 300 fs and 1 ps after excitation, which is due to free carrier generation, and the Drude model calculations indicated that the maximum free electron density was greater than 3.3 x 10(21) cm(-3). After a few picoseconds, there was direct evidence of the production of a rarefaction wave propagating towards the bulk, whose propagation velocity was estimated to be 3286 m/s. At delay times between a few hundreds of picoseconds and 1 ns, characteristic transient ring patterns were clearly observed and were related to the optical interference of the probe laser pulse reflected at the front surface of the ablating layer and at the interface of the non-ablating substrate. The estimated expansion velocity of the ablation front in 6H-SiC was found to be comparable to or slower than those reported for other semiconductors and dielectrics.