Capture of femtosecond plasmon excitation on transient nonequilibrium states of the metal surface

Abstract

Understanding of laser-material interactions is a scientific evergreen in the fundamental research of physics and optics. We report here that the ultrafast dynamics of the Cu(110) crystal surface is permanently captured by the formation of subwavelength periodic structures using two collinear femtosecond laser irradiations with different linear polarizations. Surprisingly, such periodic structures are found to have slantwise orientation that is anomalously change as a function of the time delay between two laser beams. In the case of the shorter time delays, the time-dependent slantwise orientations oscillated with terahertz frequency, depending on the pulse width and the intersection angle of two polarization directions, whereas it only presents monotonic change for the larger time delays. Analyses suggests that the former case is attributed to the surface plasmon excitation of the temporally delayed femtosecond laser irradiation on the transient state of the metal surface, which is consequently modulated by some nonthermal effects such as shock wave and bond hardening, while the latter situation is predominated by thermal relaxation of the material lattice. The simulation results agree with the experimental measurements. This investigation not only allows us to sensitively record the transient spatiotemporal evolution on superheated metal surfaces, but also provides insights for the control of material microprocessing.