Transient Laser‐Induced Periodic Surface Structures Revealed by Time‐Resolved EUV Diffuse Scattering

The formation of permanent laser‐induced periodic surface structures (LIPSS) on solid surfaces under impulsive laser irradiation above the damage threshold has been the subject of extensive research. Here, we report the formation of transient quasi‐periodic surface modulation patterns under femtosecond laser irradiation at fluences well below the ablation threshold. Time‐resolved extreme ultraviolet scattering measurements reveal distinct reciprocal‐space features similar to those observed for permanent LIPSS, but that dissipate on a timescale of hundreds of picoseconds. We show that the observed transient surface displacement patterns responsible for these features arise from thermal expansion driven by the spatial modulation of absorbed laser energy caused by scattering of the laser radiation by surface roughness. Moreover, we present an analytical model able to capture the main experimental observations. We experimentally demonstrate that such transient LIPSS‐like responses arise in a broad class of metallic thin film and multilayer systems under ultrafast low‐fluence irradiation.