Fluence-Dependent Transient Reflectance of Stainless Steel Investigated by Ultrafast Imaging Pump–Probe Reflectometry

The ablation efficiency during laser processing strongly depends on the initial and transient reflectance of the irradiated material surface. This article reports on the transient relative change of the reflectance Δ R / R of stainless steel during and after ultrashort pulsed laser excitation (800 nm, 40 fs) by spatially resolved pump–probe reflectometry. The spatial resolution of the setup in combination with the spatial Gaussian intensity distribution of the pump radiation enables a fluence-resolved detection of Δ R / R . Within the first picosecond after irradiation with a peak fluence of 2 J/cm 2, the spatially resolved Δ R / R of stainless steel evolves into an annular shape, in which the center almost remains at its initial reflectance, whereas the outer region features a decreased reflectance. The decreasing trend of Δ R / R is qualitatively supported by applying a two-temperature model, considering the transient optical properties of stainless steel from the literature. At larger fluences and thus higher electron temperatures, the experimental data deviate from the transient reflectance given in the literature. A drastically decreased occupation of the states below the Fermi energy and the subsequent excitation of electrons into these new vacant states by the probe radiation are considered being the most probable origin for this behavior at high fluences.