Pulse-Dependent Transient Reflectivity and Simulated Subsurface Density Evolution in Titanium Under Femtosecond Laser Irradiation

Successive femtosecond laser pulses interact with titanium surfaces modified by earlier irradiation. We combined time-resolved pump–probe imaging with molecular dynamics assisted by a two-temperature model (TTM–MD) to examine the resulting optical and structural changes. At 19.4 and 31 mW, delayed reflectivity oscillations weakened after the first irradiation, although surface restructuring continued. At 45 mW, spatial rings appeared at selected delays without a clear periodic component in the central reflectivity trace. Simulations at absorbed fluences of 2000 and 3000 J m−2 produced internal low-density bands with maximum first-pulse thicknesses of 37 and 59 nm, respectively. The corresponding second-pulse bands reached only 3 and 9 nm in a cooled residual target prepared from the first-pulse calculation. Polyhedral template matching revealed substantial loss of hexagonal close-packed local order in the sampled surface region. These results support a qualitative interpretation linking prior irradiation, structural modification and altered optical response. Changes in interface geometry and material optical properties provide possible contributions to the pulse-dependent signal.