A study of the formation mechanisms of periodic surface structures on rutile TiO₂ single crystal induced by delayed, cross-polarized femtosecond ultraviolet laser pulses

Laser-induced periodic surface structuring (LIPSS) on (001) surfaces of TiO₂ single crystals was investigated using two linearly polarized, time-delayed ultraviolet femtosecond laser pulses (λ = 248 nm, pulse duration = 450 fs). The influence of the angle between the polarization directions and of the interpulse delay on the formation and orientation of low spatial frequency LIPSS (LSFL) was systematically investigated. For temporally superposed pulses, LSFL formed after a small number of pulses ( N ≈ 10). Their formation is consistent with a mechanism based on the coherent superposition of surface plasmon polaritons excited by each pulse. In contrast, when the pulses were temporally delayed, LSFL formation required a significantly larger number of pulses ( N ≥ 100) and proceeded through intermediate stages involving the formation of randomly oriented ripples and surface bumps. The delayed-pulse regime is shown to be governed by the interaction of the second pulse with a transient surface state induced by the first pulse. From the delay-dependent rotation of the LSFL orientation, the lifetime of this transient state was determined to be 3.6 ps. This state is attributed to a transient refractive index grating associated with photoinduced carrier dynamics. A physical model describing the interference of temporally shifted surface plasmon polariton contributions, supported by multiscale simulations, is proposed to account for the distinct LSFL formation pathways observed in the zero-delay and delayed-pulse regimes.