Plasma-Mediated Ablation in 200 MHz Burst-Mode Femtosecond Laser: Energy Partition and Time-scanned Plasma Diagnostics

Ablation with burst-mode femtosecond lasers at hundreds of MHz introduces inter-pulse residual plasma that significantly alters the dynamics of energy deposition. We present experimental measurements of the evolution of such inter-pulse plasmas during 50-pulse bursts at 200 MHz on fused silica. A pulse-resolved energy-partition measurement quantified specular and diffuse reflection, transmission, and side-scatter during irradiation, revealing progressively increasing absorptivity throughout the burst. The measured divergence of the transmitted pulses increased as the burst progressed, likely due to negative lensing from the accumulating plasma plume. A time-scanned pump-probe experiment further characterized the plasma's specular reflectivity and direct transmissivity from –30 ps to +300 ps around each of the 60 pulses. The observed optical dynamics are interpreted using a thin-layer Fresnel-Drude model, in which the ratio of plasma thickness to collisionless skin-depth governs response. Our results demonstrate that, at these repetition rates, ablation becomes strongly plasma-mediated: later pulses encounter an evolving underdense plume that leads to increased absorption and beam distortion. These findings highlight the role of plasma persistence and hydrodynamic expansion in dictating energy coupling, motivating future diagnostics such as frequency-domain interferometry (FDI) to time-resolve the evolution of the residual plume density profile.