Focusing and light collection effects on plasma-induced frequency-resolved optical switching (PI-FROSt) traces

Plasma-Induced Frequency-Resolved Optical Switching (PI-FROSt) is a promising and recently proposed phase-matching-free technique for characterising ultrafast pulses across broad spectral ranges. We investigate the mechanisms of PI-FROSt trace formation through numerical simulations and experimental validation. The results reveal that trace characteristics are highly sensitive to the relative focusing geometry between pump and probe pulses, as well as the spatial region selected for signal collection. Depending on these conditions, the interplay between plasma defocusing and positive lens-like nonlinear effects causes either intensity depletion or enhancement in the probe beam, flipping the PI-FROSt trace. Simulations demonstrate that optimal gate stability also depends strongly on the focusing scheme and the collecting region. This study highlights that precise spatial and temporal optimisation is essential to properly exploit the benefits of this broadband pulse characterisation technique.