Energy deposition and the transition from linear to nonlinear focusing regimes in ultrafast laser filamentation

This study examines the dynamics of filamentation during ultrafast laser propagation using loosely focused ∼800 nm, ∼35 fs laser pulses through lenses with varying f-numbers. The transition between linear and nonlinear regimes—driven by the interplay of strong plasma generation, geometrical focusing, and Kerr self-focusing—introduces noticeable changes in filament signatures and properties, including energy deposition, fluorescence intensity, supercontinuum broadening, filament length, and morphology. Low f-number focusing is dominated by geometrical effects, producing shorter filaments with intense fluorescence, whereas higher f-number configurations favor nonlinear effects, resulting in longer, narrower filaments with more symmetric spectral broadening. Energy deposition analysis reveals a sharp increase at lower laser energies followed by saturation at higher energies, suggesting possible transitions to multifilamentation. Evaluating transition from linear to nonlinear regime using multiple filament signatures and energy absorption provided deeper insights into filament behavior across various regimes that would not have been achievable through individual diagnostic methods alone.