A finite-difference model for intense light interactions with dielectrics in the ultrafast ionization regime

Abstract

We present a computationally efficient model that describes the interaction of intense, ultrashort infrared laser pulses with transparent materials in the strong ionization regime. The model is augmented with a detailed self-consistent description of the local response due to ionization and collisional plasma dynamics. It incorporates the direct solution of Maxwell's equations without approximations and rigorous boundary conditions for the input pulse, allowing us to study the ultrafast formation of over-critical nanoscaled plasmas in dielectric materials under the influence of intense tightly focused laser pulses. We perform a scan of the parameter space, find unexpected optima regimes for different experientially relevant parameters, and explain these maxima based on spatiotemporal dynamics.

Figures (4)

• Figure 1: Comparison of pulse propagation results with (left) and without (right) nonlinear feedback for the shortest pulse ($3\,\mathrm{fs}$) and strongest focusing ($w_0=400\,\mathrm{nm}$) case. a,b) Relative ionization degree $\rho$ at the end of the simulation with regions of overcritical plasma density indicated by white-dashed lines. c,d) Maximum local intensity reached during pulse propagation. (e,f) Snap-shot of the field amplitude at pulse peak.
• Figure 2: Top row: Systematic study of (a) peak intensity, (b) total absorbed energy and (c) overcritical plasma volume as function of pulse duration and focus spot size for calculations including plasma feedback. All three observables show maxima at different beam conditions. Bottom row: Corresponding ratio of values without and with feedback included. White areas indicate parameter regions where feedback is of minor importance.
• Figure 3: Analysis of maximum absorption scenario (3fs, 400nm). a) Final plasma density map with overcritical regions indicated by white-dashed lines. b) Local (solid) and accumulated (dashed) energy deposition along the propagation axis. Results for the shortest pulse and strongest focusing case are shown in gray for comparison. c) Local peak intensities reached during pulse propagation. d) Electric field amplitude snapshot at pulse peak.
• Figure 4: Electric field evolution (left) and plasma generation dynamics (right) for the maximum critical plasma generation scenario (30fs, 400nm). The increasing overcritical plasma volume forces electromagnetic fields to propagate around these regions, leading to a transient hollow field profile in the focus.