Persistence of self-limited conditions in bulk silicon induced by mid-infrared femtosecond laser pulses

With the advent of intense infrared laser solutions, three-dimensional laser writing progressively becomes applicable to silicon (Si) and other semiconductors. However, the nonlinearities required for bulk-energy deposition also induce severe beam distortions, causing processing difficulties in the short-wave infrared domain. Here, we assess the potential of longer wavelengths in the mid-infrared (MIR) domain by systematically comparing microplasma formation and material modification in Si using sub-200-fs pulses at 1.4 and 4 μm. Under identical tight-focusing conditions, both wavelengths generate well-defined plasmas with similar maximum densities at ≈1020 cm−3. Z-scan writing experiments exploring energies exceeding the observed ionization limit show that neither wavelength enables internal structuring of Si, in contrast to temporally stretched picosecond pulses. While ionization dynamics and characteristics can be varied significantly with wavelength, we show that pronounced filamentary effects restrict the potentially expected advantages of the MIR domain for ultrafast laser internal structuring of semiconductors.