Abstract

In general, ionization-induced free-electron plasma is considered to increase spectral blueshift during femtosecond laser filamentation. Here we theoretically show that the enhancement of plasma density via avalanche ionization decreases the blueshift of supercontinuum (SC) spectra associated with a ultraviolet filament in fused silica. By numerically solving the forward Maxwell equations, our simulations show that the arrest of beam collapse is ascribed to multiphoton absorption rather than plasma defocusing. In addition, SC spectral broadening is mainly dominated by Kerr self-phase modulation (SPM), while the plasma generated by ionization plays a more significant role in absorbing the laser pulse energy, which would suppress the Kerr SPM effect, than in reducing the refractive index. Our work provides a clearer understanding of ultraviolet laser propagation dynamics in condensed media.

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