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 broadenin...
