Multi-octave spanning nonlinear interactions induced by femtosecond filamentation in polycrystalline ZnSe

We report on filamentation and supercontinuum generation in polycrystalline ZnSe by intense femtosecond laser pulses whose center wavelength is tunable in the 1.5-2.4 μm range. By varying the input pulse energy, single and multiple filamentation regimes were accessed, as verified by the measurements of near field intensity distributions. Along with supercontinuum generation, efficient simultaneous generation of broadband second, third, and fourth harmonics is observed. We uncover that extremely efficient harmonics generation stems from randomly quasi phase matched three wave mixing processes, which are facilitated by filamentation of the input beam, as verified by the spectral and energy conversion dynamics, and by polarization measurements.