Extreme nonlinear optics in a long pulse regime: High harmonic generation of picosecond mid-IR pulses in polycrystalline zinc selenide.

High harmonic generation (HHG) in semiconductors has been extensively studied recently in the high-intensity limit using middle infrared (mid-IR) femtosecond laser pulses resulting in emission spectra of self-phase modulated harmonics resting on top of a broadband continuum. In this report, a different approach to HHG in polycrystalline zinc selenide (poly-ZnSe) was explored utilizing a relatively low power regime (1-40 GW/cm 2 ) and much longer (30 ps) mid-IR laser pulses. Through a combination of low power, picosecond excitation, and narrowband (<10 nm full width at half maximum) mid-IR excitation, the nonlinear optical effects in poly-ZnSe could be isolated and studied independently. From the clearly distinguishable HHG peaks, harmonic conversion efficiencies of 10 -4 -10 -12 for second to ninth harmonic in poly-ZnSe were measured, and the relationship between the Nth harmonic intensity and excitation intensity (I 0 ) was found to follow a power law, I 0 x with x ≤ N/2, as a result of the random quasi-phase matching process.