High harmonic generation in ZnO with a high-power mid-IR OPA

We generate high-order harmonics in a-cut (11–20) ZnO at a high repetition rate of 50 kHz, using the tunable mid-infrared pulses (3–4 μm wavelength) from a high-power optical parametric amplifier. For driving laser pulses with 3.8 μm central wavelength, we observe nonperturbative harmonic spectra that well exceed the material band gap. The harmonic spectra depend strongly on the orientation of the crystal with respect to the laser polarization, with odd harmonics exhibiting periodicities of π/2 for a polarization within the (11–20) crystal plane. Energy conversion efficiencies of ∼10−6 per harmonic are measured for the 9th–13th harmonics, yielding an average power of more than 0.2 μW for the 13th harmonic.