Second-harmonic generation and self-phase modulation of few-cycle mid-infrared pulses

Near-single-cycle mid-infrared pulses with a spectrum covering 5.4-11 μm are efficiently frequency-doubled in different GaSe crystals. The second-harmonic spectrum spans 3-4.3 μm at a power conversion efficiency of >20%. We measure an effective nonlinear coefficient of d_eff≈35  pm/V. We also report on self-phase modulation and spectral broadening of the mid-infrared pulses in various bulk materials and find an increase of 45% of spectral width for 5 mm of Ge. These results demonstrate that nonlinear optical conversions can efficiently be driven by few-cycle mid-infrared radiation.