Multi-Watt mid-IR femtosecond polycrystalline Cr(2+):ZnS and Cr(2+):ZnSe laser amplifiers with the spectrum spanning 2.0-2.6 µm.

We demonstrate efficient amplification of few-optical-cycle mid-IR pulses in single-pass continuously pumped laser amplifiers based on polycrystalline Cr(2+):ZnS and Cr(2+):ZnSe. The 1.7 W output of a Kerr-lens mode-locked master oscillator at 2.4 µm central wavelength, 79 MHz repetition rate was amplified to 7.1 W and 2.7 W in Cr(2+):ZnS and Cr(2+):ZnSe, respectively. High peak power of the input pulses (0.5 MW) and high nonlinearity of the amplifiers' gain media resulted in a significant shortening of the output pulses and in spectral broadening. Transform-limited 40 fs pulses of the master oscillator were compressed to about 27-30 fs. The spectrum of the pulses was broadened from 136 nm to 450 nm (at -3 dB level); the span of the spectra exceeds 600 nm at -10 dB level.