Three optical cycle mid-IR Kerr-lens mode-locked polycrystalline Cr(2+):ZnS laser

We report Kerr-lens mode-locked polycrystalline Cr(2+):ZnS lasers at 2.4 μm central wavelength optimized for short pulse duration. By control of the second- and third-order dispersion within 500 nm bandwidth we obtained pulses of three optical cycles (<29  fs) at 100 MHz repetition rate with 0.44 W average power. The emission spectrum is 240 nm broad at -3  dB level and spans 950 nm at -30  dB level. Transform-limited 38 fs pulses were obtained at 300 MHz repetition rate with 700 mW average power. To the best of our knowledge these are the shortest reported to-date pulses from Cr(2+):ZnS and Cr(2+):ZnSe lasers.