We report Kerr-lens mode-locked polycrystalline Cr2+: 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 300MHz repetition rate with 700mWaverage power. To the best of our knowledge these are the shortest reported to-date pulses from Cr2+:ZnS and Cr2+:ZnSe lasers.
