The conversion efficiency of femtosecond ytterbium fiber laser pulses to the mid-IR range in AgGaS2, BaGa4Se7, and HgGa2S4 crystals

The efficiency of AgGaS₂, BaGa₄Se₇, and HgGa₂S₄ crystals has been investigated in a mid-IR frequency difference generator based on a femtosecond ytterbium fiber laser with a wavelength of 1.03 μm, pulse duration of 0.25 ps, pulse repetition rate of 10 kHz, and average radiation power of 5 W. It is shown that under the given experimental conditions the highest efficiency of difference-frequency generation (up to ∼7.5%) is achieved with a HgGa₂S₄ crystal that provides 2 or 3 times higher energy of pulses with a wavelength of 4 to 7 μm (up to 6 μJ) without damaging it. In addition, this crystal yielded the longest wavelength of converted radiation – 11 μm with pulse energy ∼1 μJ.