BaGa4Se7-based 7-15 µm tunable broadband optical parametric amplifier pumped around 2 µm.

The non-oxide BaGa₄Se₇ (BGSe) crystal with broad transparency range, large nonlinearity, and high damage threshold has been widely utilized to build optical parametric oscillators/amplifiers that convert the well-developed near-infrared pump laser around 1 µm into the developing mid-infrared radiation. However, the inherent narrow phase-matching bandwidth of BGSe with a pump around 1 µm hampers the generation of ultrashort mid-infrared pulses. Here, we demonstrate that by pumping the BGSe crystal around 2 µm, it is possible to achieve a sufficient phase-matching bandwidth to support ultrashort pulses across a broad mid-infrared spectral range. In the experiments, two synchronized 1 kHz optical parametric chirped-pulse amplification sources centered at 2.35 µm and 3.1 µm are used to pump and seed a BGSe-based optical parametric amplifier, generating 52 µJ sub-four-cycle pulses at 9.7 µm. The central wavelength of the generated mid-infrared pulse can be tuned from 7 to 15 µm by finely adjusting the pump and seed wavelengths as well as the crystal orientation. These results reveal the enormous potential and bright prospects of BGSe for generating ultrashort intense pulses in the long-wave infrared region.