Efficient and broadband long-wavelength infrared generation driven by erbium femtosecond laser

Broadband long-wavelength infrared (LWIR) light sources in the molecular fingerprint spectral region are highly desirable for advanced spectroscopy and remote sensing applications. Intra-pulse difference frequency generation (IPDFG) driven by well-established erbium-doped fiber lasers has recently emerged as an efficient and compact approach for multi-octave LWIR radiation generation. Here, we demonstrate a highly efficient IPDFG with a spectrum spanning from 7.5 to 13 µm in a newly developed ZnGeP₂ (ZGP) crystal namely, YS-ZGP, pumped by a few-cycle erbium-doped fiber laser. Benefitted from the high nonlinearity and reduced two-photon absorption in YS-ZGP, an output power of 5.48 mW and a conversion efficiency of 0.67% are obtained, which both represent record values in LWIR IPDFG systems, pumped by 1.55 µm femtosecond lasers. This result overturns the conventional perception that IPDFG in ZGP crystals can only be pumped at wavelengths longer than 2 µm. Furthermore, to extend the spectral coverage into longer wavelengths, IPDFGs in BaGa₄Se₇ and BaGa₂GeSe₆ crystals are investigated too, achieving octave-spanning LWIR outputs covering 8-16 µm and 7-16.5 µm, respectively. We anticipate that YS-ZGP-based LWIR sources pumped by 1.55 µm lasers will significantly advance practical applications in molecular spectroscopy and sensing.