High-efficiency narrowband terahertz generation in novel wide-bandgap barium chalcogenide crystals

We report an extensive experimental investigation of novel barium chalcogenides (BaGa4Se7, BaGa2GeSe6, and BaGa2GeS6) for the generation of strong narrowband terahertz (THz) radiation. These Ba-containing compounds demonstrate large bandgap values, high optical damage thresholds, and broad transparency from the visible to mid-infrared range, coupled with narrowband transparency windows in the THz frequency range. Within these windows, THz generation occurs via the second-order nonlinear process of optical rectification under optical pumping by a 50-GW high-power Cr:forsterite laser, producing intrinsically narrowband THz pulses. The highest optical-to-THz conversion efficiency of 0.8 × 10−5 is achieved in the Z-cut BaGa2GeS6 crystal in the saturation regime and corresponds to an output energy of 20 nJ. Ba compounds provide narrow-bandwidth THz pulses at frequencies in the range 1.9–2.5 THz with a relative bandwidth of 3%–4%, necessary for imaging and spectroscopy applications in security and medicine.