High-efficiency narrowband terahertz generation in novel wide-bandgap barium chalcogenide crystals
We report on extensive experimental investigation of novel barium chalcogenides (BaGa4Se7, BaGa2GeSe6 and BaGa2GeS6) for generation of a strong narrowband terahertz (THz) radiation. The Ba-containing compounds demonstrate the large bandgap values, high optical damage threshold and wide transparency range from the visible to mid-infrared region, coupled with narrowband transparency windows in the THz frequency range. In these windows a THz generation via the second-order nonlinear process of optical rectification occurs under optical pumping by a 50-GW high-power Cr:forsterite laser leading to intrinsically narrowband THz pulses. The highest optical-to-THz conversion efficiency of 0.8^10(-5) is reached 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 the frequencies in the range from 1.9 to 2.5 THz with a relative bandwidth of 3-4%, necessary for imaging and spectroscopy for security or medical applications.