PbGa2MSe6 (M = Si, Ge): Two Exceptional Infrared Nonlinear Optical Crystals

Two noncentrosymmetric (NCS) quaternary selenides, PbGa2SiSe6 (1) and PbGa2GeSe6 (2), with second-order nonlinear optical (NLO) responses, were synthesized by a conventional high-temperature solid-state reaction method. Compounds 1 and 2 are constructed by three NCS chromophores, [PbSe4], [GaSe4], and [Ga/SiSe4] or [Ga/GeSe4], with the covalent interactions between the X and Se atoms (X = Pb, Ga, Ga/Si, or Ga/Ge). They crystallize in the polar space groups Cc and Fdd2, respectively. Inspiringly, compound 2 is phase-matchable (PM) and shows high laser-induced damage threshold (LIDT) of 3.7 × AgGaS2 and wide transparent region (0.63–25 μm) in the mid-infrared (MIR) region. Most importantly, it presents extraordinary strong second harmonic generation (SHG) at 2.05 μm radiation of about 12 × AgGaS2 at the particle size of 25–45 μm, which represents the strongest SHG among PM chalcogenides to date. The calculated major SHG tensor elements of compounds 1 and 2 are d31 = 224.7 and d12 = 222.1 pm/V, respectively, while the calculated d36 of AgGaS2 is only 21.2 pm/V.