BaHgGeSe4 and SrHgGeSe4: Two New Hg-Based Infrared Nonlinear Optical Materials

New practically usable infrared (IR) nonlinear optical (NLO) crystals, especially those suitable for highly efficient pumping of the commercial 1 mu m laser, are in urgent demand. However, only a few new IR NLO materials have been grown into bulk crystals and realized IR laser output during the past 20 years due to the extreme difficulty in achieving the coexistence of various strongly correlated properties. In this work, on the basis of the bonding characteristics of the Hg element and an efficient screening strategy for high-performance new crystals, we identified two new Hg-based IR NLO materials, BaHgGeSe4 and SrHgGeSe4. They crystallize in polar space group Ama2 with the distorted HgSe4 and GeSe4 tetrahedra aligned in parallel. They exhibit exceptional balance in terms of all of the preferred properties for practical applications, especially for highly efficient pumping of the commercial 1 mu m laser. Their large band gap (similar to 2.5 eV) can avoid the two-photon absorption of the 1 mu m laser and increase the laser damage threshold. They are phase matchable with a very strong NLO response (5 x AgGaS2). They can cover the 3-5 and 8-12 mu m atmospheric windows. Moreover, they melt congruently, which indicates that bulk crystals can be obtained by the Bridgman method. Their overall properties are better than those of the traditional AgGaQ(2) (Q= S or Se) materials to a large degree. Detailed structural analysis and calculations elucidate the crucial role of cations in regulating the packing of the anionic groups and that of the highly polarizable HgSe4 in balancing optical properties.