Li2K4TiOGe4O12: A Stable Mid-Infrared Nonlinear Optical Material

Discovering new mid-IR nonlinear optical (NLO) materials with high laser damage thresholds and easy crystal growth is an urgent and challenging task. Through cationic substitution of Si4+ in Li2K4(TiO)­Si4O12, a noncentrosymmetric and polar titanogermanate material, Li2K4TiOGe4O12 has been successfully synthesized. Physical property measurements indicate that this compound can exhibit a wide transparency range extending from 0.28 to 5.8 μm, a high laser damage threshold (LDT) of 910 MW/cm2 (1064 nm, 6 ns, 15 Hz), and a moderate second harmonic generation (SHG) response, ∼2 × KDP at 1064 nm. More importantly, a high-quality single crystal with a size of 35 × 10 × 7 mm3 has been grown by a top-seeded solution growth method from a stoichiometric melt. In addition, this compound also exhibits small anisotropic thermal expansion and desirable specific heat and thermal conductivity. All of these physical properties indicate that Li2K4TiOGe4O12 is a stable mid-IR NLO material. The discovery of Li2K4TiOGe4O12 also suggests that cationic substitution using heavier elements in the same group is a simple but effective strategy to design new mid-IR NLO oxides.