Rb${}_4$Li${}_2$TiOGe${}_4$O${}_{12}$: A titanyl nonlinear optical material with the widest transparency range

Practical mid-infrared (MIR) coherent light beams generated by frequency conversion in nonlinear optical (NLO) crystals are indispensable in time-resolved infrared vibrational spectroscopy, remote light detection and ranging, and free-space communications. Herein, a new titanyl germanate Rb${}_4$Li${}_2$TiOGe${}_4$O${}_{12}$ (RLTG) MIR NLO crystal was obtained by heavier element substitution. It features a complicated structure network composed of compressed TiO${}_5$ square pyramids and distorted GeO${}_4$ tetrahedra, separated by Rb${}^{+}$ and Li${}^{+}$ cations. More importantly, RLTG exhibits concurrently short ultraviolet (0.28 μm) and long IR (5.58 μm) transmittance cutoffs, fully covering the atmospheric transparent window of 3–5 μm. Related to the short UV cutoff, it shows a higher laser-induced damage threshold in comparison to commercial MIR NLO crystals, about twice that of KTiOPO${}_4$ (KTP) and 50 times that of AgGaS${}_2$ (AGS). (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)