Rb4 Li2 TiOGe4 O12 : 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 Rb4 Li2 TiOGe4 O12 (RLTG) MIR NLO crystal was obtained by heavier element substitution. It features a complicated structure network composed of compressed TiO5 square pyramids and distorted GeO4 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 KTiOPO4 (KTP) and 50 times that of AgGaS2 (AGS).