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Phosphogermanate Crystal: A New Ultraviolet-Infrared Nonlinear Optical Crystal with Excellent Optical Performances.

The phase matching ability is a key factor for nonlinear optical crystals to realize coherent output. Herein, a new design strategy combining ultraviolet and infrared functional groups into a ferroelectric was put forward. Thus, a phosphogermanate crystal, KGeOPO4, was designed and studied. It exhibits a wide transparency window (0.22-9.70 μm), a strong second harmonic generation response (5× KH2PO4), a high laser-induced damage threshold (1.61 GW/cm2), and the typical ferroelectricity (coercive field ∼ 9.8 kV/cm and remnant polarization ∼7.6 μC/cm2). In the infrared region, it could realize coherent output by the birefringence phase matching method, while it could generate ultraviolet coherent lights by the quasi-phase matching technique. Therefore, this work designs a promising ultraviolet-infrared nonlinear optical crystal and provides a new perspective for exploring nonlinear optical crystals.