Inorganic Ge-Based Ferroelectric Perovskites towards Quasi-Phase-Matching in Infrared Region

Nonlinear optical crystals are crucial materials in laser technology. In the field of infrared quasi-phase-matching, high-quality output has been achieved using a limited set of materials, such as periodically poled LiNbO3 and LiTaO3. However, the scarcity of high performance infrared nonlinear optical crystals and suitable periodically structured materials has restricted further progress in infrared quasi-phase-matching. Herein, two Ge-based ferroelectric perovskites of CsGeCl3 (CGC) and CsGeBr3 (CGB) were synthesized, which exhibit a broad transmission window (covering 0.5-25 µm) and strong second harmonic generation responses (1.6 × AGS of CGC and 12.0 × AGS of CGB at 2050 nm). Furthermore, both compounds show ferroelectricity with spontaneous polarizations of 6.6 μC/cm2 and 19.3 μC/cm2, respectively. The induction and erasure processes of periodic ferroelectric domain structures in CGC were observed. These properties underscore their great potential for broadband infrared quasi-phase-matching applications. This work highlights the significance of ferroelectrics in quasi-phase-matching, and provides new opportunities for halide perovskites in nonlinear optics and integrated photonics.