Inorganic Ge-based ferroelectric perovskites towards quasi-phase-matching in the infrared region

Abstract

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, 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 for CGC and 12.0 × AGS for CGB at 2050 nm). Furthermore, both compounds show ferroelectricity with spontaneous polarizations of 6.6 µC cm⁻² and 19.3 µC cm⁻², respectively. The induction and erasure processes of periodic ferroelectric domain structures in CGC were observed. These properties underscore its 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.