1560‐nm‐Driven Second‐Order Nonlinear Optical Response in CsGeI ₃ : a Room‐Temperature Ferroelectric Halide Perovskite

Electro‐optic (EO) crystals that exhibit strong second‐order nonlinear optical responses in the near‐infrared region, especially around 1560 nm, are essential for the advancement of high‐speed optical communication technologies. Metal halide perovskites (MHPs), with their low‐temperature processability, tunable optoelectronic properties, and emerging ferroelectricity, offer a promising platform for next‐generation EO devices. In this study, we report a strong nonlinear optical response; terahertz pulse emission from solution‐grown single crystals of ferroelectric halide perovskite CsGeI 3 . Under 1560 nm femtosecond laser excitation, CsGeI 3 exhibited broadband terahertz emission by optical rectification with the estimated second‐order nonlinear optical susceptibility of 3.9 × 10 2 pm/V. The mapping of the terahertz amplitude revealed ferroelectric domain structures exceeding 100 µm in size. These results position CsGeI 3 as a scalable nonlinear optical material with an electronic EO response, suitable for integrated photonic devices, high‐speed Mach‐Zehnder modulators in the telecom band.