Ultra-high nonlinear absorption coefficients based on multiphoton-excited self-trapped excitons in perovskite-inspired copper halides

Abstract

Lead-free copper halides have garnered significant attention due to their remarkable advancements in optoelectronic devices. While there have been numerous reports on linear optical characteristics and device implementations for lead-free copper halides, the lack of robust research on nonlinear optical properties impedes their potential applications in nonlinear photonic devices. In this work, lead-free copper halide CsCu₂I₃ and Cs₃Cu₂I₅ single crystals were synthesized using an inverse temperature crystallization method, and their nonlinear upconversion luminescence properties arising from three-photon absorption were investigated under excitation of a near-infrared femtosecond laser. Surprisingly, in comparison with the single crystals, the CsCu₂I₃ and Cs₃Cu₂I₅ thin films were both found to exhibit completely different nonlinear optical properties with a distinct two-photon absorption mechanism under the same femtosecond laser. The two-photon absorption coefficients were obtained as high as 2.63 × 10⁷ and 1.76 × 10⁷ cm MW⁻¹ for the CsCu₂I₃ and Cs₃Cu₂I₅ thin films, respectively. Moreover, the nonlinear refractive indices of the CsCu₂I₃ and Cs₃Cu₂I₅ thin films were achieved as large as 1.25 × 10⁴ and 0.33 × 10⁴ cm² MW⁻¹, respectively. These values were at least several orders of magnitude higher than the reported two-photon absorption results of the perovskite thin films. These findings highlight the great potential of the perovskite-inspired CsCu₂I₃ and Cs₃Cu₂I₅ thin films for advanced applications in nonlinear optoelectronic devices.