Tunable near-infrared emission and three-photon absorption in lanthanide-doped double perovskite nanocrystals

Cs₂AgInCl₆ double perovskite (DP) nanocrystals (NCs) are an emerging class of materials with promising application potential in photonics/optoelectronics owing to their nontoxicity, direct bandgap, and high thermal and moisture stability. These NCs are, however, rarely explored for nonlinear optical (NLO) applications. Herein, we present a comprehensive investigation of the photophysical and nonlinear optical properties of erbium- (Er) and ytterbium (Yb)-doped Cs₂AgInCl₆ nanocrystals (denoted as Er-DP and Yb-DP, respectively). Temperature-dependent photoluminescence of these NCs was analyzed to estimate their exciton binding energy, Huang-Rhys parameter (S), and electron-phonon coupling strength, which are of fundamental interest to gain an in-depth understanding of the material systems. Femtosecond Z-scan experiments with 800 nm excitation revealed the reverse saturable absorption (RSA) behavior owing to three-photon absorption (3PA). The obtained values of the 3PA coefficients were 1.35 × 10^-4 and 1.64 × 10^-4 cm³ GW^-2, respectively, and the nonlinear refractive indices were estimated to be 1.02 × 10^-15 and 1.15 × 10^-15 cm² W^-1, respectively, for Er-DP and Yb-DP. These values are superior to those obtained in undoped Cs₂AgInCl₆ NCs. The physical parameter, Kane energy, which is closely related to the magnitude of the oscillator strength, was estimated to be 25 eV and 26 eV for Er-DP and Yb-DP, respectively. As a proof-of-concept application, we further obtained the optical limiting onset and figure of merit to reveal their prospect as an optical limiter and in photonic switching application. With such emission and nonlinear optical properties, we anticipate that lanthanide-doped Cs₂AgInCl₆ NCs can be used for designing eco-friendly nonlinear optoelectronic/photonic devices.