Broadband ultrafast nonlinear optical studies revealing exciting multi-photon absorption coefficients in phase pure zero-dimensional Cs4PbBr6 perovskite films

Abstract

Lead halide perovskite nanocrystals (NCs) apart from their overwhelming optoelectronic applications have recently demonstrated promising nonlinear optical (NLO) properties such as strong two-photon absorption cross-sections (∼10⁵ GM), two-photon fluorescence, and saturable absorption even at very high peak intensity. Zero-dimensional perovskite-related materials (0-D PRMs) are a new class of materials offering a high exciton binding energy (E_g ≥ 180 meV) with a strong photoluminescence (PL) quantum yield in few cases. Herein, we report the broadband third-order NLO properties of phase pure Cs₄PbBr₆ 0-D PRM achieved using the Z-scan and degenerate four-wave mixing techniques in the femtosecond regime. Considering the growing content of the fluorescent and non-fluorescent forms of this material, we have performed our studies on both of them. These perovskite NCs exhibited strong multi-photon absorption properties in the near-infrared region with two-photon absorption (2PA) (cross-section, σ₂ = 10⁻⁴³–10⁻⁴⁴ cm⁴ s equivalent to ∼10⁶ GM) in the 500–800 nm region, three-photon absorption (3PA) (cross-section, σ₃ ∼10⁻⁷³ cm⁶ s²) in the 900–1200 nm region and four-photon absorption (4PA) (cross-section, σ₄ ∼10⁻¹⁰⁰ cm⁸ s³) in the 1300–1500 nm spectral region. These multi-photon absorption processes are explained using a simple band diagram. The measured NLO coefficients and cross-sections are fairly large when compared to some of the earlier reports on perovskite-based NCs. Cs₄PbBr₆ 0-D PRM also demonstrated a large third-order NLO susceptibility χ⁽³⁾ (∼10⁻⁷ esu), which can be attributed to the strong quantum confinement arising from spatially isolated, exciton containing individual [PbBr₆]⁴⁻ octahedron. These results clearly suggest the potential of 0D-PRMs in applications such as photonics and ultrafast all-optical switching devices.