Linear and nonlinear optical studies of two-dimensional Perovskites

Abstract

Materials with large optical nonlinearity, especially in the visible spectral region, are in great demand for applications in all-optical information processing and quantum optics. Two-dimensional (2D) hybrid Ruddlesden−Popper halide perovskites (RPPs) with tunable ultraviolet-to-visible direct bandgaps, exhibit large nonlinear optical properties due to the strong excitonic effects present in their multiple quantum wells (MQWs). In this thesis, the nonlinear optical properties of 2D RPP nanosheets mechanically exfoliated from five different lead halide RPP single crystals, are systematically investigated. Using a microscopic Z-scan setup with femtosecond (fs) laser pulses tunable across the visible spectrum it is demonstrated that the 2D RPP nanosheets possess unprecedentedly large nonlinear refraction and absorption coefficients near excitonic and plasma resonances. It is also demonstrated that 2D RPP nanosheets exhibit strong two-photon absorption (2PA) in the near-infrared (NIR) spectral region under two-photon-resonant excitation. By taking advantage of the large nonlinear absorption coefficients of 2D RPPs in the NIR, highly efficient polarization-resolved sub-bandgap photodetection based on 2D RPP phototransistors is realized. Moreover, the 2D RPP nanosheets are shown to exhibit ultra-strong third-harmonic generation (THG) under three-photon-resonant excitation. Using an ultrafast pump-probe spectroscopy setup it is found that, under sub-bandgap excitation, ⁓100-nm thick 2D RPP sheets allow up to ⁓2% reflectivity modulation within a 20-fs period, due to the nonlinear refraction and absorption.