Ultrafast Dynamics and Nonlinear Photonics of Perovskites and Doped Semiconductor Nanocrystals

Abstract

Metal halide perovskites, with their chemical formula ABX₃, have emerged as a game-changer in the field of optoelectronics. These versatile materials hold promise for various functional applications, from light-emitting diodes (LEDs) and lasers to photovoltaics and beyond. Their true potential lies in their outstanding optoelectronic properties and low-cost solution processability. The strategy of introducing asymmetry into the perovskite structure by incorporating chiral cations at the A-site offers a promising pathway for achieving even-order nonlinear optical responses, such as second-harmonic generation (SHG), in hybrid perovskites. Particularly exciting are low-dimensional chiral hybrid perovskites; these structures exhibit exceptional nonlinear optical responses due to their unique properties, including strong quantum confinement, pronounced exciton effects, and diverse structural possibilities. Furthermore, doped semiconductor nanoparticles, such as Sn-doped In₂O₃ (ITO), exhibit localized surface plasmon resonance (LSPR) in the near-infrared (NIR) to mid-infrared (MIR) range. Interestingly, co-doping ITO with Cr alongside Sn increases carrier density and reduces carrier scattering, leading to a significantly higher LSPR Q-factor. However, the underlying mechanism behind this dramatic improvement remains an intriguing open question for further exploration. In this thesis, we have used various ultrafast spectroscopy techniques to probe fundamental physical mechanisms in perovskites and doped semiconductors. Using time-resolved THz and transient absorption spectroscopy, we have demonstrated sub-picosecond hole injection from all inorganic CsPbBr3 perovskite NCs to Cu-based inorganic hole transporting layers, paving the way for cheaper and more stable photovoltaic devices. Further, we have explored the nonlinear optical behaviour of 0D Pb-free chiral hybrid iodides, which exhibit second and third-order up-conversion. Also, Z-scan experiments revealed large two-photon absorption coefficients, high nonlinear refractive indices, and impressive optical damage thresholds, solidifying their potential for ultrafast photonic applications. In Cr-Sn co-doped In2O3 nanocrystals, utilizing time-domain THz spectroscopy, we gained invaluable insights into their complex dielectric function, observing how it changed with increasing Cr concentration. ITO possess high optical nonlinearity, which varies with carrier concentration. We employed Optical Kerr Effect spectroscopy to understand the change in the Kerr nonlinearity with Cr doping concentration. The time-resolved optical Kerr effect (OKE) measurements reveal that the recovery time of OKE is maximum for samples with the highest Cr doping. Heavy Cr doping significantly alters the lattice parameters of the material and induces a pronounced optical phonon mode in the GHz range, as revealed by the OKE spectral density analysis.