Spectroscopic differentiation and microscopic mapping of various materials using optical pump-probe contrast

Abstract

Understanding the photophysical properties of materials is important in order to identify them, predict structural and functional changes, and improve fabrication methods. Here I demonstrate the use of time-resolved nonlinear optical microscopy, pump-probe microscopy, on various materials (e.g., historic artwork pigments, modern automotive paints, melanin, and perovskite solar cells) to investigate their photophysical dynamics non-invasively with high molecular specificity and spatial resolution. In this dissertation, I discuss the challenges in the field of material science which are difficult to investigate using conventional analytical tools but which can be solved with the pump-probe technology by accessing intrinsic electronic and vibrational dynamics contrast microscopically in 3D. In chapter 2, using Vermilion (HgS, notorious for losing its red color under light exposure) as an example, I demonstrate that pump-probe microscopy can map the distribution of α-, β-HgS, and liquid mercury on the microscopic scale. Then, I present studies investigating Vermilion’s degradation products under two light sources: an ultrafast near-infrared laser and an ultraviolet lamp. I even identify degradation products in discolored Vermilion of a 14th century painting. Then, in chapter 3, I extend its application to a series of red organic pigments (ROPs) which exemplify the challenge in the field of conservation science due to the limitation of conventional nondestructive analysis tools. I highlight that intrinsic photo-physical properties of ROPs provide molecularly specific contrast and utilize this contrast for high-resolution, three-dimensional imaging without the need for physical sample removal. Furthermore, I demonstrate the possibility of the initiation of the chemical breakdown of Carmine under NIR illumination.In chapter 4, I highlight another potential application of pump-probe microscopy as an analytical tool for forensics of other types of organic colorants such as automotive paints. In addition, I present the importance of microscopic analysis using ultramarine blue pigments and discuss the non-radiative relaxation property of melanin in hair as an example. Finally, in chapter 5, I present the progress on utilizing pump-probe microscopy for the mapping of charge carrier dynamics of organic-inorganic hybrid Perovskite (CH3NH3PbI3-xClx) layers on the femto- to pico-second timescale and sub-micron spatial scale. The effects of single- and two-photon excitation, chemical composition, and aging on the granular level variations are discussed.