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

Abstract

<p>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. </p><p>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. </p><p>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.</p><p>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. </p><p>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.</p>