Title: Stimulated Raman Excited Fluorescence Spectroscopy and Microscopy
Description: Powerful optical spectroscopic and imaging tools have revolutionized many areas of science and technology. The detection sensitivity and chemical specificity are two major considerations when characterizing an optical technique. The fluorescence spectroscopy and microscopy provide excellent sensitivity down to single molecules. However, its reliance on probing the electronic transition limits the obtainable chemical information. In contrast, vibrational spectroscopy such as Raman scattering provides exquisite chemical specificity about the molecular structure, dynamics, and coupling with the environment. However, Raman scattering is intrinsically weak, and its cross sections are many orders of magnitudes smaller than those of fluorescence. The efforts that trying to bridge these two powerful methods can be traced back to the 1980s. But it was until our attempt in 2019, by carefully tuning the electronic pre-resonance and optimizing the excitation duration, the first successful Raman-featured fluorescence spectroscopy was demonstrated.

This hybrid technique was named as stimulated Raman excited fluorescence (SREF). As expected, SREF combines both high sensitivity and exquisite chemical specificity, which enables the first all-far-field single-molecule Raman spectroscopy and imaging. This thesis is trying to provide a comprehensive interpretation of SREF, and at the same time serves as a practical guide for experiments and instrumentation. In chapter 1, the early (unsuccessful) attempts for SREF spectroscopy are reviewed. Additionally, I briefly summarize the basic theory for single- and multi-photon excitation process of a single fluorophore, from which the feasibility of SREF spectroscopy is explained. Chapter 2 is mainly focused on the instrumental details of the first successful SREF spectroscopy, and the basic spectroscopic features of SREF are summarized. The generality of SREF spectroscopy is systematically discussed in Chapter 3, and a rule-of-thumb criterion for successful SREF excitation based on the simple two-beam excitation strategy is proposed.

Aimed at background-free SREF microscopy, a three-beam system based on nonlinear fiber optics and lock-in detection is illustrated in Chapter 4. In chapter 5, the first attempt to combine SREF and the stimulated emission depletion (STED) for an all-far-field super-resolution vibrational imaging is proposed. At last, as a simple application of SREF microscopy, SREF-based vibrational Stark spectroscopy on visualizing the electrostatic field at the water-oil interface of microdroplets is discussed (Chapter 6).