Electronically Preresonant Coherent Anti-Stokes Raman Scattering Microscopy

Abstract

Coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) microscopy are key techniques for label-free vibrational imaging. However, their application is often limited by poor sensitivities. Recent advancements have shown that similar to spontaneous resonance Raman spectroscopy, SRS microscopy sensitivity can be greatly enhanced under electronic preresonance conditions. While electronic enhancement of CARS signals in spectroscopy was reported in the 1970s, its application in microscopy remains less explored. This study presents experimental results demonstrating significant sensitivity enhancements in CARS microscopy by choosing excitation wavelengths near electronic transitions in sample molecules, achieving detection levels down to low micromolar concentrations. We also conducted both experimental and theoretical investigations on spectra with and without electronic resonances, addressing the background signals encountered when using this method. Our findings advance the understanding of electronic preresonance effects in CARS and offer practical sensitivity enhancements, broadening the scope for coherent Raman imaging at lower concentrations.