Heterodyne Interferometric Polarization-Sensitive Coherent Anti-Stokes Raman Scattering (HIP-CARS) Spectroscopy

Abstract

Coherent anti-Stokes Raman scattering, better known as CARS, is a powerful and versatile spectroscopic and microscopic technique, with many advantages over traditional Raman and fluorescence methods. It is immune to autofluorescence, does not require labeling, and returns molecules to their ground states, which reduces sample heating. However, CARS suffers from two negative traits compared to Raman and fluorescence measurements. First, a non-resonant background is frequently more intense than the resonant contributions in a sample. Second, CARS signals are orders of magnitude weaker than Stokes Raman lines, rendering the detection of small concentrations of weakly-resonant molecules difficult. Our current research focuses on these two problems in particular: inherent polarization-sensitivity of CARS allows isolation and rejection of the non-resonant component of CARS, though at the cost of a significant reduction in resonant signal I; heterodyning techniques provide simultaneous enhancement of CARS signals and rejection of the non-resonant background, but require explicit knowledge of the phase relations in the CARS process2. Using a home-built optical parametric oscillator3 we have developed a method that exploits both heterodyning and polarization sensitivity that offers resonant-only enhancement of CARS signals without and specific knowledge of the phases. This "HIP-CARS" procedure significantly simplifies the detection elements of the system and provides for future development of fast, resonant-only CARS imaging.