Polarization-enabled spectral-focusing CARS microscopy

Abstract

We describe a spectral-focusing-based polarization-resolved coherent anti-Stokes Raman scattering (SFP-CARS) microscopy system developed by making simple and inexpensive modifications to an existing spectral focusing CARS setup. By using the system to study polarization dependent features in the CARS spectrum of benzonitrile, we assess its capabilities and demonstrate its ability to accurately determine Raman depolarization ratios. Ultimately, the detected anti-Stokes signals are more elliptically polarized than expected, hindering a complete suppression of the non-resonant background. Furthermore, the fact that resonant signals polarized in directions similar to that of the non-resonant background are also substantially suppressed when extinguishing the non-resonant background remains a serious limitation. We conclude that non-resonant background suppression using the SFP-CARS system is best suited for studying Raman modes that generate signals polarized in directions far from that of the non-resonant background instead of for obtaining background-free CARS spectra. In all, we find that the SFP-CARS setup is a useful tool for studying polarization dependent features in the CARS spectra of various samples that is worthy of further investigation. This work aims to illuminate several technical aspects of polarization dependent CARS and inform researchers of the benefits and constraints of integrating polarization dependent detection as an add-on to existing CARS microscopy setups.