Tutorial for analyzing coherent Raman spectra excited with chirped femtosecond sources

Abstract

Coherent Raman spectroscopy (CRS) and imaging have been used successfully for the label-free visualization and analysis of functional and endogenous biomolecules in complex biological samples. The interest in implementing these methodologies worldwide in research laboratories has increased in recent years; however, many practical challenges are faced in the implementation process. One of them is identifying from scratch different coherent Raman signals when broadband femtosecond sources are utilized. We present a straightforward methodology to identify and analyze the multiple excited resonant SRS/CARS signals using a simple spectrophotometer in this work to expand the understanding of this particular branch of CRS. The coherent Raman experiments are conducted in the so-called temporal slit configuration using a broadband femtosecond laser source to obtain the excitation pump and Stokes beams and high group velocity dispersion glass to temporally stretch the pulses; but in a non-conventional and non-optimum configuration for CRS experiments neither for spectral focusing (single-frequency measurements) nor for mixing of a broadband pulse with a narrow band pulse (multiplexed detection). Despite that, we demonstrate our methodology's feasibility to extract and analyze complex to discern coherent Raman spectra. We use dimethyl sulfoxide (DMSO) and β-phase barium meta-borate crystal (BBO) and discuss molecular vibrations identified in the spectral range from 300 to 800 cm−1 from both kinds of CRS spectra, stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS).