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Background-free time-resolved coherent Raman spectroscopy (CSRS and CARS): heterodyne detection of low-energy vibrations and recognition of excited-state contributions

Coherent Raman scattering (CRS) spectroscopy techniques have been widely developed and optimized for different applications in biomedicine and fundamental science. The most utilized CRS technique has been coherent anti-Stokes Raman scattering (CARS), and more recently, stimulated Raman scattering (SRS). Coherent Stokes Raman scattering (CSRS) has been largely ignored mainly because it is often strongly affected by fluorescence, particularly for resonance enhanced measurements. However, in the cases of resonant excitation the information contained in the CSRS signal can be different and complementary to that of CARS. Here we combine the approaches of pulse shaping, interferometric heterodyne detection, 8-step phase cycling and Fourier-transform of time-domain measurements, developed in CARS and 2D electronic spectroscopy communities, to measure resonant CSRS and CARS spectra using a Titanium:sapphire oscillator. The signal is essentially background-free (both fluorescent and non-resonant background signals are suppressed) with high spectral resolution and high sensitivity, and can access low-energy modes down to ~30 cm-1. We demonstrate the ability to easily select between CSRS and CARS schemes and show an example in which acquisition of both CSRS and CARS spectra allows vibrational modes on the excited electronic state to be distinguished from those on the ground electronic state.