Single pulse interferometric coherent anti-Stokes Raman scattering (CARS)

Abstract

The non-resonant background signal has been the major obstacle in coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy. This unwanted background is generated by the electronic response of the sample. It not only obscures the desired signal but also results in spectral interference with the desired vibrationally resonant CARS signal, making it difficult to assign vibrational peaks using characteristic spontaneous Raman spectra. We show that the non-resonant background can be used as a local oscillator for spectral interferometric CARS spectroscopy. Two different techniques are discussed to extract the vibrationally resonant multiplex CARS spectrum and discriminate it against the much larger non-resonant background. The pump, Stokes and probe pulses are all selected inside a single broadband ultrafast pulse (bandwidth ~1800 cm^-1) by a phase- and/or polarization-controlled pulse-shaping technique. The first technique generates two spectral interference CARS signals simultaneously, and the normalized difference of these two signals provides an amplified background-free broadband resonant CARS spectrum over 400-1500 cm^-1. The second method generates a single spectral interference CARS signal by a phase-only pulse shaping. A Fourier transform spectral interferometry (FTSI) method is used to retrieve the Raman-equivalent CARS spectrum from the measured spectral signal. Both methods enhance the resonant CARS signal by utilizing the non-resonant background as a local oscillator for homodyne mixing.