Efficient heterodyne CARS measurement by combining spectral phase modulation with temporal delay technique

We propose and demonstrate an improved heterodyne coherent anti-Stokes Raman scattering (CARS) measurement with a rapid phase modulation and temporal displacement of the background, to simplify signal extraction and effectively reduce a nonresonant background (NRB). This method is a modification of the single-beam CARS spectroscopy originally proposed by Oron et al. in which a narrowband phase modulation is used to enhance contrast between resonant signals and the NRB through heterodyne detection [Phys. Rev. Lett. 89, 273001 (2002)]. In our scheme, a large delay between the narrow- and broadband components enables us to reduce the NRB while maintaining signal enhancement by heterodyne detection. We develop a frequency-resolved Michelson interferometer in which the narrow- and broadband components are spatially separated and recombined with an arbitrary delay. We show that sharp Raman lines can be obtained from chloroform molecules by the observation of difference spectra and phase sensitive detection. The spectral resolution achieved, which is limited by that of the spectrometer we used, is < 8 cm(-1). This method can potentially be extended to make real-time measurements by further developing a spectrometer that directly accumulates difference spectra.