Single-beam CARS spectroscopy applied to low-wavenumber vibrational modes

In single-beam coherent anti-Stokes Raman scattering (CARS) spectroscopy, a complete CARS scheme is accomplished with tailored pulses from a fs-oscillator by coherent control of the nonlinear signal generation. Here, the application of single-beam CARS to low-wavenumber vibrational modes in the range of 50-400 cm(-1) is demonstrated by investigating pure liquid-phase samples and molecules in solution. The spectral resolution (17 cm(-1)) achieved by coherent control of the CARS process is an order of magnitude better than the spectral width of the excitation pulses (150 cm(-1) FWHM). At the same time, nonresonant background is suppressed by more than 1 order of magnitude. It is shown that spectral shaping of the excitation spectrum can drastically extend the accessible range of vibrational modes in single-beam CARS. The implemented scheme is very well suited for CARS microscopy because of its conceptual simplicity and flexibility, and presents the opportunity for spatially resolved studies of conformational modes in biological samples. Copyright (C) 2006 John Wiley & Sons, Ltd.