Bioimaging by Fourier-transform coherent anti-Stokes Raman scattering microscopy

Coherent anti-Stokes Raman scattering (CARS) is a nonlinear Raman process in which pulsed light of at least two frequencies is mixed within a sample. By overlapping pump light at a frequency of omega_p and Stokes light at omega_s temporally and spatially, CARS light at a frequency of 2omega_p-omega_s is produced. When the frequency difference (omega_p-omega_s) can be tuned to specific vibrational resonances within a sample, CARS microscopy provides intrinsic vibrational contrast. However, in the case of the use of ultrabroadband pulses as an excitation light source, the resonant CARS signal is accompanied by significant nonresonant CARS signals from the electronic contribution. Fourier-transform CARS (FT-CARS) spectroscopy provides the suppression of the nonresonant CARS signals and the measurement of broadband CARS spectra with high spectral resolution at a time. In this paper, we demonstrate that FT-CARS microscopy employing 5-fs ultrabroadband pulses can be applied to imaging of unstained HeLa cells.