Nonlinear interferometric vibrational imaging: A method for distinguishing coherent anti-stokes Raman scattering from nonresonant four-wave-mixing processes and retrieving Raman spectra using broadband pulses

When utilizing broadband, short pulses to stimulate Coherent Anti-Stokes Raman Scattering (CARS), frequently the peak power is sufficient to excite other nonlinear, nonresonant processes in the material. These processes produce a four-wave-mixing component in the same frequency band as the CARS signal, so that the two signal types cannot be distinguished on the basis of frequency band alone. Typically in biological materials, the nonresonant component produced by the bulk medium can overwhelm the CARS signals produced by the usually much lower concentration target molecular species. Resonant processes can be distinguished from nonresonant processes in that molecular vibrations and rotations typically last longer than a picosecond, while nonresonant processes are not persistent and last shorter than 10 fs. Interferometry allows the arrival time of the signal to be determined to extremely high accuracy, limited by the bandwidth of the reference pulse. Because resonances are persistent, they can produce anti-Stokes radiation that persists after the nonresonant excitation is produced. Interferometry can distinguish the later arrival of the anti-Stokes radiation and therefore distinguish the resonant and nonresonant signals. More complicated pulse-shaping and interferometry schemes can allow additional flexibility to allow simultaneous sampling of the Raman spectrum while rejecting the nonresonant component. This technique enables an entire region of the Raman spectrum of a sample to be measured with a single brief broadband pulse.