Broadband Coherent Raman Scattering Microscopy

Spontaneous Raman (SR) microscopy allows label-free chemically specific imaging based on the vibrational response of molecules; however, due to the low Raman scattering cross section, it is intrinsically slow. Coherent Raman scattering (CRS) techniques, by coherently exciting vibrational oscillators in the focal volume, increase signal levels by several orders of magnitude under appropriate conditions. In its single-frequency version, CRS microscopy has reached very high imaging speeds, up to the video rate; however, it provides information which is not sufficient to distinguish spectrally overlapped chemical species within complex heterogeneous systems, such as cells and tissues. Broadband CRS combines the acquisition speed of CRS with the information content of SR, but it is technically very demanding. In this Review, the current state of the art in broadband CRS microscopy, both in the coherent anti-Stokes Raman scattering (CARS) and the stimulated Raman scattering (SRS) versions are reviewed. Different technical solutions for broadband CARS and SRS, working both in the frequency and in the time domains, are compared and their merits and drawbacks assessed.