Broadband Coherent Raman Scattering Microscopy

Spontaneous Raman (SR) microscopy allows label-free chemically specificimaging based on the vibrational response of molecules; however, due to thelow Raman scattering cross section, it is intrinsically slow. Coherent Ramanscattering (CRS) techniques, by coherently exciting vibrational oscillators inthe focal volume, increase signal levels by several orders of magnitude underappropriate conditions. In its single-frequency version, CRS microscopy hasreached very high imaging speeds, up to the video rate; however, it providesinformation which is not sufficient to distinguish spectrally overlappedchemical species within complex heterogeneous systems, such as cells andtissues. Broadband CRS combines the acquisition speed of CRS with theinformation 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 coherentanti-Stokes Raman scattering (CARS) and the stimulated Raman scattering(SRS) versions are reviewed. Different technical solutions for broadband CARSand SRS, working both in the frequency and in the time domains, arecompared and their merits and drawbacks assessed