I. Introduction
As a nonlinear vibrational imaging technique developed very recently [1]–[5], stimulated Raman scattering (SRS) microscopy probes the intensity gain in the Stokes beam or the intensity loss in the pump beam. The optical heterodyne detection used in this technique not only amplifies the signal but also renders the SRS intensity linearly proportional to the molecular concentration, thus making it sensitive to detect low-concentration molecules. Unlike the widely used coherent anti-Stokes Raman scattering microscopy [6]–[8] in which the signal contains a nonresonant electronic contribution and a Raman-resonance contribution, the SRS provides identical spectral information as spontaneous Raman scattering. The recent applications in vibrational mapping of biomass conversion, pharmaceutical tablets, and human skin show the potential of SRS microscopy for high-speed chemical imaging based on fingerprint Raman bands [5], [9], [10].