Stimulated Raman photothermal microscopy: theory and implementation

Stimulated Raman scattering (SRS) microscopy has shown enormous potential in revealing molecular structures, dynamics, and couplings in complex systems. For most biomolecules, the detection sensitivity of SRS is fundamentally limited to the milli-molar level due to the shot noise and the small modulation depth. Additionally, the operation of SRS imaging is complicated by cross phase modulation. We recently revisited SRS from the perspective of energy deposition. Via intensity gain in the Stokes beam and loss in the pump beam, the SRS process pumps molecules to their vibrationally excited states. The thereafter relaxation heats up the surroundings and induces refractive index changes. By probing the refractive index changes with a laser beam, stimulated Raman photothermal (SRP) microscopy is developed, where a >500-fold boost of modulation depth is achieved. Moreover, SRP imaging can be operated with a noisy fiber laser for excitation and a long working distance air condenser for signal collection. Two implementations and broad biological applications are reviewed. In summary, SRP microscopy opens a new way to perform chemical imaging with ultrahigh sensitivity and long working distance optics toward clinical translation.