Label-free optical imaging of nonfluorescent molecules by stimulated radiation

Imaging contrasts other than fluorescence are highly desirable for label-free detection and interrogation of nonfluorescent molecular species inside live cells, tissues, and organisms. The recently developed stimulated Raman scattering (SRS) and stimulated emission microscopy techniques provide sensitive and specific contrast mechanisms for nonfluorescent species, by employing the light amplification aspect of stimulated radiation. Compared to their spontaneous counterparts, stimulated radiation can enhance the imaging performance significantly, making the previously ‘dark’ molecules observable. Here we review and summarize the underlying principles of this emerging class of molecular imaging techniques.

► Various chemical bonds can be imaged by stimulated Raman scattering microscopy without any exogenous labeling.
► Stimulated emission microscopy is capable of image nonfluorescent chromophores with superb sensitivity.
► Both stimulated Raman and stimulated emission microscopy take advantage of the optical amplification.