Stimulated Raman scattering (SRS) microscopy generates chemical maps of live cells or tissues. A handheld imaging system using an optical fiber for laser delivery further enables in situ and in vivo measurements. However, the non-resonant background caused by the interaction between two ultrafast pulses inside an optical fiber overwhelms the stimulated Raman signal from the sample. Here, we report a background-free handheld SRS microscope. By temporally separating the two ultrashort pulses propagating in the fiber and then overlapping them on a sample through a dispersive material, we detected stimulated Raman signal that is 200 times weaker than the non-resonant background. The handheld microscope allowed ambient-light mapping of pesticide on a spinach leaf, cancerous tissue versus healthy brain tissue in a canine model, and cosmetic distribution on live human skin.
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