Coherent anti-Stokes Raman scattering (CARS) microscopy is a powerful technique that enables chemical imaging based on the vibration of molecular bonds. Usually CARS microscopy is performed with two focused and collinear beams (namely pump and Stokes) whose frequency difference equals the targeted molecular vibration. This requires pulses of nanojoule energy that are focused in a micron‑size area and raster scanned to perform imaging. In this JOSA B article, the authors take advantage of the strong electric field associated with surface plasmon polaritons (SPPs) that are excited and propagate at a metal/dielectric interface. Usually SPPs are used for molecular sensing because the SPP electromagnetic resonance is very sensitive to the refractive index of the dielectric material next to the metal. Here, the authors study in detail the generation of the CARS signal when the molecular sample is located very close to the metal surface and can be excited and coupled into the SPP. They find that phase matching conditions lead to a very peculiar CARS signal generation. Based on these investigations, the authors conceive a wide-field surface-enhanced CARS imaging scheme that requires just femtojoule energy per µm2
due to the strong SPP resonant field enhancement. This work opens the way to plasmon-enhanced CARS imaging that can find applications in surface chemistry or cell membrane biology.
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