A major challenge in tissue imaging is the degradation of resolution with increased depth, due to multiple scattering and refraction. By using long-wavelength lasers, penetration depth can be improved, while resolution further degrades. Recently, superresolution techniques emerged to enhance spatial resolution by switching or saturation of fluorescence. However, fluorescence suffers from photobleaching, and current techniques do not provide deep-tissue imaging capability due to the lack of optical sectioning or the requirement of special beam manipulation.
We have recently demonstrated that scattering from a single gold nanoparticle exhibits saturation behavior, which was adopted to significantly enhance resolution by saturated excitation (SAX) microscopy. Compared to fluorophores, scattering from plasmonic nanoparticles is free from bleaching, the cross-section is much larger, and the plasmonic resonance band is broadly tunable with particle shape and size, making it an ideal and robust contrast agent for long-term observation. On the other hand, SAX microscopy does not need any beam engineering and provides intrinsic sectioning with its confocal scheme, suitable for deep-tissue imaging.
In this work, we combine the advantages of plasmonics and SAX microscopy to demonstrate resolution enhancement underneath a very deep tissue. One general concern of scattering-based imaging is the background from the strong scattering of the surrounding tissue. Since tissue scattering is linear, and SAX allows the extraction of only nonlinear responses, the background can be fully eliminated, leaving only nanoparticle visible. Therefore, such combination provides a novel tool for not only high-resolution, but also high-contrast, background-free, and long-term imaging deep inside biological tissues.