Super-resolution imaging of SERS hot spots.

Surface-enhanced Raman scattering (SERS) hot spots occur when molecules are positioned near regions of strongly enhanced electromagnetic fields on the surface of nano-featured plasmonic substrates. The emission from the molecule is coupled out into the far field by the plasmon modes of the substrate, but due to the diffraction-limit of light, the properties of this coupled molecule-plasmon emitter cannot be resolved using typical far-field optical microscopy techniques. However, by fitting the emission to a model function such as 2-dimensional Gaussian, the relative position of the emitter can be determined with precision better than 5 nm in a process known as super-resolution imaging. This tutorial review describes the basic principles of super-resolution imaging of SERS hot spots using single molecules to probe local electromagnetic field enhancements. New advances using dipole-based fitting functions and spectrally- and spatially-resolved measurements are described, providing new insight into SERS hot spots and the important roles of both the molecule and the substrate in defining their properties.