text: Tip-enhanced Raman spectroscopy requires a confocal microscope, and a scanning probe microscope. The optical microscope is used to align the laser focal point with the tip coated with a SERS active metal. The three typical experimental configurations are bottom illumination, side illumination, and top illumination, depending on which direction the incident laser propagates towards the sample, with respect to the substrate. In the case of STM-TERS, only side and top illumination configurations can be applied, since the substrate is required to be conductive, therefore typically being non-transparent. In this case, the incident laser is usually linearly polarized and aligned parallel to the tip, in order to generate confined surface plasmon at the tip apex. The sample is moved rather than the tip so that the laser remains focused on the tip. The sample can be moved systematically to build up a series of tip enhanced Raman spectra from which a Raman map of the surface can be built allowing for surface heterogeneity to be assessed with up to 1.7 nm resolution. Subnanometer resolution has been demonstrated in certain cases allowing for submolecular features to be resolved.
  In 2019, Yan group and Liu group at University of California, Riverside developed a lens-free nanofocusing technique, which concentrates the incident light from a tapered optical fiber to the tip apex of a metallic nanowire and collects the Raman signal through the same optical fiber. Fiber-in-fiber-out NSOM-TERS has been developed.