The TERS effect comes from the strong local enhancement of the electromagnetic field occurring at the apex of a sharp noble-metal tip when illuminated with a focused laser light2. The phenomenon results from the combination of an electromagnetic ‘lightning rod effect’ and a localized surface plasmon (LSP) excitation.
This electromagnetic enhancement (EM) mechanism is associated with the excitation of surface plasmons and the strength of their EM fields near the surface. These fields can be significantly stronger than the incident fields. Theory has shown that if the tip is illuminated, a strong enhancement of the EM field can occur in the narrow space between the tip itself and the sample (consisting of an ideally metallic substrate on which are deposited adsorbates or nanomaterials). The metalized tip acts as an optical antenna that enhances both the incident and the emitted fields, in a region defined by the size of the tip apex (typically less than 30 nm).
Let us describe the field enhancement of the incident electromagnetic wave by a factor gi, and the enhancement of the scattered field by gsc. For g ≈ gsc ≈ gi (the so-called “g4 law”), the EM part of the enhancement is simplified to FEM = gi 2 gsc 2 ≈ g4. Then, a hundredfold increase of the EM field relative to the incident one thanks to the presence of the tip (i.e. gi = 100) would result in a local 10,000-fold intensity enhancement of the Raman signal (Iloc = gi 2 I0).
To summarize, in TERS, the Raman scattering process boosted by this local enhancement at the tip apex, since the scattering cross-section scales with the fourth power of the local electromagnetic field enhancement.