A practical implementation of the extra enhancement.
A gold-coated AFM tip can support more than one plasmon resonance , similar to . A strong field, localized at the edge of the tip, interacts with a Raman-active molecule (blue) and leads to an enhanced Raman signal. When the tip is decorated (it can as well be called “contaminated”) with auxiliary QEs (purple), of level spacing not close to either the exciting or the Stokes-shifted frequencies, the localized field on the Raman-active molecule remains unchanged. Still, QEs give rise to extra enhancement of the Raman signal from Raman-active molecule(s) due to Fano (interference) effect.