When photons interact with matter, they can scatter inelastically and gain or lose energy. Their frequency accordingly shifts by an amount that corresponds to a vibrational excitation. That Raman effect is the basis for a decades-old spectroscopic tool that identifies molecules by their unique vibrational fingerprints. Because the scattering cross section is low, the Raman signal is feeble. But it can be amplified more than a millionfold by, for example, placing the molecules in the tight space between a surface and the sharp metal tip of a scanning tunneling microscope (STM).
The simple act of shining light on the tip excites surface plasmons, collective electron-density oscillations in the metal. Those plasmons enhance the optical fields and localize them to a scale—typically tens of nanometers—far below the light’s diffraction limit. The sharper the tip, the greater the localization (see the article on near-field imaging by Lukas Novotny, Physics Today, July...
