The Raman scattering of an individual molecule is so small that the Raman-scattered light cannot be detected with a confoncal microscope. In order to enhance the light-matter interaction, researchers have developed over the past decades a powerful approach: using metallic nanostructures as “plasmonic antenna”. When light impinges on a metal, the electromagnetic field cannot penetrate. Instead, it excites electron oscillations at the surface, which creates a very localized electric field oscillating at the light frequency. This is called a surface plasmon.
By suitably engineering the shape and size of the metallic structure, we can obtain localized plasmonic resonances, for example in the gap between two gold spheres spaced by a few nanometers (picture below). These localized plasmons are akin to nano-cavities, trapping the electromagnetic field into deep sub-wavelength volumes. Note that plasmons quickly dissipate their energy by radiation and absorption, meaning that the quality factor of these nano-cavities is quite poor (Q~10).
In a theoretical work led by Phillipe Roelli, in the group of Tobias Kippenberg, we have shown that laser light can be used to drive the vibrational mode of a molecule coupled to a plasmonic cavity out of equilibrium, possibly reaching the regime of parametric instability (akin to a “phonon laser”). We are currently developing plasmonic nanostructures hosting molecular monolayers as test systems to demonstrate this phenomenon.