A giant optomechanical spring effect in plasmonic nanocavities

Molecular vibrations couple to visible light only weakly, have small mutual interactions, and hence are often ignored for non-linear optics. Still, molecular vibrations dominate electronic, thermal, and spin transport in a wide range of devices from photovoltaics to molecular electronics as well as being of fundamental interest. Surface-Enhanced Raman Spectroscopy (SERS) is well-established for studying molecular vibrations.

When light encounters molecules, the predominant mode of scattering is elastic scattering (Rayleigh). This scattering is responsible for the blue colour of the sky, for example. It is also possible for the incident photons to interact with the molecules in such a way that energy is either gained or lost so that the scattered photons are shifted in frequency and in phase. Such inelastic scattering is called Raman scattering. With the development of the laser, the effect was put to use.

In Raman espectroscopy light from a laser is passed through a substance. The resulting inelastically scattered photons can be of either lower (Stokes) or higher (anti-Stokes) frequency than the incoming photon. As the new frequencies in the Raman spectrum of monochromatic light scattered by the substance are characteristic of the substance, the technique is widely used as a way of determining molecular structure and is a common tool in chemical analysis.

SERS is a spectroscopic technique in which the Raman scattering is increased by placing a single molecule in a hotspot of a plasmonic cavity, where the electric fields associated with the incident and the scattered photons are strongly enhanced. The difference between the energy of those two photons provides a fingerprint of the molecule, that is, detailed chemical information about its vibrational structure. Hence, plasmonic nanostructures are designed to maximize the nanocavity optical field confinement in intense localized hot-spots in which molecules are immersed.

Recently, it was shown that SERS can be described as an instantiation of molecular optomechanics, in which molecular vibrations and the optical nanocavity are highly coupled. So far, optomechanical models for plasmonic cavities used descriptions based on cavity-QED, extended to account for plasmonic losses, and were often restricted to a single resonant photonic mode. Now, a team of researchers has shown ¹ that this approximation is incomplete, and that a full multimodal treatment of the nanocavity is needed.

The researchers show that the extreme confinement provided by plasmonic nano- and pico-cavities can sufficiently enhance optomechanical coupling so that intense laser illumination drastically softens the molecular bonds. This optomechanical pumping regime produces strong distortions of the Raman vibrational spectrum related to giant vibrational frequency shifts from an optical spring effect which is hundred-fold larger than in traditional cavities, a novel effect in the context of molecular nanotechnology.

The theoretical simulations are consistent with the experimentally-observed non-linear behaviour exhibited in the Raman spectra of nanoparticle-on-mirror constructs illuminated by ultrafast laser pulses. Importantly, plasmonic picocavities allows access to the optical spring effect in single molecules with continuous illumination.

These results are important for the optomechanics of phonons in thin crystals when integrated into plasmonic nanocavities, such as perovskites or 2D layered materials. But, in a broader sense, they mean that there is a possibility to control reversible bond softening, as well as irreversible chemistry.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.