An overview of new effects in molecular optomechanics

Resumen del trabajo presentado a la 9th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS), celebrada online del 11 al 13 de febrero de 2021.The interaction between molecular vibrations at infrared frequencies and optical nanocavities that support plasmonic resonances at significantly larger energies can be understood in terms of quantum optomechanics. This molecular optomechanical framework offers a very complete description of the Raman scattering that occurs in the system, whereby a plasmon excited by a laser of frequency ωl interchanges energy with the molecular vibrations that oscillate with frequency ωv, resulting in an increase of the vibrational population and the emission of Stokes and anti-Stokes photons at a shifted energy ħ(ωl - ωv) or ħ(ωl ωv), respectively . For simple conditions, this description fully recovers the well-known scaling of the Raman signal with the fourth power of the classical enhancement of the local electric fields near the plasmonic particle, which is at the origin of the huge increase of the emitted signal in Surface Enhanced Raman Spectroscopy. It also correctly predicts the quadratic increase of the anti-Stokes signal with laser intensity that is found when the vibrational population is induced by the Stokes scattering instead of by a thermal process, in the vibrational pumping regime. This contribution presents the molecular optomechanics framework and describes how it can be used to explore a variety of novel or less well understood effects in Surface Enhanced Raman spectroscopy, which include a signal divergence called parametric instability, collective effects in the presence of many molecules, a rich scenery of correlations of the emitted photons and coupling between vibrational and electronic degrees of freedom of the molecule at large intensities. We also describe how, although a single-mode description of the plasmonic response is advantageous to understand the main optomechanical effects, it is essential in many situations to consider the full plasmonic behaviour. This work can thus serve as a guide of experiments investigating new effects in Surface Enhanced Raman Spectroscopy.Peer reviewe