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Atomic-scale photonics for super-resolution and molecular optomechanics in SERS

AuthorsAizpurua, Javier
Issue Date2017
CitationInternational Conference on Surface-Enhanced Raman Spectroscopy (2017)
AbstractSurface-Enhanced Raman Scattering (SERS) is a fundamental spectroscopic technique that allows to access the rich vibrational structure of molecules. In a typical SERS configuration, a plasmonic cavity can enhance the vibrational signal of a molecule located in its proximity, providing a great opportunity to explore complex photochemical processes by optically monitoring the excitation of their vibrational fingerprints. In recent years, optical spectroscopy of these cavities has proven to be extremely sensitive to atomic-scale features that determine the chemistry and the optoelectronics in the gaps. In this extreme regime of photonics, quantum theoretical approaches need to be exploited to address the optics of metallic nanogaps and the interaction with molecular species at the atomic scale. Several examples of how strongly inhomogeneous near-fields around atomic features determine the optical response in plasmon-enhanced spectroscopy, and how this can set completely new selection rules in SERS will be shown. Furthermore, the extreme subnanometric localization of light allows to understand ultraresolution in molecular vibrational spectroscopy, as recently achieved in intramolecule-resolved Tip-Enhanced Raman Spectroscopy (TERS). The ultra-small effective volumes associated to atomic-scale hot spots within plasmonic cavities, here termed 'picocavities', are of particular interest in quantum nanooptics, as they provide particularly large values of the coupling strength of the photons in the cavity with excitons of an emitter, or with the mechanical vibrations of the molecule located in the cavity. Along these lines, we present a novel analogy between molecular off-resonant SERS and typical optomechanical processes. By adopting an optomechanical hamiltonian which describes the interaction between plasmons and vibrations, the quantum dynamics of both excitations can be traced. Within this framework, and under certain experimental conditions, we identify the existence of different regimes of molecular vibrational build-up in SERS as a function of incident laser intensity: (i) a thermal vibrational regime, (ii) a vibrational pumping regime, and (iii) a strongly nonlinear vibrational regime. Furthermore, as in other optomechanical systems, a detuning between the incident laser and the plasmonic cavity is shown to enhance or reduce the Raman signal depending on the sign of the detuning (heating or cooling). The second-order correlation between the Stokes and anti-Stokes photons can be also traced within this formalism. Last, we extend our study to consider resonant SERS situations, and find the possibility to selectively activate particular molecular vibrational fingerprints by modifying the incident laser detuning or its intensity. The action of "picocavities" in SERS provides a powerful tool to boost resolution of chemical maps, as well as pushes the limits of plasmon-molecule coupling to new regimes of interaction.
DescriptionResumen del trabajo presentado a la International Conference on Surface-Enhanced Raman Spectroscopy (SERS), celebrada en Xiamen (China) del 5 al 9 de diciembre de 2017.
Appears in Collections:(CFM) Comunicaciones congresos
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