Quantum description of surface-enhanced resonant Raman scattering within a hybrid-optomechanical model

Surface-Enhanced Raman Scattering (SERS) allows for detection and identification of molecular vibrational fingerprints in minute sample quantities. The SERS process can be also exploited for optical manipulation of molecular vibrations. We present a quantum description of Surface-Enhanced Resonant Raman scattering (SERRS), in analogy to hybrid cavity optomechanics, and compare the resonant situation with the off-resonant SERS. Our model predicts the existence of a regime of coherent interaction between electronic and vibrational degrees of freedom of a molecule, mediated by a plasmonic nanocavity. This coherent mechanism can be achieved by parametrically tuning the frequency and intensity of the incident pumping laser and is related to the optomechanical pumping of molecular vibrations. We find that vibrational pumping is able to selectively activate a particular vibrational mode, thus providing a mechanism to control its population and drive plasmon-assisted chemistry

Molecular optomechanics in the anharmonic regime: from nonclassical mechanical states to mechanical lasing

Cavity optomechanics aims to establish optical control over vibrations of mechanical systems, to heat, cool or to drive them toward coherent, or nonclassical states. This field was recently extended to include molecular optomechanics, which describes the dynamics of THz molecular vibrations coupled to the optical fields of lossy cavities via Raman transitions, and was developed to understand the anomalous amplification of optical phonons in Surface-Enhanced Raman Scattering experiments. But the molecular platform should prove suitable for demonstrating more sophisticated optomechanical effects, including engineering of nonclassical mechanical states, or inducing coherent molecular vibrations. In this work, we propose two pathways towards implementing these effects, enabled or revealed by the strong intrinsic anharmonicities of molecular vibrations. First, to prepare a nonclassical mechanical state, we propose an incoherent analogue of the mechanical blockade, in which the molecular aharmonicity and optical response of hybrid cavities isolate the two lowest-energy vibrational states. Secondly, we show that for a strongly driven optomechanical system, the anharmonicity can effectively suppress the mechanical amplification, shifting and reshaping the onset of coherent mechanical oscillations. Our estimates indicate that both effects should be within reach of the existing implementations of the Surface Enhanced Raman Scattering, opening the pathway towards the coherent and nonclassical effects in molecular optomechanics

Identifying unbound strong bunching and the breakdown of the Rotating Wave Approximation in the quantum Rabi model

We use a recently derived gauge-invariant formulation of the problem of an incoherently-driven two-level system coupled to an optical cavity, to explore the transition between different coupling regimes -- in particular, between the weak, and the ultra-strong coupling regimes. We explore this transition using the second-order intensity correlation $g^{(2)}(\tau)$ of the emitted light, and find strong, unbounded bunching of the emission from systems governed by the Rabi Hamiltonian. Surprisingly, this effect is observed not only in the ultra-strong coupling regime, but also in the regime of coupling typically recognized as weak coupling, where the Jaynes-Cummings Hamiltonian predicts the opposite, strongly antibunched emission. This suggests that the intensity correlations are a particularly sensitive probe of the divergence between the Jaynes-Cummings and Rabi Hamiltonians, and can serve as an indicator of the breakdown of the rotating wave approximation

Nanocavities: Optomechanics goes molecular

News and Views.A theoretical framework that interprets Raman scattering as an optomechanical process can be used to understand, and guide, experiments in surface-enhanced Raman spectroscopy.Peer Reviewe

Absorption Enhancement in Peridinin–Chlorophyll–Protein Light-Harvesting Complexes Coupled to Semicontinuous Silver Film

We report on experimental and theoretical studies of plasmon-induced effects in a hybrid nanostructure composed of light-harvesting complexes and metallic nanoparticles in the form of semicontinuous silver film. The results of continuous-wave and time-resolved spectroscopy indicate that absorption of the light-harvesting complexes is strongly enhanced upon coupling with the metallic film spaced by 25 nm of a dielectric silica layer. This conclusion is corroborated by modeling, which confirms the morphology of the silver island film

Frequency-resolved photon correlations in cavity optomechanics

Frequency-resolved photon correlations have proven to be a useful resource to unveil nonlinearities hidden in standard observables such as the spectrum or the standard (color-blind) photon correlations. In this manuscript, we analyze the frequency-resolved correlations of the photons being emitted from an optomechanical system where light is nonlinearly coupled to the quantized motion of a mechanical mode of a resonator, but where the quantum nonlinear response is typically hard to evidence. We present and unravel a rich landscape of frequency-resolved correlations, and discuss how the time-delayed correlations can reveal information about the dynamics of the system. We also study the dependence of correlations on relevant parameters such as the single-photon coupling strength, the filtering linewidth, or the thermal noise in the environment. This enriched understanding of the system can trigger new experiments to probe nonlinear phenomena in optomechanics, and provide insights into dynamics of generic nonlinear systems

The convergence of cavity optomechanics and Brillouin scattering

Cavity optomechanics and Brillouin scattering have historically developed as separate fields of study, focused on distinct optoacoustic interaction effects, and realized in different physical platforms. These gaps are now closing rapidly, as researchers embrace the fundamental similarities between the two fields. Both fields study the three-wave mixing between electromagnetic and acoustic waves. Here, we review this convergence by showing how optoacoustic platforms increasingly blur the traditional distinctions between cavity optomechanics and Brillouin scattering. We discuss how the theoretical formalisms used by the two communities can be directly mapped between each other in both waveguides and cavities