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arXiv:2608.03307

Single-molecule strong optomechanical regimes in SERS via hybrid plasmonic cavities

Miguel Á. Martínez-García, Johannes Feist, Diego Martín-Cano

quant-phcond-mat.mes-hallphysics.opticsSubmitted Aug 4, 2026• Announced Aug 5, 2026
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Abstract

We identify enhanced strong coherent Surface-Enhanced Raman Scattering (SERS) interactions facilitated by hybridized metallo-dielectric cavities with a resonant fluorescent molecule. By illuminating a detuned electronic transition near saturation via a narrowband hybrid plasmonic mode, we observe enhanced splittings in the Stokes and anti-Stokes spectra, revealing strong optomechanical coupling and nonlinear vibrational modifications at intensities far below irreversible SERS damage thresholds. We compute the second-order photon correlations that allows identifying the nonclassical character associated to these strong optomechanical interactions and SERS configurations that enables them to optically characterize the anharmonic character of the vibrational modes.

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We identify enhanced strong coherent Surface-Enhanced Raman Scattering (SERS) interactions facilitated by hybridized metallo-dielectric cavities with a resonant fluorescent molecule. By illuminating a detuned electronic transition near saturation via a narrowband hybrid plasmonic mode, we observe enhanced splittings in the Stokes and anti-Stokes spectra, revealing strong optomechanical coupling and nonlinear vibrational modifications at intensities far below irreversible SERS damage thresholds. We compute the second-order photon correlations that allows identifying the nonclassical character associated to these strong optomechanical interactions and SERS configurations that enables them to optically characterize the anharmonic character of the vibrational modes.
In the context of the quantum-mechanical description of single-molecule surface-enhanced Raman scattering, intensity-field correlation measurements of photons emitted from a plasmonic cavity are explored, theoretically, using the technique of conditional homodyne detection. The inelastic interplay between plasmons and vibrations of a diatomic molecule placed inside the cavity can be manifested in phase-dependent third-order fluctuations of the light recorded by the aforesaid technique, allowing us to reveal signatures of non-classicality (indicatives of squeezing) of the outgoing Raman photons.
As is well known, light arising from Raman scattering on molecules shows nonclassical mutual correlations between the Stokes and anti-Stokes components. However, the practical use of nonclassical Raman light is limited, due to its low intensity. The Raman signal can be enhanced in surface-enhanced Raman scattering (SERS) experiments by placing Raman active molecules near a plasmonic nanoantenna, due to the Purcell effect. Since the main contribution to emission in SERS experiments is re-emitted light from the plasmonic nanoantenna with no nonclassical correlations, the question arises as to whether it is possible to simultaneously amplify the Raman signal and preserve the nonclassical correlations. In this work, we give a positive answer to this question. We show that plasmonic nanoantennas can be used to enhance the nonclassical light intensity. We demonstrate that the main factor causing deterioration of the cross correlations of SERS light is the fluorescent background of the plasmonic nanoantenna. We analyze the way in which the geometry of a plasmonic nanoantenna affects the nonclassical cross correlations of the Raman light, and show that there is an optimal size of plasmonic nanoantenna that both increases the Raman signal and preserves the nonclassical correlations of the light at the level of a single molecule. The obtained results pave the way for the creation of sources with controlled intensity and a degree of nonclassicality.
We propose a molecular optomechanical platform to generate robust entanglement among bosonic modes-photons, phonons, and plasmons-under ambient conditions. The system integrates a high-Q whispering-gallery-mode (WGM) optical resonator with a plasmonic nanocavity formed by a metallic nanoparticle and a single molecule. This hybrid architecture offers two critical advantages over stand-alone plasmonic systems: (i) efficient redirection of Stokes photons from the lossy plasmonic mode into the long-lived WGM resonator and (ii) suppression of molecular absorption and sustaining vibrational ground states via plasmon-WGM interactions. These features enable entanglement to redistribute from the fragile plasmon-phonon bipartition to a robust photon-phonon bipartition in the blue-detuned regime, yielding robust stationary entanglement resilient to environmental noise. Remarkably, the achieved entanglement surpasses the theoretical bound for conventional two-mode squeezing in certain parameter regimes. Our scheme establishes a universal approach to safeguard entanglement in open quantum systems and opens avenues for noise-resilient quantum information technologies.
The development of accurate tools to characterize Intramolecular Vibrational Redistribution (IVR) is of major interest in chemistry. In this context, surface-enhanced vibrational spectroscopies stand up as well-established techniques to study molecular vibrational lines and populations with a sensitivity that can reach the single-molecule level. However, to date, this possibility has not been fully developed to address IVR. Here, we establish a quantum mechanical framework based on molecular optomechanics that accounts for IVR, and we adopt it to analyze strategies to optimize IVR characterization by surface-enhanced vibrational spectroscopy. In particular, we model two different pump-and-probe configurations where the vibrational pumping is provided either by infrared laser illumination or by Stokes scattering processes in surface-enhanced Raman spectroscopy (SERS). We show for the two pumping configurations the existence of clear signatures on the anti-Stokes SERS spectra of population transfer between coupled vibrational modes in a molecule. Our calculations adopt realistic molecular and SERS parameters, suggesting that these signatures of IVR could be accessible at the single-molecule level within realistic experimental platforms.
We theoretically investigate a single fluorescent molecule as a hybrid quantum optical device, in which multiple external laser sources exert control of the vibronic states. In the high-saturation regime, a coherent interaction is established between the vibrational and electronic degrees of freedom, and molecules can simulate several cavity QED models, whereby a specific vibrational mode plays the role of the cavity mode. Focusing on the specific example where the system is turned into an analogue simulator of the quantum Rabi model, the steady state exhibits vibrational bi-modality resulting in a statistical mixture of highly non-classical vibronic cat states. Applying our paradigm to molecules with prominent spatial asymmetry and combining an optical excitation with a THz(IR) driving, the system can be turned into a single photon transducer. Two possible implementations are discussed based on the coupling to a subwavelength THz patch antenna or a resonant metamaterial. In a nutshell, this work assesses the role of molecules as an optomechanical quantum toolbox for creating hybrid entangled states of electrons, photons, and vibrations, hence enabling frequency conversion over very different energy scales.
Chaos is central to secure communication and physical random-number generation. Conventional cavity-optomechanical implementations, however, usually rely on weak single-photon optomechanical coupling and low-frequency mechanical modes, so access to deterministic chaotic dynamics often requires large driving power and careful suppression of thermal noise. Here we theoretically study a hybrid molecular optomechanical system formed by coupling a plasmonic nanocavity to a whispering-gallery-mode (WGM) microcavity. The plasmonic nanocavity provides terahertz-scale single-photon optomechanical coupling to a molecular vibration, while the WGM resonator offers a low-loss photonic channel that mitigates the short plasmon lifetime. By integrating the semiclassical equations of motion and evaluating the largest Lyapunov exponent, we map the nonlinear dynamical regimes in the parameter spaces of WGM detuning, plasmon--WGM coupling, and plasmon--vibration optomechanical coupling. We show that increasing the plasmon--vibration coupling drives the system from self-sustained oscillations to chaos through a period-doubling cascade. At moderate coupling strengths, isolated chaos windows can be opened or closed by tuning the WGM detuning and the inter-cavity coupling. These results identify molecular optomechanics as a controllable room-temperature platform for on-chip chaotic light generation and random-signal applications.