Molecular cavity optomechanics: a theory of plasmon-enhanced Raman scattering (1407.1518v3)

Abstract: The conventional explanation of plasmon-enhanced Raman scattering attributes the enhancement to the antenna effect focusing the electromagnetic field into sub-wavelength volumes. Here we introduce a new model that additionally accounts for the dynamical and coherent nature of the plasmon-molecule interaction and thereby reveals an enhancement mechanism not contemplated before: dynamical backaction amplification of molecular vibrations. We first map the problem onto the canonical model of cavity optomechanics, in which the molecular vibration and the plasmon are \textit{parametrically coupled}. The optomechanical coupling rate, from which we derive the Raman cross section, is computed from the molecules Raman activities and the plasmonic field distribution. When the plasmon decay rate is comparable or smaller than the vibrational frequency and the excitation laser is blue-detuned from the plasmon onto the vibrational sideband, the resulting delayed feedback force can lead to efficient parametric amplification of molecular vibrations. The optomechanical theory provides a quantitative framework for the calculation of enhanced cross-sections, recovers known results, and enables the design of novel systems that leverage dynamical backaction to achieve additional, mode-selective enhancement. It yields a new understanding of plasmon-enhanced Raman scattering and opens a route to molecular quantum optomechanics.

• The paper introduces a novel theoretical framework mapping plasmon-enhanced Raman scattering to cavity optomechanics.
• It quantifies optomechanical coupling rates to derive Raman cross-sections and demonstrates dynamical backaction amplification.
• The results outline practical conditions for refined molecular detection and high-precision spectroscopic applications.

Overview of the Theory of Plasmon-Enhanced Raman Scattering

The paper "Molecular cavity optomechanics: a theory of plasmon-enhanced Raman scattering" presents a novel theoretical framework by aligning plasmon-enhanced Raman scattering with the principles of cavity optomechanics. The enhancement of Raman scattering via localized plasmonic resonances is a pivotal concept in spectroscopic applications, enabling single-molecule sensitivity. Historically, this enhancement was attributed primarily to the electromagnetic field concentration in small regions, known as 'hot spots.' However, the authors propose an additional mechanism based on a dynamic and coherent interaction between molecular vibrations and the plasmonic cavity, a process they term "dynamical backaction amplification."

Key Concepts and Findings

1. Model Construction: The authors map the interaction between plasmonic fields and molecular vibrations onto a cavity optomechanical framework. Here, a plasmonic mode is treated similarly to an optical cavity, interacting parametrically with molecular vibrations. This optomechanical coupling is quantified by the cavity optomechanical coupling rate, a critical parameter from which the Raman cross-section can be derived.
2. Dynamical Backaction Amplification: A unique insight from this work is the identification of dynamical backaction as an enhancement mechanism. When the laser is blue-detuned from the plasmonic resonance by the molecular vibrational frequency, the plasmonic field can amplify the molecular vibrations. This backaction leads to enhanced Raman scattering cross-sections, providing new interpretative power beyond the conventional 'E^4 law.'
3. Quantitative Framework: The presented theory provides a quantitative means to compute enhanced Raman cross-sections. By aligning with known optomechanical principles, the model recovers conventional results while offering predictions for systems with added dynamical backaction coupling, thus emphasizing the coherence and retardation impacts of the plasmonic fields.
4. Practical and Theoretical Implications: With such optomechanical interactions, the authors posit the potential for high-frequency vibrational modes to achieve out-of-equilibrium occupancies, providing significant nonlinear effects useful for applications like super-resolution imaging. Furthermore, by refining the understanding of plasmonic mode designs, this theory could influence the future development of spectroscopic and sensing applications where precise mode-selective enhancements are desired.
5. Conditions for Enhancement: The paper outlines the conditions under which dynamical backaction amplification is most effective, stressing the importance of plasmon decay rates that align well with vibrational frequencies and sufficiently large optomechanical couplings achievable in certain nanoscale environments.

Future Directions

The inclusion of molecular cavity optomechanics brings promising avenues for exploration, such as the development of systems facilitating greater control over vibrational states for quantum optomechanical applications. The high coupling rates suggested in this paper imply feasible experiments for observing phenomena like optomechanical dampening or amplification in molecular systems, even at room temperatures. This capability heralds new opportunities for integrating vibrational spectroscopy with quantum technologies, particularly in areas requiring extreme sensitivity and precision.

In conclusion, the application of optomechanical principles to plasmon-enhanced Raman scattering presents both a broadening of the theoretical landscape and a practical toolkit for enhancing Raman spectroscopy's sensitivity and resolution. By revisiting fundamental assumptions with this integrated optomechanical approach, researchers can innovatively address longstanding challenges in molecular detection and imaging.