Molecular Optomechanics Research Articles

Nanocavities for Molecular Optomechanics: Their Fundamental Description and Applications

Nanocavities for Molecular Optomechanics: Their Fundamental Description and Applications

Boosting Light-Matter Interactions in Plasmonic Nanogaps.

Plasmonic nanogaps in strongly coupled metal nanostructures can confine light to nanoscale regions, leading to huge electric field enhancement. This unique capability makes plasmonic nanogaps powerful platforms for boosting light-matter interactions, thereby enabling the rapid development of novel phenomena and applications. This review traces the progress of nanogap systems characterized by well-defined morphologies, controllable optical responses, and a focus on achieving extreme performance. The properties of plasmonic gap modes in far-field resonance and near-field enhancement are explored and a detailed comparative analysis of nanogap fabrication techniques down to sub-nanometer scales is provided, including bottom-up, top-down, and their combined approaches. Additionally, recent advancements and applications across various frontier research areas are highlighted, including surface-enhanced spectroscopy, plasmon-exciton strong coupling, nonlinear optics, optoelectronic devices, and other applications beyond photonics. Finally, the challenges and promising emerging directions in the field are discussed, such as light-driven atomic effects, molecular optomechanics, and alternative new materials.

Single-photon generation at room temperature using molecular optomechanics in a hybrid photonic-plasmonic cavity

Single-photon generation at room temperature using molecular optomechanics in a hybrid photonic-plasmonic cavity

Accelerated molecular vibrational decay and suppressed electronic nonlinearities in plasmonic cavities through coherent Raman scattering

Molecular vibrations and their dynamics are of outstanding importance for electronic and thermal transport in nanoscale devices as well as for molecular catalysis. The vibrational dynamics of <100 molecules are studied through three-color time-resolved coherent anti-Stokes Raman spectroscopy using plasmonic nanoantennas. This isolates molecular signals from four-wave mixing (FWM) while using exceptionally low nanowatt powers to avoid molecular damage via single-photon lock-in detection. FWM is found to be strongly suppressed in nanometer-wide plasmonic gaps compared to plasmonic nanoparticles. Simultaneous time-resolved incoherent anti-Stokes Raman spectroscopy allows us to separate the contributions of vibrational population decay (T1) and dephasing (T2). With increasing illumination intensity, the ultrafast vibrational dephasing rates of biphenyl-4-thiol molecules are accelerated at least tenfold, while phonon population decay rates remain constant. The extreme plasmonic field enhancement within nanogaps opens up prospects for measuring single-molecule vibrationally coupled dynamics and diverse molecular optomechanics phenomena. Published by the American Physical Society 2024

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

Cavity optomechanics aims to establish optical control over vibrations of nanoscale mechanical systems, to heat, cool or to drive them toward coherent, or nonclassical states. This field was recently extended to encompass molecular optomechanics: the dynamics of THz molecular vibrations coupled to the optical fields of lossy cavities via Raman transitions. The molecular platform should prove suitable for demonstrating more sophisticated optomechanical effects, including engineering of nonclassical mechanical states, or inducing coherent molecular vibrations. We propose two schemes for implementing these effects, exploiting 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 anharmonicity 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 suppress the mechanical amplification, shifting and reshaping the onset of coherent mechanical oscillations. Our estimates indicate that both effects should be within reach of existing platforms for Surface Enhanced Raman Scattering.

Hybrid cavity-antenna architecture for strong and tunable sideband-selective molecular Raman scattering enhancement.

Plasmon resonances at the surface of metallic antennas allow for extreme enhancement of Raman scattering. Intrinsic to plasmonics, however, is that extreme field confinement lacks precise spectral control, which would hold great promise in shaping the optomechanical interaction between light and molecular vibrations. We demonstrate an experimental platform composed of a plasmonic nanocube-on-mirror antenna coupled to an open, tunable Fabry-Perot microcavity for selective addressing of individual vibrational lines of molecules with strong Raman scattering enhancement. Multiple narrow and intense optical resonances arising from the hybridization of the cavity modes and the plasmonic broad resonance are used to simultaneously enhance the laser pump and the local density of optical states, and are characterized using rigorous modal analysis. The versatile bottom-up fabrication approach permits quantitative comparison with the bare nanocube-on-mirror system, both theoretically and experimentally. This shows that the hybrid system allows for similar SERS enhancement ratios with narrow optical modes, paving the way for dynamical backaction effects in molecular optomechanics.

A molecular optomechanics approach reveals functional relevance of force transduction across talin and desmoplakin

Many mechanobiological processes that govern development and tissue homeostasis are regulated on the level of individual molecular linkages, and a number of proteins experiencing piconewton-scale forces in cells have been identified. However, under which conditions these force-bearing linkages become critical for a given mechanobiological process is often still unclear. Here, we established an approach to revealing the mechanical function of intracellular molecules using molecular optomechanics. When applied to the integrin activator talin, the technique provides direct evidence that its role as a mechanical linker is indispensable for the maintenance of cell-matrix adhesions and overall cell integrity. Applying the technique to desmoplakin shows that mechanical engagement of desmosomes to intermediate filaments is expendable under homeostatic conditions yet strictly required for preserving cell-cell adhesion under stress. These results reveal a central role of talin and desmoplakin as mechanical linkers in cell adhesion structures and demonstrate that molecular optomechanics is a powerful tool to investigate the molecular details of mechanobiological processes.

Molecular Optomechanics Induced Hybrid Properties in Soft Materials Filled Plasmonic Nanocavities.

The optomechanical interaction between nanocavity plasmons and molecular vibrations can result in interfacial phenomena that can be tailored for sensing and photocatalytic applications. Here, we report for the first time that plasmon-vibration interaction can induce laser-plasmon detuning dependent plasmon resonance linewidth broadening, indicating energy transfer from the plasmon field to collective vibrational modes. The linewidth broadening accompanied by the large enhancement of the Raman scattering signal is observed as the laser-plasmon blue-detuning approaches the CH vibrational frequency of the molecular systems integrated in gold nanorod-on-mirror nanocavities. The experimental observations can be explained based on the molecular optomechanics theory that predicts dynamical backaction amplification of the vibrational modes and high sensitivity of Raman scattering when the plasmon resonance overlaps with the Raman emission frequency. The results presented here suggest that molecular optomechanics coupling may be manipulated for creating hybrid properties based on interactions between molecular oscillators and nanocavity electromagnetic optical modes.

Hybrid Photonic-Plasmonic Cavity Design for Very Large Purcell Factors at Telecommunication Wavelengths

Hybrid photonic-plasmonic cavities based on nanoparticle-on-a-mirror structures simultaneously provide ultralow mode volume and high $Q$-factor, and so a very large Purcell factor, which is a key measure of light-matter interaction. Operation of such cavities has been constrained to wavelengths below 1 \ensuremath{\mu}m, with the technologically relevant telecom regime remaining elusive. This study describes a hybrid cavity operating at telecom wavelengths. The proposed design leads to extremely large Purcell factors (~10${}^{7}$--10${}^{8}$), and could impact many different applications, such as molecular optomechanics, bio- and chemosensing, efficient quantum emitters, and enhanced Raman spectroscopy.

Boundary condition for the optical force density

The optical force density as a function of position and time provides fundamental information to model local and, through integration, macroscopic kinetic motion of condensed matter. Here, the boundary condition associated with the optical force density is developed and investigated using an expression stemming from the work of Einstein and Laub, and in conjunction with Maxwell's equations to describe the electromagnetic fields. Consequently, a constraint is formed that allows a unique relationship between the total force and the force density, one that is achieved by virtue of the conservation principles for physical materials and described by locally homogenized constitutive parameters. Further insight can be garnered from new experimental studies, as summarized. The mathematical steps presented form a basis for modeling various optomechanical phenomena, including optical forces in and on solid-state systems such as membranes, beams, cantilevers, and waveguides, and can be interpreted in terms of a suite of related theoretical work. This specification of the force density boundary condition is relevant for basic scientific fields including those involved with various quantum cooling issues, molecular optomechanics, photochemistry, and biophysics (including mechanotransduction). The technologies impacted encompass integrated optomechanics (silicon photonics, where new optical device concepts can be enabled), communication systems (in which optical forces could supplant electronic switching), remote control and actuation, propulsion, sensing, and navigation.

Molecular Optomechanics Approach to Surface-Enhanced Raman Scattering.

ConspectusMolecular vibrations constitute one of the smallest mechanical oscillators available for micro-/nanoengineering. The energy and strength of molecular oscillations depend delicately on the attached specific functional groups as well as on the chemical and physical environments. By exploiting the inelastic interaction of molecules with optical photons, Raman scattering can access the information contained in molecular vibrations. However, the low efficiency of the Raman process typically allows only for characterizing large numbers of molecules. To circumvent this limitation, plasmonic resonances supported by metallic nanostructures and nanocavities can be used because they localize and enhance light at optical frequencies, enabling surface-enhanced Raman scattering (SERS), where the Raman signal is increased by many orders of magnitude. This enhancement enables few- or even single-molecule characterization. The coupling between a single molecular vibration and a plasmonic mode constitutes an example of an optomechanical interaction, analogous to that existing between cavity photons and mechanical vibrations. Optomechanical systems have been intensely studied because of their fundamental interest as well as their application in practical implementations of quantum technology and sensing. In this context, SERS brings cavity optomechanics down to the molecular scale and gives access to larger vibrational frequencies associated with molecular motion, offering new possibilities for novel optomechanical nanodevices.The molecular optomechanics description of SERS is recent, and its implications have only started to be explored. In this Account, we describe the current understanding and progress of this new description of SERS, focusing on our own contributions to the field. We first show that the quantum description of molecular optomechanics is fully consistent with standard classical and semiclassical models often used to describe SERS. Furthermore, we note that the molecular optomechanics framework naturally accounts for a rich variety of nonlinear effects in the SERS signal with increasing laser intensity.Furthermore, the molecular optomechanics framework provides a tool particularly suited to addressing novel effects of fundamental and practical interest in SERS, such as the emergence of collective phenomena involving many molecules or the modification of the effective losses and energy of the molecular vibrations due to the plasmon–vibration interaction. As compared to standard optomechanics, the plasmonic resonance often differs from a single Lorentzian mode and thus requires a more detailed description of its optical response. This quantum description of SERS also allows us to address the statistics of the Raman photons emitted, enabling the interpretation of two-color correlations of the emerging photons, with potential use in the generation of nonclassical states of light. Current SERS experimental implementations in organic molecules and two-dimensional layers suggest the interest in further exploring intense pulsed illumination, situations of strong coupling, resonant-SERS, and atomic-scale field confinement.

Surface plasmonic catalysis based on molecular optomechanics

Nowadays, researchers find that the surface plasmons can mediate some chemical reactions through the generation of the confined plasmonic field, excited electrons, and local heating effect. In this article we suggest a new surface plasmonic photocatalysis mechanism based on the molecular optomechanics which is not considered before. A reaction kinetic model was established to achieve a quantitative study of catalytic efficiency. The catalytic mechanism is not limited to a specific chemical reaction, all molecules with Raman activity can be accelerated dramatically in reaction. For molecules with different mechanical properties, the corresponding optomechanical catalytic pathway needs to be selected. We hope that this work will provide guidance for achieving strong or even ultrastrong catalyses under specific optical conditions. We further demonstrate that the optomechanical effects can also be used for the deceleration, which provides the possibility to design a highly tunable chemical reaction system. We believe that the quantum photochemistry will be further developed and widely used in future research.

Integrated Molecular Optomechanics with Hybrid Dielectric-Metallic Resonators.

Molecular optomechanics describes surface-enhanced Raman scattering using the formalism of cavity optomechanics as a parametric coupling of the molecule’s vibrational modes to the plasmonic resonance. Most of the predicted applications require intense electric field hotspots but spectrally narrow resonances, out of reach of standard plasmonic resonances. The Fano lineshapes resulting from the hybridization of dielectric–plasmonic resonators with a broad-band plasmon and narrow-band cavity mode allow reaching strong Raman enhancement with high-Q resonances, paving the way for sideband resolved molecular optomechanics. We extend the molecular optomechanics formalism to describe hybrid dielectric–plasmonic resonators with multiple optical resonances and with both free-space and waveguide addressing. We demonstrate how the Raman enhancement depends on the complex response functions of the hybrid system, and we retrieve the expression of Raman enhancement as a product of pump enhancement and the local density of states. The model allows prediction of the Raman emission ratio into different output ports and enables demonstrating a fully integrated high-Q Raman resonator exploiting multiple cavity modes coupled to the same waveguide.

Optomechanical controlling of intermolecular interaction and the application in molecular self-assembly.

In this paper, we combined cavity optomechanics and quantum mechanical mechanism of van der Waals force to study the dynamic behavior of interacting bimolecules in the plasmonic localized field, and extend it to the interacting multi-molecular system. We explored how plasmonic optomechanical coupling affects the strength of intermolecular interactions. Based on our results, we propose to use optical field to modulate the intermolecular interaction potential in plasmonic cavity, which can be utilized in the enhancement of the efficiency of the molecular self-assembly process and controlling the yield of the reaction in an optical environment. This research extends molecular optomechanics from intramolecular interactions to intermolecular interactions and may has high application potential in some nanostructure synthesis.

Addressing molecular optomechanical effects in nanocavity-enhanced Raman scattering beyond the single plasmonic mode.

The description of surface-enhanced Raman scattering (SERS) as a molecular optomechanical process has provided new insights into the vibrational dynamics and nonlinearities of this inelastic scattering process. In earlier studies, molecular vibrations have typically been assumed to couple with a single plasmonic mode of a metallic nanostructure, ignoring the complexity of the plasmonic response in many configurations of practical interest such as in metallic nanojunctions. By describing the plasmonic fields as a continuum, we demonstrate here the importance of considering the full plasmonic response to properly address the molecule-cavity optomechanical interaction. We apply the continuum-field model to calculate the Raman signal from a single molecule in a plasmonic nanocavity formed by a nanoparticle-on-a-mirror configuration, and compare the results of optomechanical parameters, vibrational populations, and Stokes and anti-Stokes signals of the continuum-field model with those obtained from the single-mode model. Our results reveal that high-order non-radiative plasmonic modes significantly modify the optomechanical behavior under strong laser illumination. Moreover, Raman linewidths, lineshifts, vibrational populations, and parametric instabilities are found to be sensitive to the energy of the molecular vibrational modes. The implications of adopting the continuum-field model to describe the plasmonic cavity response in molecular optomechanics are relevant in many other nanoantenna and nanocavity configurations commonly used to enhance SERS.

Quasinormal-Mode Perturbation Theory for Dissipative and Dispersive Optomechanics.

Despite the several novel features arising from the dissipative optomechanical coupling, such effect remains vastly unexplored due to the lack of a simple formalism that captures non-Hermiticity in the engineering of optomechanical systems. In this Letter, we show that quasinormal-mode-based perturbation theory is capable of correctly predicting both dispersive and dissipative optomechanical couplings. We validate our model through simulations and also by comparison with experimental results reported in the literature. Finally, we apply this formalism to plasmonic systems, used for molecular optomechanics, where strong dissipative coupling signatures in the amplification of vibrational modes could be observed.

Coupled quantum molecular cavity optomechanics with surface plasmon enhancement: comment

We have found an incorrect formula to estimate the root mean square (rms) amplitude of the molecular vibration in the molecular optomechanics systems reported by J. Liu et al. [Photon. Res. 5, 450–456 (2017)PRHEIZ2327-912510.1364/PRJ.5.000450]. In contrast with common optomechanical systems, the equilibrium position for molecular optomechanics systems used in the letter cannot be achieved by the equipartition theorem. Here, we achieve the effective temperature of molecules and then provide a corrected formula for the estimation of the rms amplitude of molecular motion. Using defined effective temperature into our new formula, we show that the minimal measurable force is ( F ≈ 1.7 × 10 − 14 N ), which it is 1 order bigger than the result of the main paper, and it is also in accordance with numerical calculation through the Lindblad master equation.

Molecular Optomechanics in the Anharmonic Cavity-QED Regime Using Hybrid Metal–Dielectric Cavity Modes

Using carefully designed hybrid metal-dielectric resonators, we study molecular optomechanics in the strong coupling regime ($g_{\rm }^2/\omega_m {>} \kappa$), which manifests in anharmonic emission lines in the sideband-resolved region of the cavity-emitted spectrum ($\kappa{<}\omega_m$). This nonlinear optomechanical strong coupling regime is enabled through a metal-dielectric cavity system that yields not only deep sub-wavelength plasmonic confinement, but also dielectric-like confinement times that are more than two orders of magnitude larger than those from typical localized plasmon modes. These hybrid metal-dielectric cavity modes enable one to study new avenues of quantum plasmonics for single molecule Raman scattering.

Extreme nanophotonics from ultrathin metallic gaps.

Ultrathin dielectric gaps between metals can trap plasmonic optical modes with surprisingly low loss and with volumes below 1 nm3. We review the origin and subtle properties of these modes, and show how they can be well accounted for by simple models. Particularly important is the mixing between radiating antennas and confined nanogap modes, which is extremely sensitive to precise nanogeometry, right down to the single-atom level. Coupling nanogap plasmons to electronic and vibronic transitions yields a host of phenomena including single-molecule strong coupling and molecular optomechanics, opening access to atomic-scale chemistry and materials science, as well as quantum metamaterials. Ultimate low-energy devices such as robust bottom-up assembled single-atom switches are thus in prospect.

Pulsed Molecular Optomechanics in Plasmonic Nanocavities: From Nonlinear Vibrational Instabilities to Bond-Breaking

New experiments on molecules trapped in an ultrasmall optical cavity reveal details of how molecular bonds interact with light and how this can eventually lead to their ultimate breaking, potentially providing access to new regimes of optical chemistry.