Molecular cavity optomechanics: a theory of plasmon-enhanced Raman scattering

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.Comment: Extensively revised and improved version thanks to the hard work and constructive comments of a careful Referee. Includes Supplemental Materia

Mode-specific Coupling of Nanoparticle-on-Mirror Cavities with Cylindrical Vector Beams

Nanocavities formed by ultrathin metallic gaps, such as the nanoparticle-on-mirror geometry, permit the reproducible engineering and enhancement of light-matter interaction thanks to mode volumes reaching the smallest values allowed by quantum mechanics. Although a large body of experimental data has confirmed theoretical predictions regarding the dramatically enhanced vacuum field in metallic nanogaps, much fewer studies have examined the far-field to near-field input coupling. Estimates of this quantity usually rely on numerical simulations under a plane wave background field, whereas most experiments employ a strongly focused laser beam. Moreover, it is often assumed that tuning the laser frequency to that of a particular cavity mode is a sufficient condition to resonantly excite its near-field. Here, we experimentally demonstrate selective excitation of nanocavity modes controlled by the polarization and frequency of the laser beam. We reveal mode-selectivity by recording fine confocal maps of Raman scattering intensity excited by cylindrical vector beams, which are compared to the known excitation near-field patterns. Our measurements allow unambiguous identification of the transverse vs. longitudinal character of the excited cavity mode, and of their relative input coupling rates as a function of laser wavelength. The method introduced here is easily applicable to other experimental scenarios and our results are an important step to connect far-field with near-field parameters in quantitative models of nanocavity-enhanced phenomena such as molecular cavity optomechanics, polaritonics and surface-enhanced spectroscopies.Comment: 23 pages, 11 figures (SI included

Molecular Vibration Explorer: an Online Database and Toolbox for Surface-Enhanced Frequency Conversion and Infrared and Raman Spectroscopy

We present Molecular Vibration Explorer, a freely accessible online database and interactive tool for exploring vibrational spectra and tensorial light-vibration coupling strengths of a large collection of thiolated molecules. The "Gold" version of the database gathers the results from density functional theory calculations on 2800 commercially available thiol compounds linked to a gold atom, with the main motivation to screen the best molecules for THz and mid-infrared to visible upconversion. Additionally, the "Thiol" version of the database contains results for 1900 unbound thiolated compounds. They both provide access to a comprehensive set of computed spectroscopic parameters for all vibrational modes of all molecules in the database. The user can simultaneously investigate infrared absorption, Raman scattering, and vibrational sum- and difference-frequency generation cross sections. Molecules can be screened for various parameters in custom frequency ranges, such as a large Raman cross-section under a specific molecular orientation, or a large orientation-averaged sum-frequency generation (SFG) efficiency. The user can select polarization vectors for the electromagnetic fields, set the orientation of the molecule, and customize parameters for plotting the corresponding IR, Raman, and sum-frequency spectra. We illustrate the capabilities of this tool with selected applications in the field of surface-enhanced spectroscopy

Intrinsic Luminescence Blinking from Plasmonic Nanojunctions

Plasmonic nanojunctions, consisting of adjacent metal structures with nanometre gaps, can support localised plasmon resonances that boost light matter interactions and concentrate electromagnetic fields at the nanoscale. In this regime, the optical response of the system is governed by poorly understood dynamical phenomena at the frontier between the bulk, molecular and atomic scales. Here, we report ubiquitous spectral fluctuations in the intrinsic light emission from photo-excited gold nanojunctions, which we attribute to the light-induced formation of domain boundaries and quantum-confined emitters inside the noble metal. Our data suggest that photoexcited carriers and gold adatom - molecule interactions play key roles in triggering luminescence blinking. Surprisingly, this internal restructuring of the metal has no measurable impact on the Raman signal and scattering spectrum of the plasmonic cavity. Our findings demonstrate that metal luminescence offers a valuable proxy to investigate atomic fluctuations in plasmonic cavities, complementary to other optical and electrical techniques

Continuous-wave frequency upconversion with a molecular optomechanical nanocavity

[EN] Coherent upconversion of terahertz and mid-infrared signals into visible light opens new horizons for spectroscopy, imaging, and sensing but represents a challenge for conventional nonlinear optics. Here, we used a plasmonic nanocavity hosting a few hundred molecules to demonstrate optomechanical transduction of submicrowatt continuous-wave signals from the mid-infrared (32 terahertz) onto the visible domain at ambient conditions. The incoming field resonantly drives a collective molecular vibration, which imprints a coherent modulation on a visible pump laser and results in upconverted Raman sidebands with subnatural linewidth. Our dual-band nanocavity offers an estimated 13 orders of magnitude enhancement in upconversion efficiency per molecule. Our results demonstrate that molecular cavity optomechanics is a flexible paradigm for frequency conversion leveraging tailorable molecular and plasmonic properties.This work received funding from the European Union's Horizon 2020 Research and Innovation Program under grant agreement nos. 829067 (FET Open THOR), 820196 (ERC CoG QTONE), and 732894 (HOT). C.G. acknowledges support from the Swiss National Science Foundation (project nos. 170684 and 198898). This work is part of the research program of the Netherlands Organisation for Scientific Research (NWO). A.I.B. acknowledges financial support by the Alexander von Humboldt Foundation.Chen, W.; Roelli, P.; Hu, H.; Verlekar, S.; Amirtharaj, SP.; Barreda, ÁI.; Kippenberg, TJ.... (2021). Continuous-wave frequency upconversion with a molecular optomechanical nanocavity. Science. 374:1264-1267. https://doi.org/10.1126/science.abk31061264126737

The SIB Swiss Institute of Bioinformatics' resources: focus on curated databases

The SIB Swiss Institute of Bioinformatics (www.isb-sib.ch) provides world-class bioinformatics databases, software tools, services and training to the international life science community in academia and industry. These solutions allow life scientists to turn the exponentially growing amount of data into knowledge. Here, we provide an overview of SIB's resources and competence areas, with a strong focus on curated databases and SIB's most popular and widely used resources. In particular, SIB's Bioinformatics resource portal ExPASy features over 150 resources, including UniProtKB/Swiss-Prot, ENZYME, PROSITE, neXtProt, STRING, UniCarbKB, SugarBindDB, SwissRegulon, EPD, arrayMap, Bgee, SWISS-MODEL Repository, OMA, OrthoDB and other databases, which are briefly described in this article

Molecular Optomechanics with Plasmons: Backaction at the nanoscale

The plasmonic systems used in surface-enhanced Raman scattering are shown to be quivalent to optomechanical cavities. The new backaction force of the plasmon on the molecular vibration could lead to coherent amplification of vibrational motion

Molecular Platform for Frequency Upconversion at the Single-Photon Level

Direct detection of single photons at wavelengths beyond 2 mu m under ambient conditions remains an outstanding technological challenge. One promising approach is frequency upconversion into the visible (VIS) or near-infrared (NIR) domain, where single-photon detectors are readily available. Here, we propose a nanoscale solution based on a molecular optomechanical platform to up-convert photons from the far- and mid-infrared (covering part of the terahertz gap) into the VIS-NIR domain. We perform a detailed analysis of its outgoing noise spectral density and conversion efficiency with a full quantum model. Our platform consists in doubly resonant nanoantennas focusing both the incoming long-wavelength radiation and the short-wavelength pump laser field into the same active region. There, infrared active vibrational modes are resonantly excited and couple through their Raman polarizability to the pump field. This optomechanical interaction is enhanced by the antenna and leads to the coherent transfer of the incoming low-frequency signal onto the anti-Stokes sideband of the pump laser. Our calculations demonstrate that our scheme is realizable with current technology and that optimized platforms can reach single-photon sensitivity in a spectral region where this capability remains unavailable to date

Low-Loss Integrated Nanophotonic Circuits with Layered Semiconductor Materials.

Monolayer transition-metal dichalcogenides with direct bandgaps are emerging candidates for optoelectronic devices, such as photodetectors, light-emitting diodes, and electro-optic modulators. Here we report a low-loss integrated platform incorporating molybdenum ditelluride monolayers with silicon nitride photonic microresonators. We achieve microresonator quality factors >3 × 106 in the telecommunication O- to E-bands. This paves the way for low-loss, hybrid photonic integrated circuits with layered semiconductors, not requiring heterogeneous wafer bonding