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Controlling the light emitted by individual molecules is instrumental to a number of novel nanotechnologies ranging from super-resolution bio-imaging and molecular sensing to quantum nanophotonics. Molecular emission can be tailored by modifying the local photonic environment, for example by precisely placing a single molecule inside a plasmonic nanocavity with the help of DNA origami. Here, using this scalable approach, we show that commercial fluorophores experience giant Purcell factors and Lamb shifts, reaching values on par with those recently reported in scanning tip experiments. Engineering of plasmonic modes enables cavity-mediated fluorescence far detuned from the zero-phonon-line (ZPL) - at detunings that are up to two orders of magnitude larger than the fluorescence linewidth of the bare emitter and reach into the near-infrared. Our results evidence a regime where the emission linewidth is dominated by the excited state lifetime, as required for indistinguishable photon emission, baring relevance to the development of nanoscale, ultrafast quantum light sources and to the quest toward single-molecule cavity-QED. In the future, this approach may also allow to design efficient quantum emitters at infrared wavelengths, where standard organic sources have a reduced performance., Comment: Supplementary information included

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)

Frequency upconversion is a cornerstone of electromagnetic signal processing, analysis and detection. It is used to transfer energy and information from one frequency domain to another where transmission, modulation or detection is technically easier or more efficient. Optomechanical transduction is emerging as a flexible approach to coherent frequency upconversion; it has been successfully demonstrated for conversion from radio- and microwaves (kHz to GHz) to optical fields. Nevertheless, optomechanical transduction of multi-THz and mid-infrared signals remains an open challenge. Here, we utilize molecular cavity optomechanics to demonstrate upconversion of sub-microwatt continuous-wave signals at $\sim$32~THz into the visible domain at ambient conditions. The device consists in a plasmonic nanocavity hosting a small number of molecules. The incoming field resonantly drives a collective molecular vibration, which imprints an optomechanical modulation on a visible pump laser and results in Stokes and anti-Stokes upconverted Raman sidebands with sub-natural linewidth, indicating a coherent process. The nanocavity offers 13 orders of magnitude enhancement of upconversion efficiency per molecule compared to free space, with a measured phonon-to-photon internal conversion efficiency larger than $10^{-4}$ per milliwatt of pump power. Our results establish a flexible paradigm for optomechanical frequency conversion using molecular oscillators coupled to plasmonic nanocavities, whose vibrational and electromagnetic properties can be tailored at will using chemical engineering and nanofabrication., Comment: Includes Supplementary Material, 31 pages

Immense field enhancement and nanoscale confinement of light are possible within nanoparticle-on-mirror (NPoM) plasmonic resonators, which enable novel optically-activated physical and chemical phenomena, and render these nanocavities greatly sensitive to minute structural changes, down to the atomic scale. Although a few of these structural parameters, primarily linked to the nanoparticle and the mirror morphology, have been identified, the impact of molecular assembly and organization of the spacer layer between them has often been left uncharacterized. Here, we experimentally investigate how the complex and reconfigurable nature of a thiol-based self-assembled monolayer (SAM) adsorbed on the mirror surface impacts the optical properties of the NPoMs. We fabricate NPoMs with distinct molecular organizations by controlling the incubation time of the mirror in the thiol solution. Afterwards, we investigate the structural changes that occur under laser irradiation by tracking the bonding dipole plasmon mode, while also monitoring Stokes and anti-Stokes Raman scattering from the molecules as a probe of their integrity. First, we find an effective decrease in the SAM height as the laser power increases, compatible with an irreversible change of molecule orientation caused by heating. Second, we observe that the nanocavities prepared with a densely packed and more ordered monolayer of molecules are more prone to changes in their resonance compared to samples with sparser and more disordered SAMs. Our measurements indicate that molecular orientation and packing on the mirror surface play a key role in determining the stability of NPoM structures and hence highlight the under-recognized significance of SAM characterization in the development of NPoM-based applications.

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.