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We map in real space and by purely optical means near-field optical information of localized surface plasmon polariton (LSPP) resonances excited in nanoscopic particles. We demonstrate that careful polarization control enables apertureless scanning near-field optical microscopy (aSNOM) to image dipolar and quadrupolar LSPPs of the bare sample with high fidelity in both amplitude and phase. This establishes a routine method for in situ optical microscopy of plasmonic and other resonant structures under ambient conditions. [ABSTRACT FROM AUTHOR]

Controlling the light emitted by individual molecules is instrumental to a number of advanced nanotechnologies ranging from super-resolution bioimaging 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 may 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 2 orders of magnitude larger than the fluorescence line width of the bare emitter and reach into the near-infrared. Our results point toward a regime where the emission line width can become dominated by the excited-state lifetime, as required for indistinguishable photon emission, bearing 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 the design of efficient quantum emitters at infrared wavelengths, where standard organic sources have a reduced performance.

The enhancement of the molecular Raman signal in plasmon-assisted surface-enhanced Raman scattering (SERS) results from electromagnetic and chemical mechanisms, the latter determined to a large extent by the chemical interaction between the molecules and the hosting plasmonic nanoparticles. A precise quantification of the chemical mechanism in SERS based on quantum chemistry calculations is often challenging due to the interplay between the chemical and electromagnetic effects. Based on an atomistic description of the SERS signal, which includes the effect of strong field inhomogeneities, we introduce a comprehensive approach to evaluate the chemical enhancement in SERS, which conveniently removes the electromagnetic contribution inherent to any quantum calculation of the Raman polarization. Our approach uses density functional theory (DFT) and time-dependent DFT to compute the total SERS signal, together with the electromagnetic and chemical enhancement factors. We apply this framework to study the chemical enhancement of biphenyl-4,4'-dithiol embedded between two gold clusters. Although we find that for small clusters the total SERS enhancement is mainly determined by the chemical mechanism, our procedure enables removal of the electromagnetic contribution and isolation of the contribution of the bare chemical effect. This approach can be applied to reproduce and understand Raman line activation and strength in practical and challenging SERS configurations such as in plasmonic nano- and pico-cavities., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Understanding and mastering quantum electrodynamics phenomena is essential to the development of quantum nanophotonics applications. While tailoring of the local vacuum field has been widely used to tune the luminescence rate and directionality of a quantum emitter, its impact on their transition energies is barely investigated and exploited. Fluorescent defects in nanosized diamonds constitute an attractive nanophotonic platform to investigate the Lamb shift of an emitter embedded in a dielectric nanostructure with high refractive index. Using spectral and time-resolved optical spectroscopy of single SiV defects, we unveil blue shifts (up to 80 meV) of their emission lines, which are interpreted from model calculations as giant Lamb shifts. Moreover, evidence for a positive correlation between their fluorescence decay rates and emission line widths is observed, as a signature of modifications not only of the photonic local density of states but also of the phononic one, as the nanodiamond size is decreased. Correlative light-electron microscopy of single SiVs and their host nanodiamonds further supports these findings. These results make nanodiamond-SiVs promising as optically driven spin qubits and quantum light sources tunable through nanoscale tailoring of vacuum-field fluctuations.

We show using time-dependent density functional theory (TDDFT) that light can be confined into slot waveguide modes residing between individual atomic layers of coinage metals, such as gold. As the top atomic monolayer lifts a few Å off the underlying bulk Au (111), ab initio electronic structure calculations show that for gaps >1.5 Å, visible light squeezes inside the empty slot underneath, giving optical field distributions 2 Å thick, less than the atomic diameter. Paradoxically classical electromagnetic models are also able to reproduce the resulting dispersion for these subatomic slot modes, where light reaches in-plane wavevectors ∼2 nm -1 and slows to <10 -2 c . We explain the success of these classical dispersion models for gaps ≥1.5 Å due to a quantum-well state forming in the lifted monolayer in the vicinity of the Fermi level. This extreme trapping of light may explain transient "flare" emission from plasmonic cavities where Raman scattering of metal electrons is greatly enhanced when subatomic slot confinement occurs. Such atomic restructuring of Au under illumination is relevant to many fields, from photocatalysis and molecular electronics to plasmonics and quantum optics.

Thermoelectric materials play a vital role in the pursuit of a sustainable energy system by allowing the conversion of waste heat to electric energy. Low thermal conductivity is essential to achieving high-efficiency conversion. The conductivity depends on an interplay between the phononic and electronic properties of the nonequilibrium state. Therefore, obtaining a comprehensive understanding of nonequilibrium dynamics of the electronic and phononic subsystems as well as their interactions is key for unlocking the microscopic mechanisms that ultimately govern thermal conductivity. A benchmark material that exhibits ultralow thermal conductivity is SnSe. We study the nonequilibrium phonon dynamics induced by an excited electron population using a framework combining ultrafast electron diffuse scattering and nonequilibrium kinetic theory. This in-depth approach provides a fundamental understanding of energy transfer in the spatiotemporal domain. Our analysis explains the dynamics leading to the observed low thermal conductivity, which we attribute to a mode-dependent tendency to nonconservative phonon scattering. The results offer a penetrating perspective on energy transport in condensed matter with far-reaching implications for rational design of advanced materials with tailored thermal properties.

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.

The optical response of a system formed by a quantum emitter and a plasmonic gap nanoantenna is theoretically addressed within the frameworks of classical electrodynamics and the time-dependent density functional theory (TDDFT). A fully quantum many-body description of the electron dynamics within TDDFT allows for analyzing the effect of electronic coupling between the emitter and the nanoantenna, usually ignored in classical descriptions of the optical response. We show that the hybridization between the electronic states of the quantum emitter and those of the metallic nanoparticles strongly modifies the energy, the width, and the very existence of the optical resonances of the coupled system. We thus conclude that the application of a quantum many-body treatment that correctly addresses charge-transfer processes between the emitter and the nanoantenna is crucial to address complex electronic processes involving plasmon-exciton interactions directly impacting optoelectronic applications.

As the size of a molecular emitter becomes comparable to the dimensions of a nearby optical resonator, the standard approach that considers the emitter to be a point-like dipole breaks down. By adoption of a quantum description of the electronic transitions of organic molecular emitters, coupled to a plasmonic electromagnetic field, we are able to accurately calculate the position-dependent coupling strength between a plasmon and an emitter. The spatial distribution of excitonic and photonic quantum states is found to be a key aspect in determining the dynamics of molecular emission in ultrasmall cavities both in the weak and strong coupling regimes. Moreover, we show that the extreme localization of plasmonic fields leads to the selection rule breaking of molecular excitations.

Plasmon-enhanced Raman scattering can push single-molecule vibrational spectroscopy beyond a regime addressable by classical electrodynamics. We employ a quantum electrodynamics (QED) description of the coherent interaction of plasmons and molecular vibrations that reveal the emergence of nonlinearities in the inelastic response of the system. For realistic situations, we predict the onset of phonon-stimulated Raman scattering and a counterintuitive dependence of the anti-Stokes emission on the frequency of excitation. We further show that this QED framework opens a venue to analyze the correlations of photons emitted from a plasmonic cavity.

Plasmonic multinanoparticle systems exhibit collective electric and magnetic resonances that are fundamental for the development of state-of-the-art optical nanoantennas, metamaterials, and surface-enhanced spectroscopy substrates. While electric dipolar modes have been investigated in both the classical and quantum realm, little attention has been given to magnetic and other "dark" modes at the smallest dimensions. Here, we study the collective electric, magnetic, and dark modes of colloidally synthesized silver nanosphere trimers with varying interparticle separation using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). This technique enables direct visualization and spatially selective excitation of individual trimers, as well as manipulation of the interparticle distance into the subnanometer regime with the electron beam. Our experiments reveal that bonding electric and magnetic modes are significantly impacted by quantum effects, exhibiting a relative blueshift and reduced EELS amplitude compared to classical predictions. In contrast, the trimer's electric dark mode is not affected by quantum tunneling for even Ångström-scale interparticle separations. We employ a quantum-corrected model to simulate the effect of electron tunneling in the trimer which shows excellent agreement with experimental results. This understanding of classical and quantum-influenced hybridized modes may impact the development of future quantum plasmonic materials and devices, including Fano-like molecular sensors and quantum metamaterials.

Self-assembled clusters of metallic nanoparticles separated by nanometric gaps generate strong plasmonic modes that support both intense and localized near fields. These find use in many ultrasensitive chemical and biological sensing applications through surface enhanced Raman scattering (SERS). The inability to control at the nanoscale the structure of the clusters on which the optical response crucially depends, has led to the development of general descriptions to model the various morphologies fabricated. Here, we use rigorous electrodynamic calculations to study clusters formed by a hundred nanospheres that are separated by ∼1 nm distance, set by the dimensions of the macrocyclic molecular linker employed experimentally. Three-dimensional (3D) cluster structures of moderate compactness are of special interest since they resemble self-assembled clusters grown under typical diffusion-limited aggregation conditions. We find very good agreement between the simulated and measured far-field extinction spectra, supporting the equivalence of the assumed and experimental morphologies. From these results we argue that the main features of the optical response of two- and three-dimensional clusters can be understood in terms of the excitation of simple units composed of different length resonant chains. Notably, we observe a qualitative difference between short- and long-chain modes in both spectral response and spatial distribution: dimer and short-chain modes are observed in the periphery of the cluster at higher energies, whereas inside the structure longer chain excitation occurs at lower energies. We study in detail different configurations of isolated one-dimensional chains as prototypical building blocks for large clusters, showing that the optical response of the chains is robust to disorder. This study provides an intuitive understanding of the behavior of very complex aggregates and may be generalized to other types of aggregates and systems formed by large numbers of strongly interacting particles.

Cucurbit[n]urils (CB[n]) are macrocyclic host molecules with subnanometer dimensions capable of binding to gold surfaces. Aggregation of gold nanoparticles with CB[n] produces a repeatable, fixed, and rigid interparticle separation of 0.9 nm, and thus such assemblies possess distinct and exquisitely sensitive plasmonics. Understanding the plasmonic evolution is key to their use as powerful SERS substrates. Furthermore, this unique spatial control permits fast nanoscale probing of the plasmonics of the aggregates "glued" together by CBs within different kinetic regimes using simultaneous extinction and SERS measurements. The kinetic rates determine the topology of the aggregates including the constituent structural motifs and allow the identification of discrete plasmon modes which are attributed to disordered chains of increasing lengths by theoretical simulations. The CBs directly report the near-field strength of the nanojunctions they create via their own SERS, allowing calibration of the enhancement. Owing to the unique barrel-shaped geometry of CB[n] and their ability to bind "guest" molecules, the aggregates afford a new type of in situ self-calibrated and reliable SERS substrate where molecules can be selectively trapped by the CB[n] and exposed to the nanojunction plasmonic field. Using this concept, a powerful molecular-recognition-based SERS assay is demonstrated by selective cucurbit[n]uril host-guest complexation.

In this work, a method for improved protein identification of low-abundance proteins using unstained gels, in combination with robotics and matrix-assisted laser desorption/ionization tandem mass spectrometry, has been developed and evaluated. Omitting the silver-staining process resulted in increased protein identification scores, an increase in the number of peptides observed in the MALDI mass spectrum, and improved quality of the tandem mass spectrometry data.

The effect of fermentation on antinutritional factors and also on total dietary fiber (TDF), insoluble (IDF) and soluble (SDF) dietary fiber fractions was studied in beans (Phaseolus vulgaris L.). The processes studied were two types of fermentation (lactic acid and natural), and a portion of the obtained flours were processed by autoclaving. The dietary fiber (DF) content and its components were determined using the enzymatic-gravimetric and enzymatic-chemical methods. The TDF content ranged from 24.5% dry matter (DM) in the raw to 25.2% DM in the processed beans. All the processing treatments significantly decreased the SDF content, and irrelevant changes were noticed in the IDF content of processed beans. Cellulose content of all samples was reduced by the processing treatments. Correspondingly, higher amounts of resistant starch was observed in the processed beans, except in the lactic acid fermented ones. However, the levels of pectic polysaccharides and Klason lignin were higher in the samples fermented by Lactobacillus plantarum. The action of microorganisms was determinant for the different degradation of the bean cell wall, disrupting the protein-carbohydrate integration, thus reducing the solubility of DF.

The effect of different conditions of pea germination on dietary fiber (DF) composition was studied. Insoluble dietary fiber (IDF) and soluble dietary fiber (SDF) were subjected to acid hydrolysis, and the resultant neutral sugars, uronic acids, and Klason lignin were quantified. Germinated peas exhibited significantly higher contents of total dietary fiber (TDF) than the raw sample, due to the increases of both DF fractions. Under darkness conditions, germination exhibited the highest contents of IDF and SDF. Decreasing IDF/SDF ratios showed that the carbohydrate changes did not take place to the same extent during germination, the SDF fraction being the most affected. The detailed chemical composition of fiber fractions reveals increases of cellulose in the IDF of germinated samples, whereas SDF exhibits a decrease of pectic polysaccharides and also increases of polysaccharides rich in glucose and mannose. The DF results were corroborated by a comparative examination of the cell wall carbohydrate composition.