Quantum mechanical study of the coupling of plasmon excitations to atomic-scale electron transport

Uncovering the Evolution of Low-Energy Plasmons in Nanopatterned Aluminum Plasmonics on Graphene

We report adjusting the charge-transfer-plasmon (CTP) resonances of aluminum (Al) bowties on suspended monolayer graphene via controlled nanofabrication and focused electron-beam irradiation. CTP resonances of bowties with a conductive junction blue-shift with an increase in junction width, whereas their 3λ/2 and λ resonances barely red-shift. These plasmon modes are derived and confirmed by an LC circuit model and electromagnetic simulations performed with boundary-element and frequency-domain methods. A monotonic decay of the CTP lifetime is observed, while the junction width is extended. Instead, the lifetimes of 3λ/2 and λ resonances are nearly independent of junction width. When the junction is shrunk by electron-beam irradiation, all antenna resonances red-shift. Having created an electron-beam-induced sub 5 nm gap in bowties, we monitor the unambiguous transition of a CTP into a bonding-type gap mode, which is highly sensitive to the separation distance. Meanwhile, the 3λ/2 and λ resonances evolve into dipolar bright and dipolar dark modes.

Quantum surface effects in the electromagnetic coupling between a quantum emitter and a plasmonic nanoantenna: time-dependent density functional theory vs. semiclassical Feibelman approach

We use time-dependent density functional theory (TDDFT) within the jellium model to study the impact of quantum-mechanical effects on the self-interaction Green's function that governs the electromagnetic interaction between quantum emitters and plasmonic metallic nanoantennas. A semiclassical model based on the Feibelman parameters, which incorporates quantum surface-response corrections into an otherwise classical description, confirms surface-enabled Landau damping and the spill out of the induced charges as the dominant quantum mechanisms strongly affecting the nanoantenna-emitter interaction. These quantum effects produce a redshift and broadening of plasmonic resonances not present in classical theories that consider a local dielectric response of the metals. We show that the Feibelman approach correctly reproduces the nonlocal surface response obtained by full quantum TDDFT calculations for most nanoantenna-emitter configurations. However, when the emitter is located in very close proximity to the nanoantenna surface, we show that the standard Feibelman approach fails, requiring an implementation that explicitly accounts for the nonlocality of the surface response in the direction parallel to the surface. Our study thus provides a fundamental description of the electromagnetic coupling between plasmonic nanoantennas and quantum emitters at the nanoscale.

Influence of bulky substituents on single-molecule SERS sensitivity

The surface-enhanced Raman spectroscopy (SERS) detection limit strongly depends on the molecular structure, which we demonstrate for a family of tert-butyl-substituted porphycenes. Even though the investigated species present very similar photophysical properties, the ratio between the SERS signal and fluorescence background depends on the number of bulky tert-butyl groups. Moreover, the probability of single molecule detection systematically drops with the number of the moieties attached to the pyrrole ring. As steric hindrance is the only significantly changing feature among the studied chromophores, we attribute the observed phenomena to the spatial structure. We also show that the sensitivity of the SERS technique can be improved by lowering the temperature. We managed to observe single-molecule spectra for derivatives for which this was unattainable at room temperature.

Indirect to Direct Charge Transfer Transition in Plasmon-Enabled CO2 Photoreduction

Understanding hot carrier dynamics between plasmonic nanomaterials and its adsorbate is of great importance for plasmon-enhanced photoelectronic processes such as photocatalysis, optical sensing and spectroscopic analysis. However, it is often challenging to identify specific dominant mechanisms for a given process because of the complex pathways and ultrafast interactive dynamics of the photoelectrons. Here, using CO₂ reduction as an example, the underlying mechanisms of plasmon-driven catalysis at the single-molecule level using time-dependent density functional theory calculations is clearly probed. The CO₂ molecule adsorbed on two typical nanoclusters, Ag₂₀ and Ag₁₄₇ , is photoreduced by optically excited plasmon, accompanied by the excitation of asymmetric stretching and bending modes of CO₂ . A nonlinear relationship has been identified between laser intensity and reaction rate, demonstrating a synergic interplay and transition from indirect hot-electron transfer to direct charge transfer, enacted by strong localized surface plasmons. These findings offer new insights for CO₂ photoreduction and for the design of effective pathways toward highly efficient plasmon-mediated photocatalysis.

Electronic Exciton-Plasmon Coupling in a Nanocavity Beyond the Electromagnetic Interaction Picture

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.

Hybrid theoretical models for molecular nanoplasmonics

The multidisciplinary nature of the research in molecular nanoplasmonics, i.e., the use of plasmonic nanostructures to enhance, control, or suppress properties of molecules interacting with light, led to contributions from different theory communities over the years, with the aim of understanding, interpreting, and predicting the physical and chemical phenomena occurring at molecular- and nano-scale in the presence of light. Multiscale hybrid techniques, using a different level of description for the molecule and the plasmonic nanosystems, permit a reliable representation of the atomistic details and of collective features, such as plasmons, in such complex systems. Here, we focus on a selected set of topics of current interest in molecular plasmonics (control of electronic excitations in light-harvesting systems, polaritonic chemistry, hot-carrier generation, and plasmon-enhanced catalysis). We discuss how their description may benefit from a hybrid modeling approach and what are the main challenges for the application of such models. In doing so, we also provide an introduction to such models and to the selected topics, as well as general discussions on their theoretical descriptions.

New hybridization coupling mechanism and enhanced sensitivity in a Cu2-xS@Au nanoparticle dimer

To improve the refractive index sensitivity of a localized surface plasmon resonance (LSPR) sensor, we employ a new interparticle hybridization plasmon coupled resonance in a semiconductor-metal (Cu_2-xS@Au) core-shell nanoparticle dimer (SMCSND), where the refractive index sensitivity can be improved by the generation of a tunable dual-band absorption spectrum at visible and near-infrared wavelengths. Owing to two LSPRs in different wavelength regions supported by the metal shell and semiconductor core, for the first time, we theoretically demonstrate that the new interparticle hybridization plasmon coupled mechanism in semiconductor-metal core-shell nanoparticle dimer depends not only on interparticle separation gap, but also on the nanoparticle shell thickness t. Electromagnetic model analysis reveals that there are two plasmon modes (Mode A and Mode C) associated with the interparticle hybridization plasmon coupled resonance, where the Mode C shows high sensitivity and figure of merit (FoM) to changes in the background dielectric medium. The tunability of the induced interparticle hybridization plasmon coupled resonance with different the separation distance and shell thickness can change the sensitivity and FoM of LSPR sensor in the visible to near-infrared region, which has broad application prospects.

Optical Coulomb blockade lifting in plasmonic nanoparticle dimers

If two metal nanoparticles are ultimately approached, a tunneling current prevents both an infinite redshift of the bonding dipolar plasmon and an infinite increase of the electric field in the hot spot between the nanoparticles. We argue that a Coulomb blockade suppresses the tunneling current and sustains a redshift even for sub-nanometer approach up to moderate fields. Only for stronger optical fields, the Coulomb blockade is lifted and a charge transfer plasmon is formed. Numerical simulations show that such scenarios are well in reach with manageable nanoparticle dimensions, even at room temperature. Applications may include ultrafast, optically driven switches, photo detectors operating at 500 THz, or highly nonlinear devices.

Unveiling the effect of electron tunneling on the plasmonic resonance of closely spaced gold particles

Recent experimental techniques enable the nanoparticles' (NPs) ensembles to be made with an angstrom-level of interparticle gap widths. The theoretical description of their optical properties becomes more challenging because of the nonnegligible quantum mechanism (QM) effects such as electron tunneling and nonlocal screening. To demonstrate the microscopic mechanism of QM effects and quantitatively elucidate their impact on a variety of surface plasmon resonance (SPR) models of gold particle oligomers, in this work we performed a theoretical study on closely spaced Au cluster dimers and NP oligomers by the first-principles approach, and the classical or quantum-corrected electromagnetic models (CEM or QCM), respectively. Through the first-principles calculation on a series of Au_N dimers with different interparticle distances d and N, we depicted the variation of the possibility of direct electron tunneling (DET) across the junction constructed by two nearest NPs with d and N, and found that it exactly follows an exponential decay and the decay rate linearly varies with 1/N. The impact of the QM effect on the SPR models excited along the dimer axis is much more profound than those perpendicular to the dimer axis. CEM/QCM calculations on strongly coupled NP dimers and symmetric and asymmetric trimers demonstrated the evolution of their optical properties with variable NP sizes, gap separations and light polarization, as well as the QM effect on the major SPR modes. The side-by-side comparison between the results from time-dependent density functional theory and CEM/QCM models sheds light on understanding the origin of a variety of SPR models of gold NP oligomers and the QM effect on those modes, and makes a connection between the calculations of small cluster and large NP oligomers.

New coupling mechanism of titanium nitride nanosphere dimers at short separation distances

The coupled localized plasmon modes generated by a metal plasmonic nanoparticle (NP) dimer induces a stronger plasmon enhanced electromagnetic field as well as a stronger optical response compared to that of a single NP. Owing to the small Drude damping factor, however, the absorption bandwidth of noble metallic NPs is insufficiently broad. Herein, the near-field wide band coupling absorption for 25 nm diameter TiN nanospheres is investigated in a homo-dimer arrangement for various separation distances using the finite element method. An enhancement of the wide band coupling absorption and a red-shift is found, which can be explained by an uncomplicated dipole-dipole coupling model at interparticle distances greater than 5 nm. At short separation distances, the coupling absorption of the TiN dimer exhibits a tremendous change, which is diametrically opposite the results found for a Au dimer. This unexpected change phenomenon is shown by calculation and analysis to be owing to the change of the charge distribution approach at short separation distances, which is demonstrated to play a key role in the wide band coupling characteristic variation. With decreasing separation gap, a new coupling mechanism caused by surface charge properties is responsible for the decline in coupling absorption as well as the break in the ruler equations for both plasmon shift and temperature enhancement.

Plasmon Excitations in Mixed Metallic Nanoarrays

Features of the surface plasmon from macroscopic materials emerge in molecular systems, but differentiating collective excitations from single-particle excitations in molecular systems remains elusive. The rich interactions between single-particle electron-hole and collective electron excitations produce phenomena related to the chemical physics aspects within the atomic array. We study the plasmonic properties of atomic arrays of noble (Au, Ag, and Cu) and transition-metal (Pd, Pt) homonuclear chains using time-dependent density functional theory and their Kohn-Sham transition contributions. The response to the electromagnetic radiation is related to both the geometry-dependent confinement of sp-valence electrons and the energy position of d-electrons in the different atomic species and the hybridization between d and sp electrons. It is possible to tune the position of the plasmon resonance, split it into several peaks, and eventually achieve broadband absorption of radiation. Arrays of mixed noble and transition-metal chains may have strongly attenuated plasmonic behavior. The collective nature of the excitations is ascertained using their Kohn-Sham transition contributions. To manipulate the plasmonic response and achieve the desired properties for broad applications, it is vital to understand the origins of these phenomena in atomic chains and their arrays.

Momentum-resolved TDDFT algorithm in atomic basis for real time tracking of electronic excitation

Ultrafast electronic dynamics in solids lies at the core of modern condensed matter and materials physics. To build up a practical ab initio method for studying solids under photoexcitation, we develop a momentum-resolved real-time time dependent density functional theory (rt-TDDFT) algorithm using numerical atomic basis, together with the implementation of both the length and vector gauge of the electromagnetic field. When applied to simulate elementary excitations in two-dimensional materials such as graphene, different excitation modes, only distinguishable in momentum space, are observed. The momentum-resolved rt-TDDFT is important and computationally efficient for the study of ultrafast dynamics in extended systems.

Plasmon-induced nonlinear response of silver atomic chains

Nonlinear response of a linear silver atomic chain upon ultrafast laser excitation has been studied in real time using the time-dependent density functional theory. We observe the presence of nonlinear responses up to the fifth order in tunneling current, which is ascribed to the excitation of high-energy electrons generated by Landau damping of plasmons. The nonlinear effect is enhanced after adsorption of polar molecules such as water due to the enhanced damping rates during plasmon decay. Increasing the length of atomic chains also increases the nonlinear response, favoring higher-order plasmon excitation. These findings offer new insights towards a complete understanding and ultimate control of plasmon-induced nonlinear phenomena to atomic precision.

Plasmon-Induced Ultrafast Hydrogen Production in Liquid Water

Hydrogen gas production from solar water splitting provides a renewable energy cycle to address the grand global energy challenge; however, its dynamics and fundamental mechanism remain elusive. We directly explore by first-principles the ultrafast electron-nuclear quantum dynamics on the time scale of ∼100 fs during water photosplitting on a plasmonic cluster embedded in liquid water. Water molecule splitting is assisted by rapid proton transport in liquid water in a Grotthuss-like mechanism. We identify that a plasmon-induced field enhancement effect dominates water splitting, while charge transfer from gold to the antibonding orbital of a water molecule also plays an important role. "Chain-reaction" like rapid H₂ production is observed via the combination of two hydrogen atoms from different water molecules. These results provide a route toward a complete understanding of water photosplitting in the ultimate time and spatial limit.

Tip-Enhanced Raman Spectromicroscopy of Co(II)-Tetraphenylporphyrin on Au(111): Toward the Chemists' Microscope

Atomically terminated and nanoscopically smooth silver tips effectively focus light on the angstrom scale, allowing tip-enhanced Raman spectromicroscopy (TER-sm) with single molecule sensitivity and submolecular spatial resolution. Through measurements carried out on cobalt-tetraphenylporphyrin (CoTPP) adsorbed on Au(111), we highlight peculiarities of vibrational spectromicroscopy with light confined on the angstrom scale. Field-gradient-driven spectra, orientational fingerprinting, and sculpting of local fields by atomic morphology of the junction are elucidated through measurements that range from 2D arrays at room temperature to single molecule manipulations at 5 K. Notably, vibrational Stark tuning within molecules, reflecting intramolecular charge distributions, becomes accessible when light is more localized than the interrogated normal modes. The Stark images of CoTPP reveal that it is saddled, and the distortion is accompanied by charge transfer to gold through the two anchoring pyrroles.

Self-energy matrices for electron transport calculations within the real-space finite-difference formalism

The self-energy term used in transport calculations, which describes the coupling between electrode and transition regions, is able to be evaluated only from a limited number of the propagating and evanescent waves of a bulk electrode. This obviously contributes toward the reduction of the computational expenses in transport calculations. In this paper, we present a mathematical formula for reducing the computational expenses further without using any approximation and without losing accuracy. So far, the self-energy term has been handled as a matrix with the same dimension as the Hamiltonian submatrix representing the interaction between an electrode and a transition region. In this work, through the singular-value decomposition of the submatrix, the self-energy matrix is handled as a smaller matrix, whose dimension is the rank number of the Hamiltonian submatrix. This procedure is practical in the case of using the pseudopotentials in a separable form, and the computational expenses for determining the self-energy matrix are reduced by 90% when employing a code based on the real-space finite-difference formalism and projector-augmented wave method. In addition, this technique is applicable to the transport calculations using atomic or localized basis sets. Adopting the self-energy matrices obtained from this procedure, we present the calculation of the electron transport properties of C_{20} molecular junctions. The application demonstrates that the electron transmissions are sensitive to the orientation of the molecule with respect to the electrode surface. In addition, channel decomposition of the scattering wave functions reveals that some unoccupied C_{20} molecular orbitals mainly contribute to the electron conduction through the molecular junction.

Photonics and spectroscopy in nanojunctions: a theoretical insight

Green function methods for photonics and spectroscopy in nanojunctions.

Quantum mechanical effects in plasmonic structures with subnanometre gaps

Metallic structures with nanogap features have proven highly effective as building blocks for plasmonic systems, as they can provide a wide tuning range of operating frequencies and large near-field enhancements. Recent work has shown that quantum mechanical effects such as electron tunnelling and nonlocal screening become important as the gap distances approach the subnanometre length-scale. Such quantum effects challenge the classical picture of nanogap plasmons and have stimulated a number of theoretical and experimental studies. This review outlines the findings of many groups into quantum mechanical effects in nanogap plasmons, and discusses outstanding challenges and future directions.

Free-Standing Monolayered Metallic Nanoparticle Networks as Building Blocks for Plasmonic Nanoelectronic Junctions

The effective coupling of optical surface plasmons (SPs) and electron transport in a plasmonic-electronic device is one of the fundamental issues in nanoelectronics and the emerging field of plasmonics, and offer promise in providing a solution to next generation nanocircuits in which all coupling is in the near field. Attempts toward this end, however, are limited because of the integration challenge to compatible nanodevices. To date, direct electrical detection of SP-electron coupling from metallic nanostructures alone are not reported, and thus it remains a great experimental challenge. In this paper, we succeed in preparing a new suspended-film-type nanoelectronic junction, in which free-standing 2D fractal nanoparticle networks act as plasmonically active nanocomponents. Direct electrical detection of optical collective SPs was evidenced by photocurrent response of the junction upon illumination. Room-temperature I-V characteristics, differing from nonlinear to Ohmic behaviors, are found to be sensitive to the nanometer-scale morphology changes of the nanomembranes. The finding and approach may enable the development of advanced plasmonic nanocircuits and new nanoelectronics, nanophotonics, and (solar) energy applications.

Quantum transport through a Coulomb blockaded quantum emitter coupled to a plasmonic dimer

We study the electron transmission through a Coulomb blockaded quantum emitter coupled to metal nanoparticles possessing plasmon resonances by employing the time-dependent non-crossing approximation. We find that the coupling of the nanoparticle plasmons with the excitons results in a significant enhancement of the conductance through the discrete state with higher energy beyond the unitarity limit while the other discrete state with lower energy remains Coulomb blockaded. We show that boosting the plasmon-exciton coupling well below the Kondo temperature increases the enhancement adding another quantum of counductance upon saturation. Finite bias and increasing emitter resonance energy tend to reduce this enhancement. We attribute these observations to the opening of an additional transport channel via the plasmon-exciton coupling.

Quantized Evolution of the Plasmonic Response in a Stretched Nanorod

Quantum aspects, such as electron tunneling between closely separated metallic nanoparticles, are crucial for understanding the plasmonic response of nanoscale systems. We explore quantum effects on the response of the conductively coupled metallic nanoparticle dimer. This is realized by stretching a nanorod, which leads to the formation of a narrowing atomic contact between the two nanorod ends. Based on first-principles time-dependent density-functional-theory calculations, we find a discontinuous evolution of the plasmonic response as the nanorod is stretched. This is especially pronounced for the intensity of the main charge-transfer plasmon mode. We show the correlation between the observed discontinuities and the discrete nature of the conduction channels supported by the formed atomic-sized junction.

Plasmon-Mediated Electron Transport in Tip-Enhanced Raman Spectroscopic Junctions

We combine experiment, theory, and first-principles-based calculations to study the light-induced plasmon-mediated electron transport characteristics of a molecular-scale junction. The experimental data show a nonlinear increase in electronic current perturbation when the focus of a chopped laser beam moves laterally toward the tip-sample junction. To understand this behavior and generalize it, we apply a combined theory of the electronic nonequilibrium formed upon decoherence of an optically triggered plasmon and first-principles transport calculations. Our model illustrates that the current via an adsorbed molecular monolayer increases nonlinearly as more energy is pumped into the junction due to the increasing availability of virtual molecular orbital channels for transport with higher injection energies. Our results thus illustrate light-triggered, plasmon-enhanced tunneling current in the presence of a molecular linker.

Theory of light emission from quantum noise in plasmonic contacts: above-threshold emission from higher-order electron-plasmon scattering

We develop a theoretical framework for the description of light emission from plasmonic contacts based on the nonequilibrium Green function formalism. Our theory establishes a fundamental link between the finite-frequency quantum noise and ac conductance of the contact and the light emission. Calculating the quantum noise to higher orders in the electron-plasmon interaction, we identify a plasmon-induced electron-electron interaction as the source of experimentally observed above-threshold light emission from biased STM contacts. Our findings provide important insight into the effect of interactions on the light emission from atomic-scale contacts.

A classical treatment of optical tunneling in plasmonic gaps: extending the quantum corrected model to practical situations

The optical response of plasmonic nanogaps is challenging to address when the separation between the two nanoparticles forming the gap is reduced to a few nanometers or even subnanometer distances. We have compared results of the plasmon response within different levels of approximation, and identified a classical local regime, a nonlocal regime and a quantum regime of interaction. For separations of a few Ångstroms, in the quantum regime, optical tunneling can occur, strongly modifying the optics of the nanogap. We have considered a classical effective model, so called Quantum Corrected Model (QCM), that has been introduced to correctly describe the main features of optical transport in plasmonic nanogaps. The basics of this model are explained in detail, and its implementation is extended to include nonlocal effects and address practical situations involving different materials and temperatures of operation.

Theoretical predictions for hot-carrier generation from surface plasmon decay

Decay of surface plasmons to hot carriers finds a wide variety of applications in energy conversion, photocatalysis and photodetection. However, a detailed theoretical description of plasmonic hot-carrier generation in real materials has remained incomplete. Here we report predictions for the prompt distributions of excited 'hot' electrons and holes generated by plasmon decay, before inelastic relaxation, using a quantized plasmon model with detailed electronic structure. We find that carrier energy distributions are sensitive to the electronic band structure of the metal: gold and copper produce holes hotter than electrons by 1-2 eV, while silver and aluminium distribute energies more equitably between electrons and holes. Momentum-direction distributions for hot carriers are anisotropic, dominated by the plasmon polarization for aluminium and by the crystal orientation for noble metals. We show that in thin metallic films intraband transitions can alter the carrier distributions, producing hotter electrons in gold, but interband transitions remain dominant.

Quantum mechanical origin of the plasmon: from molecular systems to nanoparticles

The surface plasmon resonance (SPR) of noble metal nanoparticles is reviewed in terms of both classical and quantum mechanical approaches. The collective oscillation of the free electrons responsible for the plasmon is well described using classical electromagnetic theory for large systems (from about 10 to 100 nm). In cases where quantum effects are important, this theory fails and first principle approaches like time-dependent density functional theory (TDDFT) must be used. In this paper, we give an account of the current understanding of the quantum mechanical origin of plasmon resonances. We provide some insight into how the discrete absorption spectrum of small noble metal clusters evolves into a strong plasmon peak with increasing particle size. The collective character of the plasmon is described in terms of the constructive addition of single-particle excitations. As the system size increases, the number of single-particle excitations increases as well. A configuration interaction (CI) approach can be applied to describe the optical properties of particles of all shapes and sizes, providing a consistent definition of plasmon resonances. Finally, we expand our analysis to thiolate-protected nanoparticles and analyze the effects of ligands on the plasmon.

Plasmon hybridization model generalized to conductively bridged nanoparticle dimers

Charge transfer introduced by a conductive junction bridging a nanoparticle dimer can have a pronounced effect on the optical properties of the system. We have extended the plasmon hybridization model to include conductive junctions and charge transfer effects. From our model, we are able to derive all the plasmon resonances of the bridged dimer, including the charge transfer plasmon (CTP), and predict their trends when system parameters are varied. In particular, we find that CTP is a result of a sufficiently narrow monopole mode interacting with multipolar (including dipolar) modes. The screening arising from charge transfer induces a blueshift of the hybridized bonding dimer modes and decreases the electric field in the junction. Our model may serve as an important guide for optical properties of bridged nanoparticle aggregates.

New basic insights into the low hot electron injection efficiency of gold-nanoparticle-photosensitized titanium dioxide

The low hot electrons injection efficiency (HEIE) from plasmonic metal to semiconductor significantly affects the performance of metal-semiconductor composite. However, there are few reports about the origin of this low HEIE. In the present work, the factors affecting the transfer process and generation efficiency of hot electron in Au@TiO2 composite are investigated using first-principles calculations and Maxwell's electrodynamics theory. The occupation of surface oxygen vacancies of TiO2 by gold atoms is found to increase the hot electrons transfer barrier and expand the space charge region, which decrease the HEIE. In addition, the existing Au@TiO2 structure going against the generation of large amount of hot electrons may also lead to low HEIE. Our results reveal that the replacement of divalent host oxygen atoms with monovalent atoms can decrease the HEIE and comparison with experimental results allows us to validate our predictions. Furthermore, proper surface treatment of semiconductor before depositing metal particles and structure optimization of the composite are suggested to improve the HEIE.

Real-space finite-difference calculation method of generalized Bloch wave functions and complex band structures with reduced computational cost

Generalized Bloch wave functions of bulk structures, which are composed of not only propagating waves but also decaying and growing evanescent waves, are known to be essential for defining the open boundary conditions in the calculations of the electronic surface states and scattering wave functions of surface and junction structures. Electronic complex band structures being derived from the generalized Bloch wave functions are also essential for studying bound states of the surface and junction structures, which do not appear in conventional band structures. We present a novel calculation method to obtain the generalized Bloch wave functions of periodic bulk structures by solving a generalized eigenvalue problem, whose dimension is drastically reduced in comparison with the conventional generalized eigenvalue problem derived by Fujimoto and Hirose [Phys. Rev. B 67, 195315 (2003)]. The generalized eigenvalue problem derived in this work is even mathematically equivalent to the conventional one, and, thus, we reduce computational cost for solving the eigenvalue problem considerably without any approximation and losing the strictness of the formulations. To exhibit the performance of the present method, we demonstrate practical calculations of electronic complex band structures and electron transport properties of Al and Cu nanoscale systems. Moreover, employing atom-structured electrodes and jellium-approximated ones for both of the Al and Si monatomic chains, we investigate how much the electron transport properties are unphysically affected by the jellium parts.

Substrate-mediated charge transfer plasmons in simple and complex nanoparticle clusters

A conductive substrate can provide a simple and straightforward way to induce charge-transfer plasmon modes in Au nanoparticle clusters. For a simple dimer structure, a remarkably narrow charge transfer plasmon, which differs dramatically from the dipolar plasmon mode of the electrically isolated nanostructure, is clearly observed. For a more complex nonamer cluster that supports a strong Fano resonance on an insulating substrate, a mixed charge transfer-dipole mode is observed, where charge transfer is induced on the outer nanoparticles, establishing an opposing dipole on the intervening central particles, resulting in a strongly damped far field response.

Interfaced metal heterodimers in the quantum size regime

Synthesis of nanoparticle dimers made of asymmetric compositions is very challenging because of the difficulty in manipulating the nanoparticles' surface chemistries in order to control the assembly and/or growth of different nanoparticles. In this Letter, we report a seed-mediated, surface-confined epitaxial overgrowth strategy that enables the synthesis of high-quality interfaced Au-Ag heterodimers in the quantum size regime (diameters <10 nm). Au and Ag share a common face-centered cubic lattice and have nearly identical lattice constants, which facilitates epitaxial overgrowth and allows direct contact between the Au and Ag domains. Quantum size effects, formation of the Au/Ag interfaces, and chemical interactions with surfactant molecules strongly influence the optical properties of the dimers and lead to the observation of unique surface plasmon resonances. In particular, we find an unusual enhancement of the characteristic Au surface plasmon resonance and the emergence of a charge transfer plasmon across the Au/Ag domains, which together lead to broad-band absorption spanning visible to near-infrared wavelengths. A model that captures the changes in optical behavior due to chemical interactions and quantum size effects is used to calculate the absorption spectra of the interfaced heterodimers, resulting in good agreement with experimental measurements.

Self-similar gold-nanoparticle antennas for a cascaded enhancement of the optical field

We experimentally demonstrate cascaded field enhancement by means of gold nanoparticle dimer and trimer antennas. The local field enhancement is probed by single-molecule fluorescence using fluorophores with high intrinsic quantum efficiency (Q(0)>80%). Using a self-similar trimer antenna consisting of 80, 40, and 20 nm gold nanoparticles, we demonstrate a fluorescence enhancement of 40 and a spatial confinement of 15 nm. Compared with a single gold nanoparticle, the self-similar trimer antenna improves the enhancement-confinement ratio by more than an order of magnitude. Self-similar antennas hold promise for high-resolution imaging and spectroscopy, ultrasensitive detection, and efficient single-photon sources.

Bridging quantum and classical plasmonics with a quantum-corrected model

Electromagnetic coupling between plasmonic resonances in metallic nanoparticles allows for engineering of the optical response and generation of strong localized near-fields. Classical electrodynamics fails to describe this coupling across sub-nanometer gaps, where quantum effects become important owing to non-local screening and the spill-out of electrons. However, full quantum simulations are not presently feasible for realistically sized systems. Here we present a novel approach, the quantum-corrected model (QCM), that incorporates quantum-mechanical effects within a classical electrodynamic framework. The QCM approach models the junction between adjacent nanoparticles by means of a local dielectric response that includes electron tunnelling and tunnelling resistivity at the gap and can be integrated within a classical electrodynamical description of large and complex structures. The QCM predicts optical properties in excellent agreement with fully quantum mechanical calculations for small interacting systems, opening a new venue for addressing quantum effects in realistic plasmonic systems.

Near field of strongly coupled plasmons: uncovering dark modes

Strongly coupled plasmons in a system of individual gold nanoparticles placed at subnanometer distance to a gold film (nanoparticle-on-plane, NPOP) are investigated using two complementary single particle spectroscopy techniques. Optical scattering spectroscopy exclusively detects plasmon modes that couple to the far field via their dipole moment (bright modes). By using photoemission electron microscopy (PEEM), we detect in the identical NPOPs near-field modes that do not couple to the scattered far field (dark modes) and are characterized by a strongly enhanced nonlinear electron emission process. To our knowledge, this is the first time that both far- and near-field spectroscopy are carried out for identical individual nanostructures interacting via a subnanometer gap. Strongly resonant electron emission occurs at excitation wavelengths far off-resonant in the scattering spectra.

Coherent amplification and noise in gain-enhanced nanoplasmonic metamaterials: a Maxwell-Bloch Langevin approach

Nanoplasmonic metamaterials are an exciting new class of engineered media that promise a range of important applications, such as subwavelength focusing, cloaking, and slowing/stopping of light. At optical frequencies, using gain to overcome potentially not insignificant losses has recently emerged as a viable solution to ultra-low-loss operation that may lead to next-generation active metamaterials. Maxwell-Bloch models for active nanoplasmonic metamaterials are able to describe the coherent spatiotemporal and nonlinear gain-plasmon dynamics. Here, we extend the Maxwell-Bloch theory to a Maxwell-Bloch Langevin approach-a spatially resolved model that describes the light field and noise dynamics in gain-enhanced nanoplasmonic structures. Using the example of an optically pumped nanofishnet metamaterial with an embedded laser dye (four-level) medium exhibiting a negative refractive index, we demonstrate the transition from loss-compensation to amplification and to nanolasing. We observe ultrafast relaxation oscillations of the bright negative-index mode with frequencies just below the THz regime. The influence of noise on mode competition and the onset and magnitude of the relaxation oscillations is elucidated, and the dynamics and spectra of the emitted light indicate that coherent amplification and lasing are maintained even in the presence of noise and amplified spontaneous emission.

Nonresonant broadband funneling of light via ultrasubwavelength channels

Enhancing and funneling light efficiently through deep subwavelength apertures is essential in harnessing light-matter interaction. Thus far, this has been accomplished resonantly, by exciting the structural surface plasmons of perforated nanostructured metal films, a phenomenon known as extraordinary optical transmission. Here, we present a new paradigm structure which possesses all the capabilities of extraordinary optical transmission platforms, yet operates nonresonantly on a distinctly different mechanism. Our proposed platform demonstrates efficient ultrabroadband funneling of optical power confined in an area as small as ∼(λ/500)(2), where optical fields are enhanced, thus exhibiting functional possibilities beyond resonant platforms. We analyze the nonresonant mechanism underpinning such a phenomenon with a simple quasistatic picture, which shows excellent agreement with our numerical simulations.