Plasmon Excitation Research Articles

THz Surface Plasmons in Wide and Freestanding Graphene Nanoribbon Arrays

Graphene is a thin-film carbon material that has immense potential as a key ingredient in new nanoelectronic and nanophotonic devices due to its unique characteristics. In particular, plasmons in graphene appear as a practical tool for the manipulation of light with potential applications from cancer treatment to solar cells. A motivating tunability of graphene properties has been observed in graphene nanoribbons (GNRs) due to their geometrically controllable bandgaps that, in turn, influence the plasmonic properties. The formidable effort made over recent years in developing GNR-based technologies is, however, weakened by a lack of predictive approaches that draw upon available semi-analytical electromagnetic models. An example of such a framework is used here, focusing on experimentally realized GNRs from 155 to 480 nm wide and organized as two-dimensional (2D) GNR arrays. The results show that the plasmon frequency behavior is highly affected by the experimental setup or geometrical factors. In particular, the bandgap of the analyzed systems is of the order of a few meV with a density of states opening around zero energy (Fermi level) in contrast to what is observed in graphene. From the plasmonic part, it is observed in all 2D GNR arrays that the frequency–momentum trend follows a q-like plasmon dispersion whose plasmon frequency can be increased substantially by increasing the ribbon width or charge density concentration. Forbidden plasmon regions are observed for high values of plasmon excitation angle or electron relaxation rate. From a sensing point of view, the important finding is the fact that 2D GNR arrays of 155 nm wide with high values of electron relaxation rate have plasmon responses similar to those observed for α−thrombin in water. Our predictions are projected to be of fast support for detecting plasmons in more complex designs of ribbon nanodevices with potential applications in molecular sensing of aqueous molecules.

Propagating charge carrier plasmons in Sr2RuO4

We report on studies of charge carrier plasmon excitations in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ by transmission electron energy-loss spectroscopy. In particular, we present results on the plasmon dispersion and its width as a function of momentum transfer. The dispersion can be qualitatively explained in the framework of random phase approximation calculations, using an unrenormalized tight-binding band structure. The constant long-wavelength width of the plasmon indicates that it is caused by a decay into interband transitions and not by quantum critical fluctuations. The results from these studies on a prototypical correlated metal system show that the long-wavelength plasmon excitations near 1 eV are caused by resilient quasiparticles and are not influenced by correlation effects.

Plasmon-Exciton Interaction in Perspective Hetero-Systems

Surface plasmons and excitons have been widely studied experimentally and theoretically for various material systems. However, a number of aspects require further deeper study and understanding, among which the connection of these quasi-particles occupies an important place. New physical effects arise when plasmons and excitons in nanostructures begin to be localized at certain small distances, as a result, we can talk about their coupling. Complex systems containing the excitation of plasmons and excitons, as well as their coupling, show interesting optical properties that they cannot exhibit individually. In this type of system, the plasmon enhances the coupling between the system and the external field, and the exciton controls certain spectral properties, which opens up new possibilities for tuning their optical response. The transferred energy between plasmons and excitons becomes an important factor affecting their interaction when the resonance frequency of the localized plasmon is very close to the molecular energy transition frequency. Two types of coupling can occur depending on the ratio between the strength of the coupling and the energy losses of individual components in the system, namely strong and weak. In addition to the mutual coupling between the plasmon and the exciton, their different linewidths and ability to couple to an external field provide a variety of means to tune the optical properties of hybrid systems. Thus, it enables precise control of light at the nanometer scale, opening up possibilities for new electronics and photonics applications. In this review, we highlight the features of weak and strong modes of plasmon-exciton coupling, modern trends, and perspectives in the study of hetero-systems semiconductor–metal, metal–2D material, semiconductor–molecule, etc. Semiconductor-metal hybrid nanostructures open up exciting opportunities for the study of quantum phenomena, optical processes, and multiparticle interactions and confidently lead to application in new photonics devices.

Tale of Two Resonances: Waveguide–Plasmon Coupling and High Q-Factor Engineering on the Nanoscale

Localized surface plasmon (LSP) excitations provide an efficient strategy for advancing nanophotonic designs and applications where strong field enhancement and confinement are often required on the nanoscale. They represent an important plasmonic paradigm for achieving strong light–matter interactions in both linear and nonlinear regimes, enabling the development of high-performance chemical and biological sensing approaches and nonlinear optics with low light intensities. However, the LSP resonance line width, limited by both radiative and resistive losses of metallic nanostructures, is significantly larger than the line width of the waveguided modes supported by low-loss dielectric microcavities with a significantly lower field confinement. Hybrid microcavity–plasmonic systems are, therefore, often used to reduce the resonant line width which improves the detection spectral resolution while maintaining strong confinement. Employing the remarkable quality factors of whispering gallery mode (WGM) microresonators, the hybrid LSP-WGM systems demonstrate sensing capabilities down to the single-molecule level. In this Perspective, we review the recent advances in the hybridization of LSPs and WGMs, focusing on the fundamental understanding of the underlying coupling mechanisms and corresponding mode hybridization regimes. We further discuss opportunities for applying heterogeneous plasmonic–photonic integration to tailor the nanoscale light–matter interactions and realize novel waveguide–plasmon coupling based nontrivial responses and highlight their prospective applications in quantum optics, chiral spin-optics, nonlinear nanophotonics, and sensing.

Investigation of plasmon relaxation mechanisms using nonadiabatic molecular dynamics.

Hot carriers generated from the decay of plasmon excitation can be harvested to drive a wide range of physical or chemical processes. However, their generation efficiency is limited by the concomitant phonon-induced relaxation processes by which the energy in excited carriers is transformed into heat. However, simulations of dynamics of nanoscale clusters are challenging due to the computational complexity involved. Here, we adopt our newly developed Trajectory Surface Hopping (TSH) nonadiabatic molecular dynamics algorithm to simulate plasmon relaxation in Au20 clusters, taking the atomistic details into account. The electronic properties are treated within the Linear Response Time-Dependent Tight-binding Density Functional Theory (LR-TDDFTB) framework. The relaxation of plasmon due to coupling to phonon modes in Au20 beyond the Born-Oppenheimer approximation is described by the TSH algorithm. The numerically efficient LR-TDDFTB method allows us to address a dense manifold of excited states to ensure the inclusion of plasmon excitation. Starting from the photoexcited plasmon states in Au20 cluster, we find that the time constant for relaxation from plasmon excited states to the lowest excited states is about 2.7ps, mainly resulting from a stepwise decay process caused by low-frequency phonons of the Au20 cluster. Furthermore, our simulations show that the lifetime of the phonon-induced plasmon dephasing process is ∼10.4fs and that such a swift process can be attributed to the strong nonadiabatic effect in small clusters. Our simulations demonstrate a detailed description of the dynamic processes in nanoclusters, including plasmon excitation, hot carrier generation from plasmon excitation dephasing, and the subsequent phonon-induced relaxation process.

Calculate The Resonance Angle of Surface Plasmon Resonance Gold film Configured with Kretschmann

The current work includes measurements of the resonance angle and reflectance for p-polarization of the electric field by using the Fresnel equation at a given length. Gold's surface plasmon wave can be seen at the metal-to-air boundary. We try to determine the greatest (SPR) angle for a metal thin layer that is most suitable for the surface plasmon excitation while stimulated by a laser. SPR was performed of a single film of gold placed on a glass prism; there are SPR modes in this structure, which match the surface plasmon. We also suggest that the SPR mode associated with the Au surface, which is extremely sensitive to changes in the surrounding environment, particularly (dielectric). A few considerations to be taken into account to attain the SPR, like the incident angle of light rays addressed and analyzed for the purpose of finding the essential value for the plasmon to be emerge; the gold/air resonance angle. Furthermore, we can compare our result with other work, which was performed by using the Finite-Element-Method (FEM), the simulation is done by FDTD (Finite Difference Time Domain) software. SPR was applied in a variety of domains, containing biomedicine science, optics, biomedicine, photo-thermal plasmon, and health.

Electrochemical Fabrication of Inverse Opals of Silver with Cyanide-Free Electrolytes

Silver inverse opals were prepared electrochemically using non-toxic water–ethanol rhodanic and sulfite electrolytes for silver plating. Electrochemical crystallization of silver occurred in the pores of opal matrices made of 350 nm polystyrene microspheres. Samples with the most uniform structure and lowest percentage of surface admixtures were obtained from rhodanic electrolytes. Reflectance optical spectra of the silver inverse opals with a high surface periodicity showed various plasmon excitation modes in a visible spectral range making the silver opals attractive for plasmonics.

Electronic effects on the radiation damage in high-entropy alloys

High-entropy alloys (HEAs) are exceptional candidates for radiation-resistant materials due to their complex local chemical environment and slow defect migration. Despite commonly overlooked, electronic effects on defects evolution in radiation environments also play a crucial role by dissipating excess energy through electron-phonon coupling and electronic heat conduction during cascade events. We present a systematic study on electronic properties in random-solid solutions (RSS) in four and five principal elements HEAs and their effect on defect formation, clustering, and recombination. Electronic properties, including electron-phonon coupling factor (Ge−ph), the electronic specific heat (Ce), and the electronic thermal conductivity (κe), are computed within first-principles calculations. Using the two-temperature molecular dynamics simulations, we show that the electron-phonon coupling factor and electronic specific heat play a critical role in Frenkel pairs formation. Specifically, the electron-phonon coupling factor quickly dissipates the kinetic energy during primary knock-on atom events via plasmon excitations and is subsequently dissipated via the free-electrons conduction. We show that these effects are more critical than the elastic distortion effects produced by the atomic mismatch. Of tremendous interest, we show that including lighter elements helps to increase Ge−ph suggesting the possibility to improve radiation resistance in HEA through optimal composition.

Plasmonic dichroism and all-optical magnetization switching in nanophotonic structures with GdFeCo.

We report on a phenomenon of plasmonic dichroism observed in magnetic materials with transverse magnetization under excitation of surface plasmon polariton waves. The effect originates from the interplay of the two magnetization-dependent contributions to the material absorption, both of which are enhanced under plasmon excitation. Plasmonic dichroism is similar to circular magnetic dichroism, which is at the base of all-optical helicity-dependent switching (AO-HDS) but observed for linearly polarized light, and the dichroism acts upon in-plane magnetized films, where AO-HDS does not take place. We show by electromagnetic modeling that laser pulses exciting counter-propagating plasmons can be used to write +M or -M states in a deterministic way independent of the initial magnetization state. The presented approach applies to various ferrimagnetic materials with in-plane magnetization, exhibiting the phenomenon of all-optical switching of a thermal nature and broadens the horizons of their applications in data storage devices.

Enhanced and tunable Goos-Hänchen shift of reflected light due to Tamm surface plasmons with Dirac semimetals

In this paper, enhanced and tunable Goos-Hänchen (GH) shift of reflected light beam at mid-infrared band can be theoretically achieved by using a multilayer structure where a bulk Dirac semimetal (BDS) layer is coated on a distributed bragg reflector (DBR) separated by a spacer layer. This enhanced GH phenomenon results from the local field enhancement owing to the excitation of Tamm surface plasmons at the interface between BDS layer and spacer layer. Numerical calculation shows that the GH shift can be switched from negative to positive by harnessing the bulk conductivity properties of BDS, and can also be actively tuned through the Fermi energy of BDS or by controlling the structural parameters. These enhanced and tunable GH shifts are promising for fabricating BDS-based infrared shift devices and other applications in nanophotonics.

Graphene plasmons on attenuated total reflection structures: a route to achieve large optical pushing or pulling force intensities in the terahertz region

The ability to control optomechanical interactions at subwavelength levels is of key relevance in modern optics. The present work focuses on the improvement of the force exerted on a dielectric nanoparticle placed near an attenuated total reflection structure containing graphene. Our results show that this force is highly dependent on the angle of incidence of the impinging plane wave, and it can be enhanced by nearly three orders of magnitude when this angle coincides with that of the critical total internal reflection and that corresponding to the excitation of surface plasmons along the graphene layer. New effects are found in these angular regions where the induced elliptical polarizability on the nanoparticle provides an asymmetric excitation of electromagnetic modes that generates a pulling (attractive) component whose magnitude is comparable to that of the pushing (repulsive) force provided by the incident field. As a consequence and contrary to what happens for large separation distances, an asymmetric response with respect to the angle of incidence arises for short distances for which the force on the particle is described by curves having a maximum and a minimum. Our results can contribute to a better understanding of polarization dependent interactions between nanoparticles and electromagnetic mode fields for taking advantage of them in nanoparticle manipulation applications.

Graphene metasurfaces for terahertz wavefront shaping and light emission [Invited

Graphene is a promising materials platform for metasurface flat optics at terahertz wavelengths, with the important advantage of active tunability. Here we review recent work aimed at the development of tunable graphene metasurfaces for THz wavefront shaping (including beam-steering metamirrors and metalenses) and light emission. Various design strategies for the constituent meta-units are presented, ranging from metallic phase-shifting elements combined with a nearby graphene sheet for active tuning to graphene plasmonic resonators providing the required phase control or radiation mechanism. The key challenge in the development of these devices, related to the limited radiative coupling of graphene plasmonic excitations, is discussed in detail together with recently proposed solutions. The resulting metasurface technology can be expected to have a far-reaching impact on a wide range of device applications for THz imaging, sensing, and future wireless communications.

The Effect of Femtosecond Laser Irradiation and Plasmon Field on the Degree of Conversion of a UDMA-TEGDMA Copolymer Nanocomposite Doped with Gold Nanorods.

In this work, the effects of femtosecond laser irradiation and doping with plasmonic gold nanorods on the degree of conversion (DC) of a urethane dimethacrylate (UDMA)-triethylene glycol dimethacrylate (TEGDMA) nanocomposite were investigated. The UDMA-TEGDMA photopolymer was prepared in a 3:1 weight ratio and doped with dodecanethiol- (DDT) capped gold nanorods of 25 × 75 or 25 × 85 nm nominal diameter and length. It was found that the presence of the gold nanorods alone (without direct plasmonic excitation) can increase the DC of the photopolymer by 6-15%. This increase was found to be similar to what could be achieved with a control heat treatment of 30 min at 180 °C. It was also shown that femtosecond laser impulses (795 nm, 5 mJ pulse energy, 50 fs pulse length, 2.83 Jcm-2 fluence), applied after the photopolymerization under a standard dental curing lamp, can cause a 2-7% increase in the DC of undoped samples, even after thermal pre-treatment. The best DC values (12-15% increase) were obtained with combined nanorod doping and subsequent laser irradiation close to the plasmon resonance peak of the nanorods (760-800 nm), which proves that the excited plasmon field can directly facilitate double bond breakage (without thermoplasmonic effects due to the short pulse length) and increase the crosslink density independently from the initial photopolymerization process.

Synergistic Effect between CO2 Chemisorption Using Amino-Modified Carbon Nitride and Epoxide Activation by High-Energy Electrons for Plasmon-Assisted Synthesis of Cyclic Carbonates.

Photocatalytic CO2 cycloaddition is a promising approach for CO2 value-added processes. However, the efficiency of plasmon-assisted CO2 cycloaddition still needs to be improved and the reaction mechanism is unclear. Herein, g-C3N4/Ag (ACN-Ag) hybrids exhibited superior activity of CO2 cycloaddition by coupling a semiconductor into the plasmonic system, in which the ACN grafting amino group by the formation of carbon vacancies can enhance CO2 chemisorption; meanwhile, photo-generated electrons from ACN transfer to Ag to form high-energy electrons, which can activate propylene oxide, accelerating the ring-opening step. Importantly, photo-generated electron injection from ACN to Ag and the interaction between Ag nanoparticles and ACN were confirmed by single-particle photoluminescence spectroscopy. The wavelength-dependent activity demonstrated that the plasmon excitation is crucial for the reaction. Moreover, in situ single-particle PL quenching caused by propylene oxide and in situ electron paramagnetic resonance verified the activation of propylene oxide by ACN-Ag. This work is conducive to an in-depth understanding of the mechanism of CO2 cycloaddition at the single-particle level and provides guidance for the organic synthesis.

Massive and massless plasmons in germanene nanosheets

Atomically thin crystals may exhibit peculiar dispersive electronic states equivalent to free charged particles of ultralight to ultraheavy masses. A rare coexistence of linear and parabolic dispersions yields correlated charge density modes exploitable for nanometric light confinement. Here, we use a time-dependent density-functional approach, under several levels of increasing accuracy, from the random-phase approximation to the Bethe-Salpeter equation formalism, to assess the role of different synthesized germanene samples as platforms for these plasmon excitations. In particular, we establish that both freestanding and some supported germenene monolayers can sustain infrared massless modes, resolved into an out-of-phase (optical) and an in-phase (acoustic) component. We further indicate precise experimental geometries that naturally host infrared massive modes, involving two different families of parabolic charge carriers. We thus show that the interplay of the massless and massive plasmons can be finetuned by applied extrinsic conditions or geometry deformations, which constitutes the core mechanism of germanene-based optoelectronic and plasmonic applications.

One-Step Formation of Plasmonic Cu Nanodomains in p-Type Cu2O Matrix Films for Enhanced Photoconversion of n-ZnO/p-Cu2O Heterojunctions

Plasmonic Cu nanoparticles were in-situ grown into a Cu$_2$O semiconductor matrix by using reactive magnetron sputtering and adjusting the amount of oxygen available during the synthesis in order to prevent the oxidation of part of copper atoms landed on the film surface. Varying only the oxygen flowrate (OFR) and using a single Cu target it was possible to observe the evolution in the simultaneous formation of metallic Cu and Cu$_2$O phases for oxygen-poor conditions. Suchformation is accompanied by the development of the surface plasmon band (SPB) corresponding to Cu, as evidenced by UV-Vis spectrophotometry and spectroscopic ellipsometry. The bandgap values of the elaborated composites containing embedded Cu plasmonic nanodomains were lower than the bandgap of single-phased Cu$_2$O films, likely due to the higher defect density associated to the nanocrystalline nature of films, promoted by the presence of metallic Cu. The resistivity of the thin films increased with more oxidative deposition conditions and was associated to an increase in Cu$_2$O/Cu ratio and smaller and more isolated Cu particles, as evidenced by high resolution transmission electron microscopy and X-ray diffraction. Photoconversion devices based on the studied nanocomposites were characterized by I-V and spectral photocurrent measurements, showing an increase in the photocurrent density under light illumination as consequence of the plasmonic particles excitation leading to hot carrier's injection in the nearby ZnO and Cu$_2$O semiconductors.

CO2 adsorption and activation on Ag(1 1 1) surfaces in the presence of surface charge density: A static gas phase DFT study

The CO2 reduction reaction on Ag in the presence of surface plasmons are of strong interest in photocatalysis. Here, two Density Functional Theory (DFT) approaches are proposed to localize charges at the Ag surface to mimic surface plasmon excitation. These calculations predict charge localization at the outermost surface layer, a result confirmed via light excitation at the plasmon resonance modeled by Density Functional Tight Binding Hamiltonian (DFTB) theory. The CO2(gas) reduction initial steps are studied by DFT models showing that bonded species can be created on top of an Ag-atom in the presence of extra charges. A second CO2 molecule can assist the first molecule, decreasing the charge carrier density requirement for CO2 reduction bonded species. A Molecular Dynamics (MD) study shows a possible interaction among CO2 molecule. These results show that a static DFT simulation can mimic charge localization resulting from surface plasmon effects, thus enabling studies on surface plasmon-enhanced chemical reactions, paving the way for future time-dependent (TD) studies.

Plasmonic transverse dipole moment in chiral fermion nanowires

Plasmons are elementary quantum excitations of conducting materials with Fermi surfaces. In two dimensions they may carry a static dipole moment that is transverse to their momentum which is quantum geometric in nature, the quantum geometric dipole (QGD). We show that this property is also realized for such materials confined in nanowire geometries. Focusing on the gapless, intrasubband plasmon excitations, we compute the transverse dipole moment ${\mathcal{D}}_{x}$ of the modes for a variety of situations. We find that single chiral fermions generically host nonvanishing ${\mathcal{D}}_{x}$, even when there is no intrinsic gap in the two-dimensional spectrum, for which the corresponding two-dimensional QGD vanishes. In the limit of very wide wires, the transverse dipole moment of the highest velocity plasmon mode matches onto the two-dimensional QGD. Plasmons of multivalley systems that are time-reversal symmetric have a vanishing transverse dipole moment but can be made to carry nonvanishing values by breaking the valley symmetry, for example, via a magnetic field. The presence of a nonvanishing transverse dipole moment for nanowire plasmons in principle offers the possibility of continuously controlling their energies and velocities by the application of a static transverse electric field.

Second harmonic generation under doubly resonant lattice plasmon excitation.

Second harmonic generation is enhanced at the surface lattice resonance in plasmonic nanoparticle arrays. We carried out a parametric investigation on two-dimensional lattices composed of gold nanobars where the centrosymmetry is broken at oblique incidence. We study the influence of the periodicity, the incidence angle and the direction of the linear input polarization on the second harmonic generation. Excitation of the surface lattice resonance either at the fundamental or second harmonic wavelength, achieved by varying the incidence angle, enhance the conversion efficiency. As a special case, we demonstrate that both the wavelengths can be simultaneously in resonance for a specific period of the lattice. In this double resonant case, maximum second harmonic power is achieved.

Near-Infrared Plasmon-Induced Hot Electron Extraction Evidence in an Indium Tin Oxide Nanoparticle/Monolayer Molybdenum Disulfide Heterostructure.

In this work, we observe plasmon-induced hot electron extraction in a heterojunction between indium tin oxide nanocrystals and monolayer molybdenum disulfide. We study the sample with ultrafast differential transmission, exciting the sample at 1750 nm where the intense localized plasmon surface resonance of the indium tin oxide nanocrystals is and where the monolayer molybdenum disulfide does not absorb light. With the excitation at 1750 nm, we observe the excitonic features of molybdenum disulfide in the visible range, close to the exciton of molybdenum disulfide. Such a phenomenon can be ascribed to a charge transfer between indium tin oxide nanocrystals and monolayer molybdenum disulfide upon plasmon excitation. These results are a first step toward the implementation of near-infrared plasmonic materials for photoconversion.