Theory Of Plasmons Research Articles

Theory of intrinsic acoustic plasmons in twisted bilayer graphene

Theory of intrinsic acoustic plasmons in twisted bilayer graphene

Experimental and theoretical investigation of waveguided plasmonic surface lattice resonances.

Plasmonic nanostructures are good candidates for refractive index sensing applications through the surface plasmon resonance due to their strong dependence on the surrounding dielectric media. However, typically low quality-factor limits their application in sensing devices. To improve the quality-factor, we have experimentally and theoretically investigated two-dimensional gold nanoparticle gratings situated on top of a waveguide. The coupling between the localized surface plasmon and waveguide modes results in Fano-type resonances, with high quality-factors, very similar to plasmonic surface lattice resonances. By combining plasmonic surface lattice resonance and waveguide theory, we present a theoretical framework describing the structures. By immersing the fabricated samples in three different media we find a sensitivity of ∼50 nm/RIU and figure of merit of 8.9, and demonstrate good agreement with the theory presented. Further analysis show that the sensitivity is very dependent on the waveguide parameters, grating constant and the dielectric environment, and by tuning these parameters we obtain a theoretical sensitivity of 887 nm/RIU.

A Dual-Band Guided-Laser Absorber Based on Silicon-Nickel Metasurface

Metasurface absorbers are already used in many laser applications. To achieve laser stealth, a dual-band guided-laser absorber is presented in this paper. The absorber is composed of periodic silicon squares on a silicon film and a nickel layer. Under normal incidence, the absorber has two absorption peaks at wavelengths of 1.55 μm and 1.064 μm, with absorption rates higher than 94.4%. For wavelength-tunable 1.55 μm lasers with a large wavelength tuning range of ±50 nm, the absorption rate is still as high as 90%. Plasmonic resonance theory, as well as FDTD simulations, are used to design and study the absorber. It is found that the absorber is independent of the incident polarization and tolerant to the incident angle. The design method is flexible, and the absorber is easy to manufacture.

The Optical Response of Metallic Spherical Core-Shell Nanoparticles

The optical response of spherical nanoparticles was studied based on the quasi-static approximation of classical electrodynamics. The simple Drude model was used to calculate the complex dielectric function of the metal. We investigated the behavior of the absorption cross-section spectra of metallic spherical core–shell (including dielectric-metal and metal-dielectric) nanoparticles numerically. We found that the number of resonance peaks in the absorption cross-section spectra depends on the number of metal-dielectric interfaces of the nanostructure, as predicted by plasmonic hybridization theory. Furthermore, the absorption cross-section spectra can be precisely controlled by changing the thickness of the shell and the dielectric constant of the medium.

HIGH ENERGY KΑ SINGLE PLASMON SATELLITES OF FLUORIDES

The origin of high energy Kα single Plasmon satellites of Kα 1,2 main emission line of Fluorides viz NaF, LiF, CaF2, Na3AlF6, MgF2, AlF3, NiF2, PbF2, CuF2 have been explained using Plasmon theory with new formulation of relative intensity by considering relative interference between intrinsic and extrinsic effects. The interference ‘Plasmon-coupling’ have been obtained theoretically in terms of non-diagram line to diagram X-ray line for the satellite profiles in the X-ray photoemission spectra. It is found that our results with new intensity formulation are in better agreement with experimental results.

Plasmon Localization and Giant Fields in an Open-Resonator Metasurface forSurface-Enhanced-Raman-Scattering Sensors

We present metal-dielectric metasurfaces fabricated from metal periodical nanograting deposited on a dielectric substrate. The metasurface consists of a modulated dielectric, which is covered by a thin silver layer. The metasurface operates as an open plasmon resonator. The theory of plasmons excited in the open resonator formed by a metal nanograting is presented. The large local electromagnetic field is predicted for optical frequencies. The excitation of plasmons is experimentally demonstrated in the metasurface designed on a 4-in. $\mathrm{Si}$ wafer. The enhancement of the local electric field results in surface-enhanced Raman scattering (SERS). To investigate the SERS effect, the metasurface is covered with a thin layer of the 4-mercaptophenylboronic acid that molecules form covalent bonds with the silver nanolayer. The enhancement of the Raman scattering serves as a proof of concept. We obtain a detection limit of 230 nM for molecules of 4-mercaptophenylboronic acid.

Plasmonic characteristics of the structure with double graphene layers immersed in chiral media and applications in detection of chirality.

The propagating behavior of surface plasmons in chiral media is different from that in achiral media. By comparing the propagation behavior of SPPs, the chirality of the environment can be evaluated. We theoretically reveal the features of SPPs in a structure with two individual graphene sheets surrounded by chiral environment. A more universal dispersion relation covering the achiral cases is obtained. The effects of the chirality of medium and the chemical potential of graphene on the SPPs are explored. Besides, a novel method of detecting the chirality of the environment based on the lateral optical force and torque is proposed. The characteristics of the structure may enrich the surface plasmonic wave theory on the waveguides with multiple graphene sheets and provide new opportunities for the design of more compact nanophotonic functional devices and novel biosensors.

Quasistationary near-gate plasmons in van der Waals heterostructures

Near-gate plasmons are a new type of plasma oscillations emerging in homogeneous two-dimensional electron systems where a gate provides partial screening of electron-electron interaction. Here we develop a theory of the near-gate plasmons in van der Waals heterostructures comprising a conducting layer separated by a thin insulator from an uncharged disk-shaped gate. We show that in these structures the near-gate plasmons form gate-size-quantized quasi-stationary discrete modes even in the collisionless limit. Belonging to continuum spectrum of two-dimensional plasmons outside of the disk-gate, the near-gate plasmons are manifested as Fano-like resonances in frequency and magnetodispersions of scattering cross-section of the former scattered off the region under the gate. This enables to recover spectrum of the near-gate plasmons in the van der Waals heterostructures using near-field imaging techniques.

Plasmonic enhancement of metamaterial nanograting with high color purity and wide color gamut characteristics

We propose two designs of plasmonic enhancement using metamaterial nanograting (MNG) for color filters. MNGs are composed of metal–insulator–metal and metal–air–metal layers denoted as MNG-1 and MNG-2, respectively. MNGs exhibit polarization-dependent, high color purity, and wide color gamut characteristics. According to the surface plasmon (SP) theory, MNG-1 shows all colors filtering in the whole visible spectrum by changing the thickness of metal and insulator layers. While MNG-2 exhibits tunable color filtering characteristic by dynamically changing the distance between two metal layers. MNG-2 also exhibits tunable dual-resonance when exposed on the ambient with different refraction index. These high color purity and wide color gamut characteristics open the avenue of ultrahigh resolution imaging, visual anti-counterfeiting, and environmental sensing applications.

Theory of spoof magnetic localized surface plasmons beyond effective medium approximations

A homogeneous negative permeability sphere can support magnetic localized surface plasmons (MLSPs). Generally, negative permeability materials are metamaterial (MM) structures exhibiting very deep subwavelength spatial scales, whose effects may be detrimental in the near-field for those applications based on effective medium approximations. We suggest to overcome this fundamental limitation by demonstrating analytically that the electromagnetic spatial distribution, associated to a MLSP resonance and excited by a near-field source, can be accurately reproduced outside the sphere by substituting the negative permeability sphere with a homogeneous high-index dielectric one having the same radius. Considering that a large class of ferroelectric materials shows ultra-high dielectric constant and low-losses at low frequency (up to GHz), our spoof MLSPs theory could be a key tool for realizing high performance subwavelength magnetic photonic devices in the radiofrequency and microwave regions.

Quantization of electromagnetic modes and angular momentum on plasmonic nanowires**Project supported by the National Natural Science Foundation of China (Grant Nos. 11704058 and 11974069), the National Special Support Program for High-level Personnel Recruitment, China (Grant No. W03020231), Liaoning Revitalization Talents Program, China (Grant No. XLYC1902113), Program for Liaoning Innovation Team in University, China (Grant No. LT2016011), Science and Technique Foundation

Quantum theory of surface plasmons is very important for studying the interactions between light and different metal nanostructures in nanoplasmonics. In this work, using the canonical quantization method, the SPPs on nanowires and their orbital and spin angular momentums are investigated. The results show that the SPPs on nanowire carry both orbital and spin momentums during propagation. Later, the result is applied to the plasmonic nanowire waveguide to show the agreement of the theory. The study is helpful for the nano wire based plasmonic interactions and the quantum information based optical circuit in the future.

Microscopic Theory of Plasmons in Substrate-Supported Borophene.

We compute the dielectric properties of freestanding and metal-supported borophene from first-principles time-dependent density functional theory. We find that both the low- and high-energy plasmons of borophene are fully quenched by the presence of a metallic substrate at borophene-metal distances smaller than ≃9 Å. Based on these findings, we derive an electrodynamic model of the interacting, momentum-dependent polarizability for a two-dimensional metal on a model metallic substrate, which quantitatively captures the evolution of the dielectric properties of borophene as a function of metal-borophene distance. Applying this model to a series of metallic substrates, we show that maximizing the plasmon energy detuning between borophene and substrate is the key material descriptor for plasmonic performance.

On the Theory of Plasmon–Excitons: An Estimate of the Coupling Constant and the Optical Spectrum

The structure of the optical spectra related to the resonant interaction of quasi-two-dimensional excitons and localized plasmons is investigated theoretically. The constant of plasmon–exciton coupling is estimated in a model considering a semiconductor quantum well close to a layer of metal nanoparticles in an adjacent dielectric medium. Numerical calculations carried out for GaAs/Ag and ZnO/Al nanosystems indicate that near the plasmon–exciton resonance the spectrum features a double-peak structure which exhibits the plasmon-excitonic anticrossing behavior upon detuning from exact resonance.

Theory of localized plasmons for multiple metal nanostructures in dielectrics

A theory of localized bulk and surface plasmons for multiple metal nanostructures embedded in dielectrics is derived in the random phase approximation under the high-fRequency condition. The local electron density in multiple metal nanostructures and the local susceptibility around the nanostructures are given by the sum of those for each metal nanostructure, which gives self-consistent integral equations for the scalar potentials. Solving them in polar coordinates, the localized surface plasmon frequencies are derived for dimer and chain of metal nanospheres in dielectrics with step-function electron density at their interfaces. The light emission intensities from the dimer and chain of metal nanospheres in dielectrics are also derived using the step-function electron density model in the electric dipole approximation. Finally the retardation effect on the localized surface plasmons for multiple metal nanostructures in dielectrics is studied by applying the structural Green’s function method.

Plasmons and Plasmon–Polaritons in Finite Ionic Systems: Toward Soft-Plasmonics of Confined Electrolyte Structures

We address the field of soft plasmonics in finite electrolyte liquid systems ranged by insulating membranes by an analogy to the plasmonics of metallic nanostructures. The confined electrolyte systems can be encountered on a bio-cell organizational level, taking into account that the characteristics of ion plasmons fall to the micrometer size scale instead of the nanometer in metals because of at least three orders of magnitude larger masses of ions in comparison to electrons. The lower density of ions in electrolytes in comparison to density of electrons in metal may also reduce the energy of plasmons by several orders. We provide the fully analytical description of surface and volume plasmons in finite ionic micro-systems allowing for further applications. We next apply the theory of ionic plasmons to plasmon–polaritons in ionic periodic systems. The complete theory of ionic plasmon–polariton kinetics in the chain of micrometer-sized electrolyte spheres, confined by a dielectric membrane, is formulated and solved. The latter theory has next been applied to the explanation of a mysterious and unclear (for several dozen of years) problem of so-called saltatory conduction of the action potential in myelinated axons of nerve cells. Contrary to conventional models of nerve signaling, the plasmon–polariton model pretty well fits to the queer properties of the saltatory conduction. Moreover, the presented application of soft plasmonics to signaling in periodically myelinated axons may allow for identification of a different role in information processing of the white and gray matters in brain and spinal cord. We have outlined some perspectives to utilize the difference between the electricity of myelinated and non-myelinated nerve cells in brain to develop the topological concept of the memory functioning. The proposed ionic plasmon–polariton model of the saltatory conduction differently recognizes the role of the insulating myelin than previously was thought which may be helpful in the development of a better understanding of the demyelination diseases.

Research progress of coupling theory of metal surface plasmon

Metal surface plasmon is a collective oscillation effect of free electrons at the micro-nanostructure surface under the stimulation of incident light. Since the corresponding oscillating electric field is strongly bound below the sub-wavelength scale, it can be used as an information carrier for future micro-nano photonic circuit and device, and can also be used to enhance the interaction between light and matter on a micro-nano scale, such as surface enhanced photoluminescence, Raman scattering, nonlinear signal generation, surface enhanced catalysis, photothermal conversion, photovoltaic conversion, etc. How to theoretically understand the unique optical behavior dominated by the plasmon oscillation mode is one of the hot research spots in the field of surface plasmon photonics. In recent years, the theory of surface plasmon has been continuously improved with the support of a large number of experimental researches. In this paper, we first systematically summarize the optical behaviors and properties of metal under the excitation of incident electromagnetic waves, and then briefly describe the plasmonic modes existing in the metal and their corresponding physical natures, the oscillation dynamics process and the currently prevailing surface plasmon coupling theories. We hope that this paper can provide a theoretical basis for those researchers who have just dabbled in the field of surface plasmons and help them to master the relevant basic knowledge quickly.

Quantized plasmon excitations of electron gas in potential well

Using the Schrödinger-Poisson system in this paper, the basic quantum features of plasmon excitations in a free noninteracting electron gas with arbitrary degeneracy are investigated. The standing wave solution of the free electron gas is derived from the corresponding linearized pseudo-force system with appropriate boundary conditions. It is shown that the plasmon excitation energies for electron gas confined in an infinite potential well are quantized eigenvalues of which are obtained. It is found that any arbitrary degenerate quantum electron gas possesses two different characteristic length scales, unlike the classical dilute electron gas, with the smaller length scale corresponding to the single particle oscillation and the larger one due to the collective Langmuir excitations. The probability density of the free electron gas in a box contains fine structures which are modulated over a larger pattern. The envelope probability density profile for the electron Fermi gas confined in an impenetrable well in different energy states is found to be quite similar to that of the free electron confined to an infinite potential well. However, the illustrative features of the plasmon theory presented in this research can be further elaborated in order to illuminate a wide range of interesting physical phenomena involving both the single particle and the collective features.

Numerical analysis of high-Q multiple Fano resonances

We design and numerically analyze high-quality (Q) multiple Fano resonances based on a hybrid metamaterial waveguide structure, which consists of T-shaped gold cut wires placed on a dielectric board waveguide. There are three sharp Fano peaks arising from the interference between plasmon dipole mode and different guided modes. The physical origin of obvious Fano behaviors is explained by the three-level plasmonic system and slab waveguide theory. The largest Q-factor reaches 547, and modulation depth of the peak C can get to nearly 100%, making it possible to perfectly realize the Fano switch function. Combining the cramped spectral lines with large near-field confinement, we demonstrate an optical refractive index sensor with a sensitivity of 4920 nm/RIU and a figure of merit of 188. This work provides a way to obtain multiple high Q-factor Fano resonances, which can widen channels for fabricating devices in biochemical sensing and optical switching.

Two-fluid, hydrodynamic model for spherical electrolyte systems

Spatial interaction effects between charge carriers in ionic systems play a sizable role beyond a classical Maxwellian description. We develop a nonlocal, two-fluid, hydrodynamic theory of charges and study ionic plasmon effects, i.e. collective charge oscillations in electrolytes. Ionic spatial dispersion arises from both positive and negative charge dynamics with an impact in the (far-)infrared. Despite highly classical parameters, nonlocal quenching of up to 90% is observed for particle sizes spanning orders of magnitude. Notably, the ionic system is widely tunable via ion concentration, mass and charge, in contrast to solid metal nanoparticles. A nonlocal soft plasmonic theory for ions is relevant for biological and chemical systems bridging hard and soft matter theory and allowing the investigation of non-classical effects in electrolytes in full analogy to solid metal particles. The presented semi-classical approach allows studying plasmonic photo-catalysis introducing nonlocal aspects into electrolyte-metal interactions.

Quantum nonlocal theory of topological Fermi arc plasmons in Weyl semimetals

The surface of a Weyl semimetal (WSM) displays Fermi arcs, i.e. disjoint segments of a two-dimensional Fermi contour. We present a quantum-mechanical non-local theory of chiral Fermi arc plasmons in WSMs with broken time-reversal symmetry. These are collective excitations constructed from topological Fermi arc and bulk electron states and arising from electron-electron interactions, which are treated in the realm of the random phase approximation. Our theory includes quantum effects associated with the penetration of the Fermi arc surface states into the bulk and dissipation, which is intrinsically non-local in nature and arises from decay processes mainly involving bulk electron-hole pair excitations.