Quantum Theory of Surface Plasmon Polaritons: Planar and Spherical Geometries

A quantum theory of retarded surface plasmons on a metal–vacuum interface is formulated, by analogy with the well-known and widely exploited theory of exciton-polaritons. The Hamiltonian for mutually interacting instantaneous surface plasmons and transverse electromagnetic modes is diagonalized with recourse to a Hopfield–Bogoljubov transformation, in order to obtain a new family of modes, to be identified with retarded plasmons. The interaction with nearby dipolar emitters is treated with a full quantum formalism based on a general definition of modal effective volumes. The illustrative cases of a planar surface and of a spherical nanoparticle are considered in detail. In the ideal situation of absence of dissipation, as an effect of the conservation of in-plane wavevector, retarded plasmons on a planar surface represent true stationary states (which are usually called surface plasmon polaritons), whereas retarded plasmons in a spherical nanoparticle, characterized by frequencies that overlap with the transverse electromagnetic continuum, become resonances with a finite radiative broadening. The theory presented constitutes a suitable full quantum framework for the study of nonperturbative and nonlinear effects in plasmonic nanosystems.     

Ring Gap Resonance Modes on Disk/Film Coupling System Caused by Strong Plasmon Interaction

Ring modes with large wave vectors cannot be easily excited on a single disk by the plane wave illumination with the polarization parallel to the disk interface. In this work, we show that special antisymmetric ring gap modes on the surface of the disk in close proximity to the metallic thin film can be excited in the visible light region of the electromagnetic spectrum. In the presence of the film, the strong plasmon interaction between disk and film causes ring gap modes to have lower energies and be more easily excited. We apply the plasmon hybridization method to illustrate the ring gap modes arising from the interaction between the localized disk plasmons and the continuum surface plasmons. The calculated hybridization data show good agreement with the results of finite element simulations. The excitation of ring gap modes provides further insight into the strong coupling of plasmons and the design of novel nanostructures.

     

Sensitive label-free biosensors by using gap plasmons in gold nanoslits

The detection sensitivities of gap plasmons in gold nanoslit arrays are studied and compared with surface plasmons on outside surface. The nanoslit arrays were fabricated in a 130 nm-thick gold film with various slit widths. For transverse-magnetic (TM) incident wave, the 600 nm-period nanoslit array shows two distinguishable transmission peaks corresponding to the resonances of gap plasmons and surface plasmons, respectively. The surface sensitivities for both modes were compared by coating thin SiO(2) film and different biomolecules on the nanoslit arrays. Our experimental results verify gap plasmons are more sensitive than conventional surface plasmons. Its detection sensitivity increases with the decrease of slit width. The gap plasmon is one order of magnitude sensitive than the surface plasmon for slit widths smaller than 30 nm. We attribute this high sensitivity to the large overlap between biomolecules and nanometer-sized gap plasmons.     

Effect of Proximity in Arrays of Plasmonic Nanoantennas on Hot Spots Density: Degenerate Semiconductors vs. Conventional Metals

Using rigorous finite element electromagnetic calculations, we study the performance of a localized surface plasmons resonance array infrared sensor, from an uncommon perspective, by analyzing electromagnetic hot spots density. We show that the hot spots density strongly depends on near-field interaction between plasmonic nanoantennas. We provide important theoretical evidence that the degree of the interaction depends on both material and geometrical properties of an array. To this end, we consider arrays of conventional metal (CM) and plasmonic transparent conducting oxide (TCO) at different nanoantennas density. We establish that a single CM nanoantenna has higher field intensity enhancement than a TCO counterpart. On the other hand, an array of TCO nanoantennas provides 30 times larger density of the hotspots and 15 times higher signal-to-noise ratio, as compared to a CM array.     

Plasmon Modes Management

A new plasmonic structure based on bimetallic layer is proposed. We analyze the structure and show that bimetallic film plays a crucial role in the management of surface plasmons. The roll of the buffer is discussed, as well. Up to three surface plasmons can be excited simultaneously in the structure. Two of plasmons can be used for two-plasmon spectroscopy. The third plasmon can be used for controlling the temperature of the structure.     

Surface Plasmon Resonant THz Wave Transmission on Carbon Nanotube Film

The properties of the terahertz resonant surface plasmons wave on the carbon nanotube film and dielectric interface have been investigated. As a first step towards engineering terahertz SPPs-like surface modes, we present a computer experiment to demonstrate that the carbon nanotube film surface can also be employed to concentrate and guide the terahertz SPPs wave. The carbon nanotube film is modeled in an experimentally realizable geometry. It is shown that a unique electromagnetic surface mode in terahertz region can be supported along the carbon nanotube film/dielectric interface when the free-space broadband terahertz pulse is incident on the carbon nanotube film with subwavelength gratings. Comparing with noble metals, plasmonic nano-structure materials based on carbon nanotube film offer a potentially more versatile approach to engineering tightly confined surface modes in the THz regime.     

Enhancing the Dual-Band Guiding Capabilities of Coaxial Spoof Plasmons via use of Transmission Line Concepts

We derive closed analytical forms for the response of coaxial spoof plasmons, aided by transmission line concepts under the effective complex surface impedance framework. This constitutes a powerful platform to improve as well as to elucidate designs with enhanced performances. In particular, we propose a dual-band spoof plasmon waveguiding geometry with the higher order slow-wave mode operating well below the regime governed by dispersion of periodic guides (Bragg reflections at Brillouin zone boundaries), that is, diffraction. The analysis is supported by eigen mode numerical calculations. As an example in a waveguide device context, we demonstrate the dual-band planar routing ability of elliptical–coaxial cable-based spoof plasmons along a straight chain as well as a Y-splitter.     

Plasmon-Enhanced Fluorescence Biosensors: a Review

Surfaces of metallic films and metallic nanoparticles can strongly confine electromagnetic field through its coupling to propagating or localized surface plasmons. This interaction is associated with large enhancement of the field intensity and local optical density of states which provides means to increase excitation rate, raise quantum yield, and control far field angular distribution of fluorescence light emitted by organic dyes and quantum dots. Such emitters are commonly used as labels in assays for detection of chemical and biological species. Their interaction with surface plasmons allows amplifying fluorescence signal (brightness) that accompanies molecular binding events by several orders of magnitude. In conjunction with interfacial architectures for the specific capture of target analyte on a metallic surface, plasmon-enhanced fluorescence (PEF) that is also referred to as metal-enhanced fluorescence (MEF) represents an attractive method for shortening detection times and increasing sensitivity of various fluorescence-based analytical technologies. This review provides an introduction to fundamentals of PEF, illustrates current developments in design of metallic nanostructures for efficient fluorescence signal amplification that utilizes propagating and localized surface plasmons, and summarizes current implementations to biosensors for detection of trace amounts of biomarkers, toxins, and pathogens that are relevant to medical diagnostics and food control.

     

Experimental Study on Localized Surface Plasmon Mode Hybridization in the Near and Mid Infrared

We investigate plasmon excitations within a regular grating of double-layered gold/insulator nanoparticles in the infrared and visible spectral region. Provided a flat gold film as substrate, strong coupling between the localized surface plasmon modes and their image-like excitations in the metal is observed. The interaction results in a strong red shift of the plasmon mode as well as the splitting of the modes into levels of different angular momenta, often referred to as plasmon hybridization. The diameters of the nanoparticles are designed in a way that the splitting of the resonances occurs in the spectral region between 0.1 and 1 eV, thus being accessible using an infrared microscope. Moreover, we investigated the infrared absorption signal of gratings that contain two differently sized nanoparticles. The interaction between two autonomous localized surface plasmon excitations is investigated by analyzing their crossing behavior. In contrast to the interaction between localized surface plasmons and propagating plasmon excitations which results in pronounced anticrossing, the presented structures show no interaction between two autonomous localized surface plasmons. Finally, plasmon excitations of the nanostructured surfaces in the visible spectral region are demonstrated through photographs acquired at three different illumination angles. The change in color of the gratings demonstrates the complex interaction between propagating and localized surface plasmon modes.     

Fluorescence Enhancement Caused by Plasmonics Coupling Between Silver Nano-Cubes and Silver Film

In this article, we experimentally investigated the plasmonics interaction in the system composed of Ag nano-cubes on Ag film with controlled distance. The distance is controlled by Rhodamine B (RhB)-doped polymethylmethacrylate (PMMA) film as the spacer, whose fluorescence intensity was then enhanced by the plasmonics interaction. Experimental results show that the fluorescence enhancement is sensitive to the thickness of the spacer. The largest enhancement factor obtained is 521 with the RhB-doped PMMA film of 10 nm thickness. For comparison, we also presented the fluorescence enhancement caused by only the localized surface plasmons from Ag nano-cubes on glass substrate coated with RhB-doped PMMA film, which gives out lower enhancement factors at the same thick spacer. Our experimental results are consistent with previous theoretical investigation and shows promising applications in fluorescence based bio-sensing or bio-imaging.     

Polarization of electromagnetic radiation at the second harmonic of the electron gyrofrequency in a turbulent plasma

A study is made of the polarization of electromagnetic radiation at the second harmonic of the electron gyrofrequency. The radiation is emitted by a highly ionized collisionless plasma in which the turbulence is excited at electron gyrofrequencies in a strong magnetic field. The mechanism for the generation of electromagnetic waves during mergings of the gyrofrequency plasmons is analyzed. It is shown that, even in a strong magnetic field, the degree of circular polarization of electromagnetic radiation at the second harmonic of the electron gyrofrequency may be moderate or weak.     

Two-Dimensional Analogies to Frequency-Selective Surfaces (FSS) on the Graphene Sheet

In this paper, we propose that two-dimensional analogies to frequency-selective surfaces (FSS) can be achieved on graphene surfaces based on transformation optics. The analogies to representative FSS structures, including the anti-reflecting coating (ARC) and the high-reflecting coating (Bragg reflector), have been investigated through both analytical effective-index method (EIM)/transfer-matrix method (TMM) and numerical simulations. Both analytical and numerical solutions have shown that the propagation of plasmons on graphene surface with periodic chemical potentials can be an analogy to the interaction of incident light with traditional FSS multilayer dielectric media in which the transmission or reflection can be obtained by EIM/TMM. Combined with the tunability of graphene, the transmission or reflection of plasmons can be tuned by adjusting the bias voltage. The proposed structures and theoretical methods may provide new visions for achieving two-dimensional analogies to traditional structures on graphene.

     

Fluorescence Resonance Energy Transfer Near Thin Films on Surfaces

A theory is developed for resonant energy transfer between donor and acceptor molecules outside of a solid coated with a thin film. The energy transfer rate is expressed in terms of a second-rank tensor, allowing one to consider all possible orientations of the transition dipole moments of the molecules. The theory of images is employed to construct expressions valid in the near-field approximation. This theory is extended to the full electrodynamic theory valid over all distances. Connections are made between the expressions for the image charges and the Fresnel coefficients from optics. It is found that the energy transfer rates are strongly influenced by surface resonances, including the interfacial surface plasmons and the two-dimensional plasmon of a metallic film. The possibility of the film supporting Fabry–Perot resonator modes is discussed.     

Spectra of Electromagnetic Radiation from a Hot Plasma with Langmuir Turbulence in a Magnetic Field

A study is made of the generation of electromagnetic waves during the merging of two Langmuir plasmons in a hot plasma with a magnetic field. It is shown that the frequency of Langmuir plasmons can vary in the range from 0.8 to 1.1 of the electron Langmuir frequency. The spectrum and polarization of the emitted electromagnetic radiation are analyzed. It is found that the thermal motion of plasma particles may lead to the generation of electromagnetic waves in the frequency range from 1.6 to 2.2 of the electron Langmuir frequency. In a plasma with an isotropic Langmuir turbulence spectrum, the degree of circular polarization of the emitted radiation can amount to 50%.     

Retarded Charge–Carrier Recombination in Photoelectrochemical Cells from Plasmon‐Induced Resonance Energy Transfer

N‐type metal oxides such as hematite (α‐Fe₂O₃) and bismuth vanadate (BiVO₄) are promising candidate materials for efficient photoelectrochemical water splitting; however, their short minority carrier diffusion length and restricted carrier lifetime result in undesired rapid charge recombination. Herein, a 2D arranged globular Au nanosphere (NS) monolayer array with a highly ordered hexagonal hole pattern (hereafter, Au array) is introduced onto the surface of photoanodes comprised of metal oxide films via a facile drying and transfer‐printing process. Through plasmon‐induced resonance energy transfer, the Au array provides a strong electromagnetic field in the near‐surface area of the metal oxide film. The near‐field coupling interaction and amplification of the electromagnetic field suppress the charge recombination with long‐lived photogenerated holes and simultaneously enhance the light harvesting and charge transfer efficiencies. Consequently, an over 3.3‐fold higher photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) is achieved for the Au array/α‐Fe₂O₃. Furthermore, the high versatility of this transfer printing of Au arrays is demonstrated by introducing it on the molybdenum‐doped BiVO₄ film, resulting in 1.5‐fold higher photocurrent density at 1.23 V versus RHE. The tailored metal film design can provide a potential strategy for the versatile application in various light‐mediated energy conversion and optoelectronic devices.     

Subradiant,Superradiant Plasmon Modes and Fano Resonance in a Multilayer Nanocylinder Array Standing on a Thin Metal Film

The optical properties of a novel nanostructure consisting of a hexagonal array of aligned vertically three-layered metal-dielectric-metal nanodisks on a silver film are theoretically studied through the finite-difference time-domain method. The novel nanostructure exhibits three obvious optical transmission bands due to the excitation of subradiant plasmon modes, superradiant plasmon modes, and Fano resonances. Surface plasmon polaritons of the underlying Ag film also play a significant role on these three optical transmission bands via coupling with localized surface plasmons of nanodisk pairs. Moreover, the nanostructure also exhibits a good tunability of optical response by modifying the sizes of cylinders, the thickness of underlying metal film, and the dielectric constant of middle layer. These results demonstrate the nanostructure with great advantages in optical sensors and filters.

     

Measurements of the cross-bridge attachment/detachment process within intact sarcomeres by surface plasmon resonance.

We have developed a surface plasmon resonance (SPR) system to monitor the cross-bridge attachment/detachment process within intact sarcomeres from mouse heart muscle. SPR occurs when laser light energy is transferred to surface plasmons that are resonantly excited in a metal (gold) film. This resonance manifests itself as a minimum in the reflection of the incident laser light and occurs at a characteristic angle. The angle of the SPR occurrence depends on the dielectric permittivity of the sample medium adjacent to the gold film. Purified sarcomeric preparations are immobilized onto the gold film in the presence of a relaxing solution. Replacement of the relaxing solution with increasing Ca(2+) concentration solution activates the cross-bridge interaction and produces an increase in the SPR angle. These results imply that the interaction of myosin heads with actin within an intact sarcomere changes the dielectric permittivity of the sarcomeric structure. In addition, we further verify that SPR measurements can detect the changes in the population of the attached cross-bridges with altered concentrations of phosphate, 2,3-butanedione monoxime, or adenosine triphosphate at a fixed calcium concentration, which have been shown to reduce the force and increase the cross-bridge population in attached state. Thus, our data provide the first evidence that the SPR technique allows the monitoring of the cross-bridge attachment/detachment process within intact sarcomeres.     

Nanoantenna-Assisted Extraordinary Optical Transmission Under Radial Polarization Illumination

We numerically study the extraordinary optical transmission of a plasmonic structure that combines a circular nanoantenna and a vertical annular nanoslit etched into a gold film under radially polarized illumination. The nanoantenna collects the incident field and localizes it in a horizontal Fabry-Pérot cavity over the gold film. The vertical nanoslit positioned at the maximal field in the horizontal cavity couples the localized field and facilitates its transmission to the free space. Due to the symmetry matching between the structure and the illumination polarization, surface plasmons can be excited effectively and enhance the transmission. Through optimizing the structure parameters, the transmission efficiency can be greatly enhanced by 225 times for a resonant annular nanoslit and 251 times for a non-resonant annular nanoslit. This axisymmetric extraordinary optical transmission setup may be fabricated on the facet of an optical fiber for optical sensing applications.     

High Optical Transmission in a Hybrid Plasmonic-Optical Structure with a Continuous Metal Film

Metals are naturally opaque for electromagnetic (EM) waves below violet frequency due to the Coulomb screening effect. In this letter, we demonstrate high optical transparency of a seamless continuous metal film by sandwiching it in a hybrid plasmonic-optical structure. The proposed structure consists of a plasmonic array and an optical cavity, which exhibits magnetic plasmon (MP) resonance and optical Fabry-Perot (FP) resonance, respectively. An optical transparency of 84% in the near-IR regime is achieved making use of interaction between the plasmonic and optical modes. Furthermore, spectral tunability of the high transparency is demonstrated and robustness under oblique incidence is examined. This work may give insights into plasmonic-optical interactions and may be a potential candidate for transparent electrodes.     

Efficient Extraction of Fluorescence Emission Utilizing Multiple Surface Plasmon Modes from Gold Wire Gratings

We demonstrate directional enhanced fluorescence emission from fluorophores located above gold wire gratings. In contrast to previous studies on corrugated films, efficient coupling was recorded for multiple plasmon modes associated with both the active and substrate side of the wires. This difference is likely due to the subtle differences in how light interacts with corrugated films versus metal films with periodic subwavelength slots. For corrugated films, coupling between modes on opposite sides of the grating are out of phase, and therefore plasmon modes on the opposite side of the grating are only weakly excited. For wire gratings, transmission and reflection features have been modeled well with a dynamical diffraction model that includes surface plasmons, which allows for efficient coupling to surface plasmon modes on both sides of the grating. We also compared the two mechanisms for fluorescent enhancement, namely the intense electromagnetic field associated with surface plasmons and excited fluorophores radiating via surface plasmon modes. We found the latter mechanism clearly dominant.