Plasmonic Media Research Articles

Bulk-Plasmon-Mediated Free-Electron Radiation Beyond the Conventional Formation Time.

Free-electron radiation is a fundamental photon emission process that is induced by fast-moving electrons interacting with optical media. Historically, it has been understood that, just like any other photon emission process, free-electron radiation must be constrained within a finite time interval known as the "formation time," whose concept is applicable to both Cherenkov radiation and transition radiation, the two basic mechanisms describing radiation from a bulk medium and from an interface, respectively. Here, this work reveals an alternative mechanism of free-electron radiation far beyond the previously defined formation time. It occurs when a fast electron crosses the interface between vacuum and a plasmonic medium supporting bulk plasmons. While emitted continuously from the crossing point on the interface-thus consistent with the features of transition radiation-the extra radiation beyond the conventional formation time is supported by a long tail of bulk plasmons following the electron's trajectory deep into the plasmonic medium. Such a plasmonic tail mixes surface and bulk effects, and provides a sustained channel for electron-interface interaction. These results also settle the historical debate in Ferrell radiation, regarding whether it is a surface or bulk effect, from transition radiation or plasmonic oscillation.

Sommerfeld Pole or Brewster Zero?: Zenneck wave or surface plasmon?

The surface waves that may exist at a planar interface between a lossless dielectric and a conventional or plasmonic medium are investigated. It is shown that these surface waves are pole waves associated with the pole of the Fresnel reflection coefficient looking into the structure. Furthermore, it is shown that Zenneck waves and surface plasmon-polaritons (SPPs) are two manifestations of a pole wave corresponding to different media, and as such are closely related. It is also shown that both surface waves may be set up by illuminating the structure with an inhomogeneous plane wave incident at the complex Brewster angle. The case of a graphene sheet at the interface is also considered and it is shown that this structure supports a highly attenuated SPP. The theoretical development is illustrated and supported by representative numerical results.

Graphene‐assisted synthesis NH3 gas sensor based on silicon photonics crystal fiber and surface plasmon resonance

ObjectiveIn this article, we theoretically simulated a silicon core-based photonics crystal fiber combined with the Au/graphene coating thin-film to surround the fiber-air-holes with a plasmonic active medium for sensing NH3 gas concentrations. MethodsBy means of our previous research experiences and the related theory, the complex refractive index of graphene under different concentrations of NH3 exposing was analyzed and calculated by using the finite element method. ResultThe sensing average sensitivity of 0.0269 nm/ppm for the proposed NH3-gas sensor was theoretically predicted for the concentration of NH3 in the range from 1 ppm to 1000 ppm. In addition, the proposed photonics crystal fiber sensor can be extended into the other field applications, such as medical diagnostics, bio-chemical detection etc.

Self‐Assembly Silver Nanoparticles Decorated on Gold Nanoislands for Label‐Free Localized Surface Plasmon Resonance Biosensing

AbstractA nanostructure of self‐assembly silver nanoparticles decorated on gold nanoislands (SAM Ag@AuNIs) is proposed in this article as a sensitive sensing medium for localized surface plasmon resonance (LSPR) biosensing applications. A wide tunable plasmonic sensing range from 472 to 552 nm is found by adjusting the Au–Ag atomic ratio. Biotin is employed as an indicator to investigate the potential biosensing applicability and site‐specific bioactivity of the SAM Ag@AuNIs. In virtue of a white light common‐path sensing system, direct adsorption of biotin on the SAM Ag@AuNIs is measured in label‐free manner with detection limit (LOD) of 2.9 pg mL−1 (11.87 × 10−12 m). It is 105‐fold better than that of TiN film, which is previously reported to provide excellent LSPR LOD after functionalization with biotin. The experimental and computational results indicate that the biotin molecule can selectively immobilize onto the Ag nanoparticles (AgNPs) surface through S–Ag bonding, resulting in site‐specific functionalization. Meanwhile, the geometry of AgNPs/AuNIs provided a coupling effect that helps to improve the sensitivity. Here, AgNPs are, for the first time, reported to be directly functionalized with biotin. The SAM Ag@AuNIs can serve as a desirable plasmonic medium in the biosensing community based on the exceptionally good LOD of biotin.

Computational Electromagnetics: A Miscellany

The paper presents a miscellany of unorthodox and, in some cases, paradoxical or controversial items related to computational and applied electromagnetics. The topics include a definition of the magnetic source field via a line integral, losses in electric power transmission vs. losses in photonics, homogenization of periodic electromagnetic structures, spurious modes, models of plasmonic media, and more. It is hoped that this assortment of subjects will be of interest to a broad audience of scientists and engineers.

Outdoor Characterization of a Plasmonic Luminescent Solar Concentrator

Plasmonic luminescent solar concentrators (PLSCs) have been shown to enhance the optical performance and power conversion efficiency of LSCs, due to added plasmonic gain in the medium. Despite the promising outlook of a PLSC through plasmonic coupling, device characterization and performance have not been verified in the real outdoor conditions, with varying direct and diffuse solar irradiation. In this work, characterization of a PLSC device of dimensions 4.5 \(\times\) 4.5 \(\times\) 0.3 cm3 embedded with Lumogen Red 305 dye and plasmonic gain medium of gold core silver shell nano cuboids was carried out in outdoor conditions of Dublin, Ireland. Optimized PLSC device power output at different solar insolation was compared to a reference photovoltaic (PV) cell and an optimized luminescent solar concentrator (LSC) device. The effect of the solar disc position, solar insolation, PV cell surface temperature, diurnal, and seasonal variation on the performance of the PLSC device is studied. The key observation was that PLSC average power conversion efficiency was 1.4 times more than the PV cell in cloudy and diffuse solar conditions. The PLSC device performed 45% better than a PV cell in December than July, as December has higher diffuse solar irradiation. Even though the PLSC device absorbs only 31% and transmits 69% incident solar irradiation in the concerned visible range of 380–750 nm. The preliminary outdoor characterization on a small-scale PLSC establishes its viability in a diffuse to direct solar radiation ratio throughout the year as well as establishing its benefits for integration in buildings.

Asymptotic solution for the electromagnetic scattering of a vertical dipole over plasmonic and non‐plasmonic half‐spaces

A new asymptotic solution for the scattered electromagnetic fields of a vertical Hertzian dipole antenna in the presence of an imperfectly conducting half-plane for ordinary and plasmonic media is proposed. The scattered electric and magnetic field components are calculated from the intermediate Hertz potential expressed in terms of the Fourier-Bessel transforms associated with the Sommerfeld-type integral, which is difficult to evaluate due to the singularities of the integrand near the integration path and its oscillatory and slowly decaying integrand. Using the modified saddle point method, an approximate closed-form solution of the far-zone scattered electromagnetic fields including surface waves is presented. The new formulations are applied to calculate radiation patterns of different impedance half-planes for both ordinary and plasmonic media, that is, seawater, silty clay soil, silty loam soil and lake water as ordinary, and silver and gold as plasmonic media. Furthermore, a numerical evaluation of the proposed solution at various frequencies and comparisons with two alternative state of the art solutions shows that the proposed solution has higher accuracy in terms of the normalised root-mean-square error and the normalised maximum absolute error for plasmonic and non-plasmonic structures.

Multiple Dirac points by high-order photonic bands in plasmonic-dielectric superlattices.

The emergence of Dirac points (DPs) characterizes the topological phase transition and the gapless interface states in composite metal-dielectric metamaterials. In this work, we study a kind of compound plasmonic-dielectric periodic structure (PDPS) which sustains both plasmonic modes and multiple photonic modes. The structure has primitive cell consisting of four layers made from triple constituent components. Due to the generalized Su-Schrieffer-Heeger, DPs can emerge at the Brillouin zone center. More specifically, in weak plasmonic-photonic mode interaction regime, multiple DPs would emerge at the Brillouin zone center and edge due to the band folding, from the perspective of general effective medium. From the rigorous field analysis, the origin of these DPs is clearly demonstrated. These interleaved DPs behave as the intermediate transitions of the surface impedance for the PDPS and raise fully spanned topological interface states originated from 0 to 2nd-order photonic bands in the PDPS. The cases of combining our PDPS with either a plasmonic or dielectric homogenous medium are presented.

Discrete dipole approximation for lossy plasmonic background.

The discrete dipoles approximation method is exploited to study light scattering near a cylindrical cavity, made in an absorbing plasmonic surrounding medium. To the best of our knowledge, this is the first use of the method for the metal background. Our two-dimensional numerical simulation of the local fields demonstrates a good performance of the approach. Its high accuracy is proven by comparison with the known analytical solution.

Surface states ensured by a synthetic Weyl point in one-dimensional plasmonic dielectric crystals with broken inversion symmetry

Weyl points, i.e., peculiar nodal degenerate points in three-dimensional reciprocal space, carry topological charges, therefore giving rise to many intriguing phenomena including topologically protected surface states and chiral anomaly. More importantly, the concept of the Weyl point can be applied in systems with reduced geometrical dimensionality by means of synthetic dimension. Such generalized synthetic Weyl points therefore provide a versatile platform to explore the Weyl-point physics in one or two-dimensional structures. In this work, we study the synthetic Weyl points in one-dimensional plasmonic-dielectric crystals with broken inversion symmetry and further demonstrate that the synthetic Weyl points can ensure the emergence of topological interface states. Instead of using the Zak phase, the emergence of a Weyl point in synthetic space is helpful to understand the underlying physics and the topological properties of the interface states in one-dimensional crystals with broken inversion symmetry. Due to the opposite sign of the surface impedance for plasmonic and dielectric cover medium, the surface states demonstrate distinct properties and locate in separated parameter space regions. Our results show that the synthetic Weyl point provides a potential approach for studying novel electromagnetic modes and facilitates the formation of topologically protected interface states in systems with reduced geometrical dimensions.

Numerical simulation of electron energy loss spectroscopy accounting for nonlocal effect in plasmonic nanoparticles

Interaction between an electron in straight uniform motion and a nearby plasmonic nanoparticle deposited in homogeneous medium is examined numerically in norecoil approximation. To account for the nonlocal effect occurring in the plasmonic medium Generalized Nonlocal Optical Response scattering problem statement is considered. Discrete Source Method is employed to construct the solution. Electron energy loss probability is computed as a function of incident field frequency and obtained surface plasmon resonance energy is compared to local response simulations. It is shown that surface plasmon resonance peak frequency blueshift can be resolved within the proposed approach.

Bimetallic Au-Ag alloy nanoislands for highly sensitive localized surface plasmon resonance biosensing

High performance label-free biosensors must possess beneficial features, including outstanding sensitivity, high durability and practical functionalization approaches for specific detection. Au and Ag are common plasmonic metals, but have drawbacks in plasmonic sensitivity and chemical stability respectively. Thus, proposing a new systematic framework for high performance biosensing is imperative. In this work, monodispersed bimetallic nanoislands (BMNIs) with various Au-Ag atomic ratios, synthesized by solid-state dewetting method, show not only improvement of localized surface plasmon resonance (LSPR) sensitivity from that of either Au or Ag, but have also provided chemical stability against spontaneous corrosion. Au-Ag alloying and BMNIs formation were achieved by a one-step self-assembly method. A large tunable plasmonic sensing range from 421 nm to 578 nm was found for all possible Au-Ag atomic ratios. By employing the common-path interferometric sensing system, the Au0.6Ag0.4 BMNIs showed the best refractive index sensitivity. The as-synthesized Au-Ag BMNIs sensing chips were functionalized with various protocols for biosensing applications, i.e. thiolate functionalization with 11-Mercaptoundecanoic acid and dielectric functionalization with aminosilanes. The dielectric functionalized Au-Ag BMNIs LSPR biosensors can detect sub-picomolar concentration of human IgG with wide dynamic range from 0.89 pM to 1000 pM, whereas thiolate functionalization actually degrades the sensing performance due to Ag degradation. Moreover, the dielectric functionalization layer also provided anti-degradation coverage to protect Au-Ag BMNIs from etchant agents, e.g. H2O2, which indicated the potential in wide biosensing applications. In virtue of the dielectric functionalization method, Au-Ag BMNIs can be utilized as a desirable alternative plasmonic medium in the biosensing community.

One-Shot Measurement of the Three-Dimensional Electromagnetic Field Scattered by a Subwavelength Aperture Tip Coupled to the Environment

Near-field scanning optical microscopy (NSOM) achieves subwavelength resolution by bringing a nanosized probe close to the surface of the sample. This extends the spectrum of spatial frequencies that can be detected with respect to a diffraction limited microscope. The interaction of the probe with the sample is expected to affect its radiation to the far field in a way that is often hard to predict. Here we address this question by proposing a general method based on full-field off-axis digital holography microscopy which enables to study in detail the far-field radiation from a NSOM probe as a function of its environment. A first application is demonstrated by performing a three-dimensional (3D) tomographic reconstruction of light scattered from the subwavelength aperture tip of a NSOM, in free space or coupled to transparent and plasmonic media. A single holographic image recorded in one shot in the far field contains information on both the amplitude and the phase of the scattered light. This is suffi...

Transmission/reflection behaviors of surface plasmons at an interface between two plasmonic systems

Although surface plasmon polaritons (SPPs) have been intensively studied in past years, the transmission/reflection properties of SPPs at an interface between two plasmonic media are still not fully understood. In this article, we employ a mode expansion method (MEM) to systematically study such a problem based on a model system jointing two superlattices, each consisting of a periodic stacking of dielectric and plasmonic slabs with different material properties. Such a generic model can represent two widely used plasmonic structures (i.e. interfaces between two single dielectric/metal systems or between two metal–insulator–metal waveguides) under certain conditions. Our MEM calculations, in excellent agreement with full-wave simulations, uncover the rich physics behind the SPP reflections at generic plasmonic interfaces. In particular, we successfully derive from the MEM several analytical formulas that can quantitatively describe the SPP reflections at different plasmonic interfaces, and show that our formulas exhibit wider applicable regions than previously proposed empirical ones.

On the Far‐Zone Electromagnetic Field of a Horizontal Electric Dipole Over an Imperfectly Conducting Half‐Space With Extensions to Plasmonics

AbstractSecond‐order asymptotic formulas for the electromagnetic fields of a horizontal electric dipole over an imperfectly conducting half‐space are derived using the modified saddle‐point method. Application examples are presented for ordinary and plasmonic media, and the accuracy of the new formulation is assessed by comparisons with two alternative state‐of‐the‐art theories and with the rigorous results of numerical integration.

On the Sommerfeld half-space problem: appraisal of approximate solutions with extensions to plasmonics

The classical Sommerfeld problem of a vertical electric dipole radiating over an imperfectly conducting half-space is revisited. Two variants of the modified saddle-point integration method are employed to derive new, closed-form, second-order approximations of the far-zone electromagnetic fields. The new formulas are applied to compute radiation patterns and surface fields for sample problems including both ordinary and plasmonic media, and their accuracy is assessed by comparison with the results of rigorous numerical evaluation of the Sommerfeld integrals.

A customizable class of colloidal-quantum-dot spasers and plasmonic amplifiers.

Colloidal quantum dots are robust, efficient, and tunable emitters now used in lighting, displays, and lasers. Consequently, when the spaser-a laser-like source of high-intensity, narrow-band surface plasmons-was first proposed, quantum dots were specified as the ideal plasmonic gain medium for overcoming the significant intrinsic losses of plasmons. Many subsequent spasers, however, have required a single material to simultaneously provide gain and define the plasmonic cavity, a design unable to accommodate quantum dots and other colloidal nanomaterials. In addition, these and other designs have been ill suited for integration with other elements in a larger plasmonic circuit, limiting their use. We develop a more open architecture that decouples the gain medium from the cavity, leading to a versatile class of quantum dot-based spasers that allow controlled generation, extraction, and manipulation of plasmons. We first create aberration-corrected plasmonic cavities with high quality factors at desired locations on an ultrasmooth silver substrate. We then incorporate quantum dots into these cavities via electrohydrodynamic printing or drop-casting. Photoexcitation under ambient conditions generates monochromatic plasmons (0.65-nm linewidth at 630 nm, Q ~ 1000) above threshold. This signal is extracted, directed through an integrated amplifier, and focused at a nearby nanoscale tip, generating intense electromagnetic fields. More generally, our device platform can be straightforwardly deployed at different wavelengths, size scales, and geometries on large-area plasmonic chips for fundamental studies and applications.

Optically controlled coherent X-ray radiations from photo-excited nanotubes

Relativistic electrons propagating through a plasmonic medium such as photo-excited plasma channels with negative permittivities undergo betatron motions, emitting photons at oscillating resonance modes. The similar betatron radiation can be generated in X-ray regimes from electrons transported through optically pumped carbon nanotubes (CNTs). The X-ray radiation condition of 0.5 and 6MeV electrons phase-matched with plasmonic waves in CNTs is analyzed with a theoretical model of the CNT dispersion relation. Based on the dispersion analysis, radiation intensities and the brilliance of the coherent X-ray source averaged over the pulse duration are estimated using a typical range of system parameters of conventional electron sources and tabletop femtosecond lasers. The assessment indicates that the average brilliance of the harmonic radiation can reach 1010–1013photons/s/mm2/mrad2/0.1%BW with 0.5–6MeV electrons and X-ray energy up to a few keV.

On the computation of plane-wave impressed fields and far-zone fields of arbitrary current sources in planar multilayer dielectric and plasmonic media

A unified and succinct transmission line formulation is presented for the computation of the plane-wave impressed fields and far-zone radiation fields due to arbitrary electric and magnetic current source distributions embedded in planar layered media. The equivalence principle and the spectral-domain transmission-line network analogue of the multilayer medium are used to express the fields in terms of the voltage and current transmission-line Green functions. The new formulation is applied to sample problems involving both dielectric and plasmonic multilayer media.

Near-infrared hybrid plasmonic multiple quantum well nanowire lasers.

The lasing characteristics of hybrid plasmonic AlGaAs/GaAs multiple quantum well (MQW) nanowire (NW) lasers beyond diffraction limit have been investigated by 3D finite-difference time-domain simulations. The results show that the hybrid plasmonic MQW NW has lower threshold gain over a broad diameter range in comparison with its photonic counterpart. Beyond the diffraction limit, the hybrid plasmonic MQW NW has a lowest threshold gain of 788 cm-1 at a diameter of 130 nm, and a cutoff diameter of 80 nm, half that of the photonic lasers. In comparison with the hybrid plasmonic core-shell NWs, the hybrid plasmonic MQW NWs exhibit significantly lower threshold gain, higher Purcell factor, and smaller cutoff diameter, which are attributed to the superior overlap between the hybrid plasmonic modes and gain medium, as well as a stronger optical confinement due to the grating-like effect of MQW structures. Moreover, the hybrid plasmonic MQW NW has a lower threshold gain than that of the core-shell NW over a broad wavelength range. The hybrid plasmonic MQW NW structure is promising for ultrasmall and low-consumption near-infrared nanolasers.