Surface Plasmon Polaritons Dispersion Research Articles

Manipulation of surface plasmon polariton propagation on isotropic and anisotropic two-dimensional materials coupled to boron nitride heterostructures

We present a comprehensive analysis of surface plasmon polariton dispersion characteristics associated with isotropic and anisotropic two-dimensional atomically thin layered materials (2D sheets) coupled to h-BN heterostructures. A scattering matrix based approach is presented to compute the electromagnetic fields and related dispersion characteristics of stacked layered systems composed of anisotropic 2D sheets and uniaxial bulk materials. We analyze specifically the surface plasmon polariton (SPP) dispersion characteristics in case of isolated and coupled two-dimensional layers with isotropic and anisotropic conductivities. An analysis based on residue theorem is utilized to identify optimum optical parameters (surface conductivity) and geometrical parameters (separation between layers) to maximize the SPP field at a given position. The effect of type and degree of anisotropy on the shapes of iso-frequency curves and propagation characteristics is discussed in detail. The analysis presented in this paper gives an insight to identify optimum setup to enhance the SPP field at a given position and in a given direction on the surface of two-dimensional materials.

Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses

We demonstrate enhanced light extraction for monochrome top-emitting organic light-emitting diodes (OLEDs). The enhancement by a factor of 1.2 compared to a reference sample is caused by the use of a hole transport layer (HTL) material possessing a low refractive index (1.52). The low refractive index reduces the in-plane wave vector of the surface plasmon polariton (SPP) excited at the interface between the bottom opaque metallic electrode (anode) and the HTL. The shift of the SPP dispersion relation decreases the power dissipated into lost evanescent excitations and thus increases the outcoupling efficiency, although the SPP remains constant in intensity. The proposed method is suitable for emitter materials owning isotropic orientation of the transition dipole moments as well as anisotropic, preferentially horizontal orientation, resulting in comparable enhancement factors. Furthermore, for sufficiently low refractive indices of the HTL material, the SPP can be modeled as a propagating plane wave within other organic materials in the optical microcavity. Thus, by applying further extraction methods, such as micro lenses or Bragg gratings, it would become feasible to obtain even higher enhancements of the light extraction.

Optical properties of high-quality nanohole arrays in gold made using soft-nanoimprint lithography

We present a novel soft-nanoimprint procedure to fabricate high-quality sub-wavelength hole arrays in optically thick films of gold on glass substrates. We fabricate 0.5 × 0.5 mm2 structures composed of a square array of 180 nm-diameter holes with a 780 nm pitch. Optical angular transmission measurements on the arrays show clear extraordinary transmission peaks corresponding to the dispersion of surface plasmon polaritons propagating on either side of the metal film. The transmission features can be strongly controlled by engineering the dielectric environment around the holes. As the nanoimprint procedure enables fabrication of nanoscale patterns over wafer-scale areas at low cost, these imprinted metal nanoparticle arrays can find applications in, e.g., optical components, photovoltaics, integrated optics, and microfluidics.

Light-Triggered Control of Plasmonic Refraction and Group Delay by Photochromic Molecular Switches

An interface supporting plasmonic switching is prepared from a gold substrate coated with a polymer film doped with photochromic molecular switches. A reversible light-induced change in the surface plasmon polariton dispersion curve of the interface is experimentally demonstrated, evidencing reversible switching of the surface plasmon polariton group and phase velocity. The switching capabilities of the interface are furthermore successfully applied to achieve focus control of a plasmonic lens. The results imply the realization of nonvolatile and reversible plasmonic switching units providing complex functionalities based on surface plasmon refraction and group delay.

Spoof plasmons in corrugated semiconductors

We report on a theoretical investigation of the dispersion relation of surface plasmon polaritons (SPPs) on a periodically corrugated semiconductor surface. We assumed Drude’s permittivity model of the semiconductor, which accurately describes the loss of these spoof SPPs. In the THz frequency range, the properties of the dispersion and loss of spoof SPPs on corrugated Si surfaces are studied. A low-loss propagation of spoof SPPs can be achieved by an optimum design of the surface structure. It was found that by increasing the lattice constant or by reducing the groove depth, the investigated structure can provide a low guiding attenuation.

Silicon as a virtual plasmonic material: Acquisition of its transient optical constants and the ultrafast surface plasmon-polariton excitation

Ultrafast intense photoexcitation of a silicon surface is complementarily studied experimentally and theoretically, with its prompt optical dielectric function obtained by means of time-resolved optical reflection microscopy and the underlying electron-hole plasma dynamics modeled numerically, using a quantum kinetic approach. The corresponding transient surface plasmon-polariton (SPP) dispersion curves of the photo-excited material were simulated as a function of the electron-hole plasma density, using the derived optical dielectric function model, and directly mapped at several laser photon energies, measuring spatial periods of the corresponding SPP-mediated surface relief nanogratings. The unusual spectral dynamics of the surface plasmon resonance, initially increasing with the increase in the electron-hole plasma density but damped at high interband absorption losses induced by the high-density electron-hole plasma through instantaneous bandgap renormalization, was envisioned through the multi-color mapping.

Transformation of surface plasmon polaritons to radiation in graphene in terahertz regime

We demonstrate a concept that allows direct excitation of surface plasmon polaritons (SPPs) by a moving electron bunch above a single layer graphene sheet deposited on a dielectric substrate without any additional coupling requirements. We show that if the two-dimensional current in the graphene is dominated by the third order nonlinear effect when the surface electric field exceeds a moderate strength of ∼5 kV/cm, the SPP mode can cross the light line although the group velocity remains much smaller than the speed of light. This effect gives rise to direct transformation of SPPs into radiation. The underlying mechanism of the crossing of the SPP dispersion into the light line is the energy shift of charged particles in the nonlinear regime and the finite transport scattering time in graphene. Both the energy and lifetime of the SPPs increase with the field intensity. The radiation intensity and frequency can be tuned with an AC bias.

Optical Properties and Plasmonic Performance of Titanium Nitride

Titanium nitride (TiN) is one of the most well-established engineering materials nowadays. TiN can overcome most of the drawbacks of palsmonic metals due to its high electron conductivity and mobility, high melting point and due to the compatibility of its growth with Complementary Metal Oxide Semiconductor (CMOS) technology. In this work, we review the dielectric function spectra of TiN and we evaluate the plasmonic performance of TiN by calculating (i) the Surface Plasmon Polariton (SPP) dispersion relations and (ii) the Localized Surface Plasmon Resonance (LSPR) band of TiN nanoparticles, and we demonstrate a significant plasmonic performance of TiN.

Interference of surface plasmons and Smith-Purcell emission probed by angle-resolved cathodoluminescence spectroscopy

We investigate the interplay between geometrical lattice resonances and surface plasmons mediating the emission of Smith-Purcell visible light via angle-resolved cathodoluminescence spectroscopy. We observe strong modulations in the dispersion curves of Smith-Purcell radiation (SPR) when they intersect the surface plasmons of silver gratings using a 200-kV transmission electron microscope. The decay of the plasmons away from the grating is directly probed by controlling the electron-beam position relative to the sample surface with nanometer precision. Our measurements are in excellent agreement with numerical simulations, clearly revealing the presence of characteristic Fano profiles resulting from the interference of the light continuum and the discrete plasmon states for each direction of emission. The intensity anomaly in the SPR emission pattern can be well explained from the geometrical consideration of the intersections between the dispersion planes of the SPR and surface plasmon polariton (SPP). A strong and directional SPR beam can be realized under the condition that the SPR dispersion plane comes in contact with the band edge of the SPP dispersion plane.

Electrically tunable graphene anti-dot array terahertz plasmonic crystals exhibiting multi-band resonances

Graphene-based plasmonic structures feature large tunability, high spatial confinement, and potentially low loss, and are therefore an emerging technology for unconventional manipulation of light. In this paper, we demonstrate electrically tunable terahertz plasmonic crystals consisting of square-lattice graphene periodic anti-dot arrays on a SiO2/Si substrate. Transmission spectroscopy reveals multiple distinct resonances arising from excitations of graphene surface-plasmon–polariton (SPP) modes on different branches of the SPP dispersion curves inherent to the periodic structures. The resonance frequencies are readily tuned electrostatically with the Si back-gate and exhibit the dependency on the carrier density unique to SPP in graphene. Simulations show excellent agreement with the experiments and further illustrate the symmetry-based selection rule for the excited graphene SPP modes. Such graphene plasmonic crystals may lead to a broad range of applications including plasmonic waveguide and transformation optics. Exploiting higher-order graphene SPP modes is an effective way to further facilitate field localization and enhancement.

Temperature dependence of exciton-surface plasmon polariton coupling in Ag, Au, and Al films on InxGa1−xN/GaN quantum wells studied with time-resolved cathodoluminescence

The optical properties and coupling of excitons to surface plasmon polaritons (SPPs) in Ag, Au, and Al-coated InxGa1−xN/GaN multiple and single quantum wells (SQWs) were probed with time-resolved cathodoluminescence. Excitons were generated in the metal coated SQWs by injecting a pulsed high-energy electron beam through the thin metal films. The Purcell enhancement factor (Fp) was obtained by direct measurement of changes in the temperature-dependent radiative lifetime caused by the SQW exciton-SPP coupling. Three chosen plasmonic metals of Al, Ag, and Au facilitate an interesting comparison of the exciton-SPP coupling for energy ranges in which the SP energy is greater than, approximately equal to, and less than the excitonic transition energy for the InGaN/GaN QW emitter. A modeling of the temperature dependence of the Purcell enhancement factor, Fp, included the effects of ohmic losses of the metals and changes in the dielectric properties due to the temperature dependence of (i) the intraband behavior in the Drude model and (ii) the interband critical point transition energies which involve the d-bands of Au and Ag. We show that an inclusion of both intraband and interband effects is essential when calculating the ω vs k SPP dispersion relation, plasmon density of states (DOS), and the dependence of Fp on frequency and temperature. Moreover, the “back bending” in the SPP dispersion relation when including ohmic losses can cause a finite DOS above ωsp and lead to a measurable Fp in a limited energy range above ωsp, which can potentially be exploited in plasmonic devices utilizing Ag and Au.

Probing complex reflection coefficients in one-dimensional surface plasmon polariton waveguides and cavities using STEM EELS.

The resonant properties of a plasmonic cavity are determined by the size of the cavity, the surface plasmon polariton (SPP) dispersion relationship, and the complex reflection coefficients of the cavity boundaries. In small wavelength-scale cavities, the phase propagation due to reflections from the cavity walls is of a similar magnitude to propagation due to traversing the cavity. Until now, this reflection phase has been inferred from measurements of the resonant frequencies of a cavity of known dispersion and length. In this work, we present a method for measuring the complex reflection coefficients of a truncation in a 1D surface plasmon waveguide using electron energy loss spectroscopy in the scanning transmission electron microscope (STEM EELS) and show that this insight can be used to engineer custom cavities with engineered reflecting boundaries, whose resonant wavelengths and internal mode density profiles can be analytically predicted given knowledge of the cavity dimensions and complex reflection coefficients of the boundaries.

Identification of Fluids by the Color of Surface Plasmon Polaritons

When optical waves make the free electrons on a metal surface resonate, optical energy propagates along the surface as density waves of the free electrons. The longitudinal waves and electrical fields of the electrons are called surface plasmon polaritons (SPPs), which are widely applied in high sensitivity sensors because the excitation of SPPs sensitively depends on the refractive index of the surrounding dielectric sample. Here, we report the identification of fluids by using the color dispersion of SPPs. Silver film on a prism surface is illuminated with white light to excite SPPs. A color component in the white light is thereby selectively coupled with SPPs due to the color dispersion that depends on the refractive index of the fluid on the film. Thus, theoretically, when the refractive index is changed, the color of SPPs changes as well. Our application uses a medium consisting of fluid samples to be identified. The proposed identification method can be applied to fluid analysis for label-free visualization of or as a simple analysis method, since the refractive indices or concentrations of the sample fluids directly affect the color of the SPPs, and this color can be visually identified. We theoretically confirmed that the color of SPPs excited with white light illumination can help to differentiate between water and ethanol. Experimentally, SPPs belonging to the frequency region of the color green were detected when the sample was water, and the color changed to red when ethanol was used instead. In the future, we plan to develop simple, small, sensitive, and low-cost sensors that can determine the concentration and refractive index of fluids on the basis of the color of the SPPs.

Surface plasmons in nanowires with toroidal magnetic structure.

We consider the problem of the influence of a toroidal magnetization on a cylindrical surface plasmon polariton (SPP) propagating along a nanowire of a circular cross section. It follows from the dispersion equations that the SPP wavenumber linearly depends on the toroidal moment and the effect of magneto-optical nonreciprocity appears. The numerical solution of the dispersion equations demonstrates that the corresponding splitting of the SPP dispersion curves for two opposite directions of the toroidal moment is increased by an order of magnitude with respect to the planar case. The largest values of this splitting are observed for systems with relatively low optical losses, as is demonstrated by calculations for SPPs in a gold cylinder surrounded by rare-earth bismuth iron garnet.

Tunable THz surface plasmon polariton based on a topological insulator/layered superconductor hybrid structure

We theoretically investigate the surface plasmon polariton (SPP) at the interface between 3D strong topological insulator (TI) and layered superconductor-magnetic insulator structure. The tunability of SPP through electronic doping can be enhanced when the magnetic permeability of the layered structure becomes higher. When the interface is gapped by superconductivity or perpendicular magnetism, SPP dispersion is further distorted, accompanied by a shift of group velocity and penetration depth. Such a shift of SPP reaches maximum when the magnitude of Fermi level approaches the gap value, and may lead to observable effects. The tunable SPP at the interface between layered superconductor and magnetism materials in proximity to TI surface may provide new insight in the detection of Majorana Fermions.

Tailoring Alphabetical Metamaterials in Optical Frequency: Plasmonic Coupling, Dispersion, and Sensing

Tailoring optical properties of artificial metamaterials, whose optical properties go beyond the limitations of conventional and naturally occurring materials, is of importance in fundamental research and has led to many important applications such as security imaging, invisible cloak, negative refraction, ultrasensitive sensing, and transformable and switchable optics. Herein, by precisely controlling the size, symmetry, and topology of alphabetical metamaterials with U, S, Y, H, U-bar, and V shapes, we have obtained highly tunable optical response covering visible-to-infrared (vis-NIR) optical frequency. In addition, we show a detailed study on the physical origin of resonance modes, plasmonic coupling, the dispersion of resonance modes, and the possibility of negative refraction. We have found that all the electronic and magnetic modes follow the dispersion of surface plasmon polaritons; thus, essentially they are electronic- and magnetic-surface-plasmon-polaritons-like (ESPP-like and MSPP-like) modes resulted from diffraction coupling between localized surface plasmon and freely propagating light. On the basis of the fill factor and formula of magnetism permeability, we predict that the alphabetical metamaterials should show the negative refraction capability in visible optical frequency. Furthermore, we have demonstrated the specific ultrasensitive surface enhanced Raman spectroscopy (SERS) sensing of monolayer molecules and femtomolar food contaminants by tuning their resonance to match the laser wavelength, or by tuning the laser wavelength to match the plasmon resonance of metamaterials. Our tunable alphabetical metamaterials provide a generic platform to study the electromagnetic properties of metamaterials and explore the novel applications in optical frequency.

Morphological Tuning of the Plasmon Dispersion Relation in Dielectric-Loaded Nanofiber Waveguides

Understanding the impact of lateral mode confinement in plasmonic waveguides is of fundamental interest regarding potential applications in plasmonic devices. The knowledge of the frequency-wave vector dispersion relation provides the full information on electromagnetic field propagation in a waveguide. This Letter reports on the measurement of the real part of the surface plasmon polariton dispersion relation in the near infrared spectral regime for individual nanoscale plasmonic waveguides, which were formed by deposition of para-hexaphenylene (p-6P) based nanofibers on top of a gold film. A detailed structural characterization of the nanofibers provides accurate information on the dimensions of the investigated waveguides and enables us to quantify the effect of mode confinement by comparison with experimental results from continuous p-6P films and calculations based on the effective index method.

Near-infrared surface plasmon polariton dispersion control with hyperbolic metamaterials

We demonstrate experimentally signatures and dispersion control of surface plasmon polaritons from 1 to 1.8 µm using periodic multilayer metallo-dielectric hyperbolic metamaterials. The fabricated structures are comprised of smooth films with very low metal filling factor. The measured dispersion properties of these hyperbolic metamaterials agree well with calculations using transfer matrix, finite-difference time-domain, and effective medium approximation methods despite using only 2.5 periods. The enhancement factor in the local photonic density of states from the studied samples in the near-infrared wavelength region is determined to be 2.5-3.5. Development of this type of metamaterial is relevant to sub-wavelength imaging, spontaneous emission and thermophotovoltaic applications.

Analysis of plasmonic properties of heavily doped semiconductors using full band structure calculations

Surface plasmon polaritons (SPPs) and localized surface plasmon (LSP) resonances are not limited to noble metals. Any material with a substantial amount of free carriers will support surface plasma oscillations which, when coupled to an electromagnetic field, will result in surface plasmon polaritons and localized surface plasmon resonances in confined systems. Utilizing a full band structure approach, we analyze the plasmonic properties of several heavily doped semiconductors. We present rigorous quantum mechanical calculations of the plasma frequency, and study in detail its dependence on impurity doping concentration. Results are presented for silicon, germanium, gallium arsenide, zinc oxide, and gallium nitride. For silicon and zinc oxide, the surface plasmon resonance frequency is calculated for a large range of doping concentrations and we study the dispersion of surface plasmon polaritons on thin films. The investigated properties of heavily doped semiconductors hold promises for several interesting applications within plasmonics.

Ultrafast electron dynamics manipulation of laser induced periodic ripples via a train of shaped pulses

We present the electron dynamics manipulation of laser induced periodic nanoripples on fused silica via a train of shaped femtosecond pulses. The non-linear ionization used for supporting the surface plasmon polaritons (SPPs) is taken into account in the analysis of the ripple dynamics. It is revealed that the ripple periods are closely related to both the pulse-to-pulse separation and the laser fluence. The result is attributed to the ultrafast dynamics manipulation for the SPP dispersion wavelength through the unusual ionization arising via a train of shaped pulses. This study should be helpful for the fundamental understanding of the ripple formation mechanism arising from a train of shaped pulses, used for controlling the ripple features.