Antisymmetric Surface Plasmon Polariton Research Articles

Near-field properties of spin, Huygens and Janus sources in a narrow sandwiched structure

Near-field photonics has attracted extensive attention for its applications from classical to quantum fields, and near-field directionality is the core of many applications. Here, we use a sandwich structure to investigate the effects of waveguide width and source position on the behavior of spin, Huygens and Janus sources. Although the structure is very simple, the conclusions obtained through it can be extended to more complex structures. In a narrow waveguide, the surface modes on each interface can be coupled together to form symmetric and antisymmetric surface plasmon polaritons (SPPs) modes, connecting the evanescent fields at the two interfaces. The width of the waveguide determines the propagation constants of the two SPPs modes and will affect the near-field directional emission of the three sources. We find that the directionality of the three sources in the narrow waveguide is no longer perfect, but related to the position of the source. In addition, we also discuss the possibility of realizing near-field directional emission of three sources with only one electric dipole in different structures by selectively exciting the special SPPs mode. Our result increases the flexibility of controlling the direction of near-field emission of evanescent waves, having potential applications in nanophotonic propagation and multifunctional nanophotonic devices.

Symmetric and antisymmetric surface plasmon polariton solitons in a metal-dielectric-metal waveguide with incoherent pumping

We propose a scheme to realize stable linear and nonlinear propagation of symmetric and antisymmetric surface plasmon polaritons (SPPs) solitons by doping ladder-type three-level quantum emitters into the middle layer of a metal-dielectric-metal (MDM) waveguide. In linear propagation regime, we show that both symmetric and antisymmetric SPPs can acquire a gain from electromagnetically induced transparency (EIT) effect with an incoherent pumping. The EIT can be used not only to completely compensate the Ohmic loss in the metal but also to acquire a subluminal group velocity for the SPPs. We also show that in nonlinear propagation regime a huge enhancement of Kerr nonlinearity of the symmetric and antisymmetric SPPs can be obtained but with different incoherent pumping intensities. As a result, gain-assisted (1+1)-dimensional symmetric and antisymmetric subluminal surface polaritonic solitons may be produced based on the strong confinement of electric field in the MDM waveguide. Our study may have promising applications in light information processing and transmission at nanoscale level based on MDM waveguides.

A High-Efficiency Multispectral Filter Based on Plasmonic Hybridization between Two Cascaded Ultrathin Nanogratings.

Overcoming the disadvantages of low transmission and broad peak bandwidth of previously reported plasmonic color filters, a high-efficiency multispectral plasmonic color filter is theoretically proposed with two cascaded ultrathin metallic nanogratings separated by two heterogeneous dielectric layers, and its optical properties are theoretically investigated using the finite-difference time-domain method. The transmission spectrum presents three near-unity peak bands accompanied with three near-null dip bands adjacent around them. Both transmission efficiencies of above 90% and ultranarrow peak bandwidth of 20 nm are achieved in the visible regime. The peak band positions can be flexibly tailored by varying the structural parameters. The filter selects the visible color with high signal noise ratio at the peak bands. The outstanding spectral properties of this filter indicate significant improvement for the high-accuracy color filtering and multispectral imaging applications. The simulated near-field electromagnetic distributions suggest that the excitation of the hybrid antisymmetric surface plasmon polariton (SPP) leaky mode and metal-insulator-metal waveguide modes are responsible for the peak transmission bands, while the formation of the hybrid SPP bound modes confined on the bottom nanograting makes the dip transmission bands, all of which are the consequence of the plasmonic hybridization between the two neighboring metallic nanogratings.

Near-Perfect Spectral Transmission Properties for Two Cascaded Plasmonic Ultrathin Nanograting Structures

A high-performance plasmonic transmission structure consisting of two longitudinally cascaded ultrathin metallic nanogratings with a half period lateral dislocated and separated by two heterogeneous dielectric layers is proposed and theoretically studied. Three near-unity spectral transmission peaks are observed for the cascaded plasmonic nanograting structure due to the evanescent field coupling of surface plasmon polariton waves supported by the two neighboring plasmonic nanogratings. The physical mechanism responsible for the near-perfect peak transmissions is discussed based on the corresponding spatial distributions of electromagnetic fields and is found to be two possible ways: by the excitation of hybrid anti-symmetric surface plasmon polariton leaky mode on the incident and transmission surfaces of the cascaded plasmonic nanograting structure or by the formation of localized surface plasmon polariton resonance modes within horizontally butt-jointed metal/insulator/metal coupled waveguides between the two cascaded plasmonic nanogratings. It is the two heterogeneous dielectric layers inserted between the two cascaded plasmonic nanogratings that is indispensable for the formation of the hybrid anti-symmetric surface plasmon polariton leaky mode, resulting in the near-unity transmission peak with an ultra-narrow bandwidth of 20 nm. The high-tunability of spectral transmission behaviors with varying structural parameters and dielectric layer are explored, which promise numerous potential applications in nano-optics devices, such as plasmonic filters, sensors, and nanoscale distance ruler.

On-chip polarization splitter based on a multimode plasmonic waveguide

The miniaturization of polarization beam splitters (PBSs) is vital for ultradense chip-scale photonic integrated circuits. However, the small PBSs based on complex hybrid plasmonic structures exhibit large fabrication difficulties or high insertion losses. Here, by designing a bending multimode plasmonic waveguide, an ultrabroadband on-chip plasmonic PBS with low insertion losses is numerically and experimentally realized. The multimode plasmonic waveguide, consisting of a metal strip with a V-shaped groove on the metal surface, supports the symmetric and antisymmetric surface plasmon polariton (SPP) waveguide modes in nature. Due to the different field confinements of the two SPP waveguide modes, which result in different bending losses, the two incident SPP waveguide modes of orthogonal polarization states are efficiently split in the bending multimode plasmonic waveguide. The numerical simulations show that the operation bandwidth of the proposed PBS is as large as 430 nm because there is no resonance or interference effect in the splitting process. Compared with the complex hybrid plasmonic structure, the simple bending multimode plasmonic waveguide is much easier to fabricate. In the experiment, a broadband (Δλ≈120 nm) and low-insertion-loss (<3 dB with a minimum insertion loss of 0.7 dB) PBS is demonstrated by using the strongly confined waveguide modes as the incident sources in the bending multimode plasmonic waveguide.

Plasmonic coupled modes in metal-dielectric multilayer structures: Fano resonance and giant field enhancement.

We provide an overview of Fano resonance and plasmon induced transparency (PIT) as well as on plasmons coupling in planar structures, and we discuss their application in sensing and enhanced spectroscopy. Metal-insulator-metal (MIM) structures, which are known to support symmetric and anti-symmetric surface plasmon polaritons (SPPs) arising from the coupling between two SPPs at the metal-insulator interfaces, exhibit anticrossing behavior of the dispersion relations arising from the coupling of the symmetric SPP and the metal/air SPP. Multilayer structures, formed by a metal film and a high-index dielectric waveguide (WG), separated by a low-index dielectric spacer layer, give narrow resonances of PIT and Fano line shapes. An optimized Fano structure shows a giant field intensity enhancement value of 106 in air at the surface of the high-index dielectric WG. The calculated field enhancement factor and the figure of merit for the sensitivity of the Fano structure in air can be 104 times as large as those of the conventional surface plasmon resonance and WG sensors.

Nonlocal Effects on Optical Gradient Force from Coupled Surface Plasmon Polaritons in Wire Medium Waveguides

We analytically study optical gradient forces from the mutual coupling between surface plasmon polariton (SPP) modes at the interfaces of two wire medium metamaterial waveguides separated by a linear dielectric. Based on the nonlocal model, dispersion properties and optical gradient forces from coupled SPP modes are analyzed mainly and compared with the local case. The forces from the coupled SPP modes are calculated by Maxwell stress tensor. Numerical results show that the nonlocal effects on dispersion properties and optical forces are very large, especially in the case of large propagation constant, and that symmetric (antisymmetric) SPP modes give rise to an attractive (repulsive) interaction, respectively. In addition, the nonlocal effects on the cutoff frequency for the antisymmetric SPP modes are clarified. These properties indicate that the force from the coupled SPPs at wire medium metamaterials offer another avenue for controlling the growth and self-assembling of artificial metamaterials.

Self-phase-modulation of surface plasmon polaritons

We present an approach for calculating the nonlinear propagation of surface plasmon polaritons in one-dimensional planar waveguides consisting of a metal slab or a semi-infinite metal bounded by linear dielectrics, starting with an assumed third-order nonlinearity that characterizes the nonlinear response of the metal. With this approach we model the self-phase-modulation of surface plasmon polaritons. Expressions relating the complex nonlinear parameter of these surface waves with the third-order nonlinear susceptibility of the metal are provided. We present and discuss results pertaining to the self-phase-modulation of the symmetric and antisymmetric surface plasmon polaritons supported by a thin gold slab in vacuum and of the surface plasmon polariton supported by the single gold-vacuum interface.

Numerical investigation of coupling efficiency between a magnetic line dipole emitter and surface plasmon polariton modes of a nanometer thin metallic film

In this paper we considered a line magnetic dipole emitter near a 25nm thick silver film embedded in a homogeneous dielectric environment. By using a two dimensional magnetic Green's function, the Purcell factor and coupling efficiency factor (β factor) for symmetric and antisymmetric surface plasmon polaritons (SPP) and out of plane (OUP) propagating decay channels were calculated. For a fixed wavelength emission, the effects of source position and refractive index of the dielectric medium on the Purcell factor and β factors are investigated.The numerical results show that, where the Purcell factor is maximum (minimum) the antisymmetric (symmetric) SPP is dominant decay channel and the selective excitation of symmetric SPP is better achievable in the case in which the dielectric medium has the higher refractive index. The wavelength dependency of β factors is also investigated for different source locations. This investigation shows that how the wavelength position for the peak of β factors depends on source location. The results of this paper could have implications for metamaterials research and many lanthanides based photonic devices.

Coupling light into and out from the surface plasmon polaritons of a nanometer-thin metal film with a metal nanostrip

Scattering of light by a metal nanostrip placed near a nanometer-thin metal film in a homogeneous dielectric environment is considered. For a plane wave incident on the geometry from outside we study the scattering cross sections governing the excitation of symmetric and antisymmetric surface plasmon polariton (SPP) waves and out-of-plane (OUP) propagating waves. For a symmetric (long-range) or antisymmetric (short-range) SPP propagating along the metal film and being incident on the nanostrip, we study the fraction of power coupled into forward and backward propagating symmetric and antisymmetric SPP waves, and into OUP propagating waves, respectively. Resonance peaks in the scattering cross sections, or dips in the relative power going into all scattering channels due to resonant absorption, are related to standing-wave resonances of counterpropagating gap SPP waves in the gap between the metal nanostrip and the metal film. The resonances are studied with respect to the width of the metal nanostrip and the size of the gap between the metal film and nanostrip. For a 15-nm-thick film it is found that coupling of light in and out of the symmetric SPP is weak compared with coupling to and from OUP and antisymmetric SPP waves. Improving the in and out coupling of power in the symmetric SPP waves with thicker films (50 nm) and larger scattering objects is considered.

Green's function approach to investigate the excitation of surface plasmon polaritons in a nanometer-thin metal film

The two-dimensional magnetic-field Green's function for a nanometer-thin metal film embedded in a dielectric environment is constructed with the mode-expansion method. Explicit expressions are obtained for the Green's-function components related to the excitation of symmetric and antisymmetric surface-plasmon polariton (SPP) waves and out-of-plane propagating waves, respectively. The obtained expressions are used to study the power emitted by a magnetic dipole line source into the different radiation channels. For a wavelength of $\ensuremath{\lambda}=500$ nm the total emitted power is found to increase by a factor of three (the Purcell factor), with approximately 60% going into SPP waves when the source is placed directly on the surface of a 15-nm-thick silver film. Results obtained with and without including ohmic losses in the calculations are only negligibly different. The presented semianalytical approach allows for an exact analysis of the excitation of SPP waves on nanometer-thin metal films and may therefore find several applications within nanoplasmonics.

Imaging the Hidden Modes of Ultrathin Plasmonic Strip Antennas by Cathodoluminescence

We perform spectrally resolved cathodoluminescence (CL) imaging nanoscopy using a 30 keV electron beam to identify the resonant modes of an ultrathin (20 nm), laterally tapered plasmonic Ag nanostrip antenna. We resolve with deep-subwavelength resolution four antenna resonances (resonance orders m = 2-5) that are ascribed to surface plasmon polariton standing waves that are confined on the strip. We map the local density of states on the strip surface and show that it has contributions from symmetric and antisymmetric surface plasmon polariton modes, each with a very different mode index. This work illustrates the power of CL experiments that can visualize hidden modes that for symmetry reasons have been elusive in optical light scattering experiments.

Active modulation of plasmonic signal with a subwavelength metal/nonlinear dielectric material/metal structure

Light propagation through a metal/nonlinear dielectric material/metal (M-NL-M) structure is numerically studied. The design parameters of the M-NL-M structure are found with the waveguide theory so that the structure only supports the symmetric surface plasmon polaritons (SPP(0)) mode and the antisymmetric surface plasmon polaritons (SPP(1)) mode. The coupling between the two modes within the M-NL-M structure is exploited. Through controlling the propagation constants of the two modes with the intensity-dependent dielectric constant of the nonlinear Kerr material, an effective all-optical control of plasmonic signal modulator can be realized with this M-NL-M structure.

Modal characteristics of coupled metallic nanoscale rectangular apertures

We examine the modal characteristics of coupled metallic nanoscale rectangular apertures using a simple and accurate separation of variables method, recently proposed by us for a single aperture [Opt. Lett.33, 333 (2008)]. The study shows that the coupling between the antisymmetric surface plasmon polariton (SPP) modes of the two rectangular apertures results in a coupled SPP mode with very large propagation length, larger than 1 mm at the wavelength of 0.633 microm, provided the aperture size and the separation are sufficiently large. The effective indices and the propagation lengths of various coupled SPP modes are presented.

Sensitivity enhancement of nanoplasmonic sensors in low refractive index substrates

Metal films perforated by nanoholes constitute a powerful platform for surface plasmon resonance biosensing. We find that the refractive index sensitivity of nanohole arrays increases if their resonance is red-shifted by increasing the separation distance between holes. However, an additional sensitivity enhancement occurs if the nanohole sensors are manufactured on low index substrates, despite the fact such substrates significantly blue-shift the resonance. We find a approximately 40% higher bulk refractive index sensitivity for a system of approximately 100 nm holes in 20 nm gold films fabricated on Teflon substrates (n=1.32) compared to the case when conventional glass substrates (n=1.52) are used. A similar improvement is observed for the case when a thin layer of dielectric material is deposited on the samples. These results can be understood by considering the electric field distribution induced by the so-called antisymmetric surface plasmon polariton in the thin gold films.

Optical antennas based on coupled nanoholes in thin metal films

The ability to control optical effects at the nanoscale is a challenge that could be of great importance for a range of photonic applications. However, progress requires a deep understanding of the relationship between near-field and far-field properties of the individual elements of the nanostructure, as well as of the role of nano-optical interactions. Here, we show that the strong interaction between nanoholes in optically thin metal films can be used to readily tune their spectral response and visibility. Control of this interaction in short chains of nanoholes enables either amplification or almost total suppression of the scattered light. The phenomena are interpreted in terms of hole coupling mediated via antisymmetric surface plasmon polaritons, which makes the nanohole chains effectively behave as linear wire antennas.

Coupled Surface Plasmons in a Symmetric System

Abstract Results of an experimental study of a totally symmetric prism/silver film/air gap/silver film/prism system are presented in which both symmetric and antisymmetric surface plasmon-polaritons (SPP) are optically excited using 632·8 nm radiation. Data taken over a range of air gap thicknesses illustrate many interesting features in accordance with predictions made using Fresnel's equations. The manner in which the two coupled SPPs evolve in terms of Poynting vector distribution and electric field profiles within the structure is studied in detail. Both the transmitted and the reflected attenuated total reflection signals are investigated.