Ultra-Uniform Field Distribution in a Finite-Width Metal–Dielectric–Metal Waveguide

We investigate the propagation characteristics of the fundamental surface plasmon polariton (SPP) mode of a finite-width metal–dielectric–metal waveguide. By changing the refractive index or the thickness of the dielectric layer of the waveguide, the SPP mode can be transformed from a mode confined in the dielectric layer into a mode confined around the metal corners. There always exists a condition at which the mode field distribution in the dielectric layer becomes almost perfectly uniform along the direction parallel to the metal layers, and this condition is insensitive to the width of the waveguide. It is also possible to obtain an ultra-uniform field distribution by controlling the refractive index of a different dielectric placed on both sides of the waveguide. The waveguide can be used as a basic structure for the realization of nanosized photonic devices and sensors.     

Coated Drop-Shaped Nanowires for Guiding THz Surface Plasmon Polaritons with Nanoscale Confinement and Ultra-Long Propagation Distances

Focusing far beyond the diffraction limit is very important for terahertz (THz) wave applications due to its much longer wavelength compared with optical wave. Surface plasmon polaritons (SPPs) on metal wires are frequently used to attain this focusing. However, when the mode width is reduced down to the nanometer scale, the mode loss is very high. Here, a coated drop-shaped nanowire (CDSN) is proposed for guiding THz SPPs with both ultra-strong mode confinement (10 nm) and extreme-long propagation length (1~15 mm), which result from the distinctive mode fields around the top and bottom arcs, respectively, of the metal wire. The fantastic mode properties make the waveguide very useful in nanophotonics, bio-photonics, and highly integrated photonic circuits.     

Analysis and Optimum Design of Hybrid Plasmonic Slab Waveguides

Propagation properties of hybrid plasmonic slab waveguides are studied in detail using transfer matrix method considering structural and material aspects. Hybrid metal–insulator, hybrid metal–insulator–metal, and hybrid insulator–metal–insulator waveguides are considered. Propagation length (L _p), spatial length (L _s), and mode length (L _m) are utilized as three common figures of merit to compare and optimize the waveguides according to the layer thicknesses and metal/dielectric materials. The effect of constituting materials including metals (such as silver, gold, copper, and aluminum) and dielectrics (common dielectric materials used in photonic integrated circuit technologies such as silicon and silicon compounds, III–V compounds, and polymers) are discussed. It is found that hybrid waveguides are partially to completely superior to conventional plasmonic waveguides, providing a better balance between confinement and loss.     

Nano-Scale THz Wave Propagating with Ultra-Low Loss

A metal nanowire placed in a dielectric hole is proposed to guide THz modified surface plasmon polaritons (MSPPs). In theory, the MSPP waveguide can guide THz wave with nano-scale mode width (570 nm) and simultaneously ultra-long propagation distance (2.4 m). Compared with conventional surface plasmon polaritons (SPPs) on a bare metal nanowire, the MSPPs’ mode nanoconfinement can be maintained by keeping a part of the mode field nearly unchanged. On the other hand, by modifying the rest of the mode field, the THz power inside the metal nanowire can be significantly reduced for MSPPs, which dramatically decreases the propagation loss (3 orders of magnitude).

     

Effects of Dielectric Anisotropy on Surface Plasmon Polaritons in Three-Layer Plasmonic Nanostructures

The effects of highly anisotropic dielectric on surface plasmon polaritons (SPPs) are investigated in several three-layer plasmonic nanostructures. Dispersion relations of SPPs in anisotropic-dielectric-metal (ADM), dielectric-anisotropic-metal (DAM), and metal-anisotropic-metal (MAM) structures are analytically derived. The numerical results in the visible indicate that, in ADM, the propagation length of a conductor-gap-dielectric mode is changed from 5.9 to 91 μm and its cutoff thickness from 83 to 7 nm with varying the optical axis, while in DAM, the influences of anisotropic dielectric are reversed on propagation length and cutoff thickness. In MAM, by tuning the optical axis, the light confinement of symmetry SPPs mode varies about 10 %. Further numerical calculations show that the above results induced by the anisotropy of dielectric can be extended to the telecommunication frequency. The improved mode properties may be used in plasmonic-based nanodevices and tunable single surface plasmon sources.     

Novel Dielectric-Loaded Plasmonic Waveguide for Tight-Confined Hybrid Plasmon Mode

In this paper, a novel metal-dielectric waveguide structure is proposed to support hybrid long range surface plasmon polaritons (LRSPPs) with a highly confined mode field. The simulation results showed that our proposed structure has better mode confinement and propagation length compared to that of conventional dielectric-loaded surface plasmon polaritons (DLSPPs) waveguides. This structure offers greater flexibility for the design of surface plasmon polaritons (SPPs) waveguides by altering the trade-off between mode confinement and propagation length. The proposed structure has significant potential for application in highly integrated photonic circuits.     

Surface Plasmon Polariton Propagation at the Interface of a Metal and an Ambichiral Nanostructured Medium

The propagation of a surface plasmon polariton wave at the interface of a metal and an ambichiral nanostructured medium was theoretically investigated in the Kretschmann configuration using transfer matrix method. The dependence of optical absorption linear polarization on structural parameters was reported. The results were compared with those obtained from the interface of a metal and a chiral dielectric medium as a reference structure. We found that multiple plasmon modes are excited at the interface of metal and ambichiral dielectric medium. Our calculations revealed that there exist five plasmon modes for chiral, trigonal, and tetragonal structures; three plasmon modes for pentagonal structure; two plasmon modes for hexagonal structure; and one plasmon mode for dodecagonal structure that propagate with different phase speeds. The obtained results showed that only one plasmon mode occurs at all pitches, while other modes exist at some of the pitches of anisotropic chiral and ambichiral dielectric mediums. The time-averaged Poynting vector versus the thickness of metal film confirmed that the energy of photons of incident light is transferred to surface plasmon polariton quasiparticles and the surface plasmon polariton wave is localized at the interface of metal and ambichiral dielectric medium.     

Effective Coupling of Incident Light Through an Air Region into an S-Shape Plasmonic Ag Nanowire Waveguide with Relatively Long Propagation Length

Coupling of incident light through an air region into an S-shape silver (Ag) plasmonic nanowire waveguide (SSAPNW) is a highly difficult challenge of light guiding on the surface of metal nanowire. In this paper, we numerically analyze the coupling effect of an SSAPNW which is covered by a dielectric medium using a finite element method. The coupling effect can be modulated by adjusting the Ag nanowire diameter and the covering dielectric medium width and wavelength of incident light, and the propagation length of surface plasmon (SP) coupling can be maximized. Simulation results reveal that the field confinement can be significantly improved and the majority of the electric field can be carried on the surface of a bending Ag nanowire. The effect of electric field transport along an SSAPNW due to SP coupling and Fabry-Perot resonance is investigated for different dimensions and lengths. Accordingly, long propagation lengths of about 41.5 μm for 10?×?SSAPNW at an incident wavelength of 810 nm and longer propagation length can be achieved if more sections of an SSAPNW are used. Simulation results offer an efficient method for optimizing SP coupling into bending metal nanowire waveguides and promote the realization of highly integrated plasmonic devices.     

Near-Field Coupling Method for Subwavelength Surface Plasmon Polariton Waveguides

A near-field coupling method for studying propagation properties of surface plasmon polariton (SPP) in subwavelength dielectric-loaded SPP waveguides (DLSPPWs) is presented. In this method, a tapered fiber probe is employed to generate a nanometer optical spot. When this spot is near the entrance of the DLSPPW with its polarization parallel to the waveguide, a strong guiding wave is observed by a leakage radiation microscope. For DLSPPWs with a dielectric height of about 600 nm, we observed SPP waves with zigzag propagation patterns at 650 nm wavelength. Such zigzag propagation results in a great reduction of propagation loss. In addition, the zigzag wave has a strong optical confinement. The bending loss for an L-bend DLSPPW is only about 0.4 dB.     

Confined Synthesis of 2D Nanostructured Materials toward Electrocatalysis

2D nanostructured materials have shown great application prospects in energy conversion, owing to their unique structural features and fascinating physicochemical properties. Developing efficient approaches for the synthesis of well‐defined 2D nanostructured materials with controllable composition and morphology is critical. The emerging concept, confined synthesis, has been regarded as a promising strategy to design and synthesize novel 2D nanostructured materials. This review mainly summarizes the recent advances in confined synthesis of 2D nanostructured materials by using layered materials as host matrices (also denoted as “nanoreactors”). By virtue of the space‐ and surface‐confinement effects of these layered hosts, various well‐organized 2D nanostructured materials, including 2D metals, 2D metal compounds, 2D carbon materials, 2D polymers, 2D metal‐organic frameworks (MOFs) and covalent‐organic frameworks (COFs), as well as 2D carbon nitrides are successfully synthesized. The wide employment of these 2D materials in electrocatalytic applications (e.g., electrochemical oxygen/hydrogen evolution reactions, small molecule oxidation, and oxygen reduction reaction) is presented and discussed. In the final section, challenges and prospects in 2D confined synthesis from the viewpoint of designing new materials and exploring practical applications are commented, which would push this fast‐evolving field a step further toward greater success in both fundamental studies and ultimate industrialization.     

The Tunability of Surface Plasmon Polaritons in Graphene Waveguide Structures

The tunability of propagation properties of surface plasmon polariton (SPP) modes in a waveguide formed by two parallel graphene layers separated by a dielectric layer is studied. For this purpose, the dispersion equation of the structure is numerically solved and the effects of applied bias voltage, the role of effective structural parameters, and electron–phonon scattering rate on the propagation of symmetric and antisymmetric SPP waves are investigated. The results of calculations show that considering the electron–phonon scattering rate as a function of Fermi energy and temperature leads to a considerable decrease in the propagation length of SPPs. As the main result of this work, tuning the propagation characteristics of SPPs is possible by varying any of the parameters such as applied voltage, thickness of insulating layer between two graphene layers and permittivities of dielectric layers, and finally the temperature. It is found that antisymmetric mode benefits from a larger propagation length in comparison with that of the symmetric mode.

     

Optical Transmission Through Multilayered Ultra-Thin Metal Gratings

Optical transmission properties of multilayered ultra-thin metal gratings are numerically studied. The transmission spectrum has a broad stop-band with extremely low transmittance compared to that of a single-layer one for TM polarization. The stop-band is shown to be formed by multiple-interference tunneling and various plasmon resonance processes in ultra-thin-metal and dielectric multilayers. That is on the transmission background of non-apertured metal/dielectric multilayer structures that have low transmission in the long-wavelength range due to destructive multiple-interference tunneling, the transmission is further suppressed in the stop-band by plasmon resonances in the top metal/dielectric layers, e.g., the anti-symmetric bound surface plasmon mode in the ultra-thin metal layer and the gap surface plasmon mode in the metal-sandwiched dielectric layer. High transmission beyond the stop-band is due to coupled gap surface plasmon mode in the entire multilayer structures. Applications of the optical properties of the multilayered ultra-thin metal gratings are suggested for optical filtering (wavelength or polarization selective).     

Influence of the Local Field and Dipole-Dipole Interactions on the Spectral Characteristics of Simple Metals and Their Nanoparticles

The effect of dipole-dipole interactions of free electrons on the spectral characteristics of simple metals and their nanoparticles is analyzed using Drude theory and the model of the Lorentz local field. It is established that accounting for the dispersion of a local field under conditions of one-dimensional (1D) confinement based on the optical constants of the bulk metal allows the determination of its spectral micro-characteristics in the frequency region of the longitudinal collective motions of the free electrons. This corresponds to the spectra of the dielectric losses of bulk plasma oscillations. A similar procedure for three-dimensional (3D) confinement produces the spectrum of dielectric losses at the frequency of localized plasma oscillations. Using a number of simple metal examples, viz., Li, Na, and K, and also Al, Be, and Mg, it is shown that the frequencies of volume and localized plasma oscillations obtained from a model of dispersion of the local field in the long-wave limit are in good agreement with the actual frequencies of the plasma oscillations of the corresponding metals and the absorption maxima of their nanoparticles with a radius of 2–20 nm. It is shown that the frequencies of the main mode of longitudinal plasma oscillations and the absorption frequency of localized plasmons are well described using the dynamic theory of crystal lattice vibrations.

     

Electro-absorption Modulator with Dual Carrier Accumulation Layers Based on Epsilon-Near-Zero ITO

An electro-absorption modulator based on indium tin oxide is proposed by constructing a waveguide consisting of metal-dielectric-ITO-dielectric-Si stack. Applying a negative voltage bias on the ITO layer, carrier accumulation occurs at both dielectric-ITO interfaces, which dramatically changes the guided mode properties due to the epsilon-near-zero effect. By tuning the real part of the permittivity around zero, the guided plasmonic mode concentrates in either ITO or dielectric layers, resulting in a high propagation loss. These dual carrier accumulation layers significantly improve the extinction ratio of the modulator. A further improvement is obtained by using high refractive index dielectric thin layers, which provides a strong optical confinement in the carrier accumulation layers. The dual carrier accumulation layer device shows a 200 % increase of the modulation efficiency compared to a single accumulation layer design. A modulation depth of 9.9 dB/μm can be achieved by numerical simulation.     

Particle-in-cell simulation of multipactor discharge on a dielectric in a parallel-plate waveguide

An original 2D3V (two-dimensional in coordinate space and three-dimensional in velocity space) particle-in-cell code has been developed for simulation of multipactor discharge on a dielectric in a parallelplate metal waveguide with allowance for secondary electron emission (SEE) from the dielectric surface and waveguide walls, finite temperature of secondary electrons, electron space charge, and elastic and inelastic scattering of electrons from the dielectric and metal surfaces. The code allows one to simulate all stages of the multipactor discharge, from the onset of the electron avalanche to saturation. It is shown that the threshold for the excitation of a single-surface multipactor on a dielectric placed in a low-profile waveguide with absorbing walls increases as compared to that in the case of an unbounded dielectric surface due to escape of electrons onto the waveguide walls. It is found that, depending on the microwave field amplitude and the SEE characteristics of the waveguide walls, the multipactor may operate in two modes. In the first mode, which takes place at relatively low microwave amplitudes, a single-surface multipactor develops only on the dielectric, the surface of which acquires a positively potential with respect to the waveguide walls. In the second mode, which occurs at sufficiently high microwave intensities, a single-surface multipactor on the dielectric and a two-surface multipactor between the waveguide walls operate simultaneously. In this case, both the dielectric surface and the interwall space acquire a negative potential. It is shown that electron scattering from the dielectric surface and waveguide walls results in the appearance of high-energy tails in the electron distribution function.     

Resonant TE Transmission Through a Continuous Metal Film: Perspectives for Low-Loss Plasmonic Elements

Whereas resonant transverse magnetic transmission across an undulated continuous metal film is achieved with the mediation of plasmon modes excited by the undulation, it is shown here that transverse electric (TE) resonant transmission through a continuous metal film can also be achieved with the mediation of the second-order TE₁ mode of a dielectric slab waveguide having the metal film sandwiched at its middle. The demonstration is made by using the materials currently used in the domain of optical security and counterfeit deterrence: ZnS is shown to possibly be a lossless interface/adhesion layer between a polymer and a noble metal for plasmonic resonant elements.     

Electromagnetic pulse propagation in dispersive planar dielectrics

The responses of a plane-wave pulse train irradiating a lossy dispersive dielectric half-space are investigated. The incident pulse train is expressed as a Fourier series with summing done by the inverse fast Fourier transform. The Fourier series technique is adopted to avoid the many difficulties often encountered in finding the inverse Fourier transform when transform analyses are used. Calculations are made for propagation in pure water, and typical waveforms inside the dielectric half-space are presented. Higher harmonics are strongly attenuated, resulting in a single continuous sinusoidal waveform at the frequency of the fundamental depth in the material. The time-averaged specific absorption rate (SAR) for pulse-train propagation is shown to be the sum of the time-averaged SARs of the individual harmonic components of the pulse train. For the same average power, calculated SARs reveal that pulse trains generally penetrate deeper than carrier-frequency continuous waves but not deeper than continuous waves at frequencies approaching the fundamental of the pulse train. The effects of rise time on the propagating pulse train in the dielectrics are shown and explained. Since most practical pulsed systems are very limited in bandwidth, no pronounced differences between their response and continuous wave (CW) response would be expected. Typical results for pulse-train propagation in arrays of dispersive planar dielectric slabs are presented. Expressing the pulse train as a Fourier series provides a practical way of interpreting the dispersion characteristics from the spectral point of view.     

Dielectric behavior of polyelectrolytes. III. The role of counterion interactions

The role of the Coulomb forces between the counterions on the surface of polyelectrolytes on the dielectric response is analyzed. An estimate of the maximum dielectric increment (as a function of the number of counterions) is found as a function of the molecular length. The minimum-energy configuration of the counterions on a cylinder is found to be a double helix, suggesting the fundamental importance of electrostatic interactions in determining structure. Solutions of the dynamical equations for a few counterions indicate that a single mode dominates the relaxation which is enhanced by the inter-ion repulsions. A lower bound is found for this mode based on analysis of the system response for short lengths. Sum rules for the rates and amplitudes of the dipolar correlation function are derived and lead to an upper bound for the rate of the dominant mode. These bounds approach one another for the parameters characteristic of restriction fragments of DNA. This permits a prediction of the magnitude and time scale of the dielectric response.     

Characteristics of Symmetric Surface Plasmon Polariton Mode in Glass–Metal–Glass Waveguide

The propagation characteristics of symmetric surface plasmon polariton mode in a glass–metal–glass waveguide are presented. Gallium lanthanum sulfide has been taken as the glass and silver (Ag) has been used as the metal. The analysis has been done both numerically and analytically. A two-dimensional finite-difference time-domain-based simulation model has been developed in order to analyze the propagation characteristics numerically. The obtained results using numerical and analytical methods have been compared and a very good agreement has been found.     

A comprehensive study on the electronic structure,dielectric and optical properties of alkali-earth metals and transition metal hydroxides M(OH)2

In the present work, the physical properties of alkali-earth metal and transition metal hydroxides are comprehensively investigated using the density functional theory. Here, the alkali-earth metals Ca, Mg, and transition metals Cd, Zn are considered from the II-A and II-B groups in the periodic table of elements. The first principle electronic structure calculations show that these bulk hydroxide materials are direct band gap material. Ca(OH)₂ and Mg(OH)₂ exhibit an insulating behavior with a very large band gap. However, Cd(OH)₂ and Zn(OH)₂ are found to be wide band gap semiconductors. The dielectric and optical studies reveal that these materials have a high degree of anisotropy. Hence, the light propagation in these materials behaves differently in the direction perpendicular and parallel to the optical axis, and exhibits birefringence. Therefore, these materials may be useful for optical communication. The calculated electron energy loss suggests that these materials can also be used for unwanted signal noise suppression. The wide band gap makes them useful for high-power applications. Moreover, Ca(OH)₂ and Mg(OH)₂ are found to be suitable for dielectric medium.