Excitation Of Surface Modes Research Articles

Free-electron coupling to surface polaritons mediated by small scatterers.

The ability of surface polaritons (SPs) to enhance and manipulate light fields down to deep-subwavelength length scales enables applications in optical sensing and nonlinear optics at the nanoscale. However, the wavelength mismatch between light and SPs prevents direct optical excitation of surface-bound modes, thereby limiting the widespread development of SP-based photonics. Free electrons are a natural choice to directly excite strongly confined SPs because they can supply field components of high momentum at designated positions with subnanometer precision. Here, we theoretically explore free-electron-SP coupling mediated by small scatterers and show that low-energy electrons can efficiently excite surface modes with a maximum probability reached at an optimum surface-scatterer distance. By aligning the electron beam with a periodic array of scatterers placed near a polariton-supporting interface, in-plane Smith-Purcell emission results in the excitation of surface modes along well-defined directions. Our results support using scattering elements to excite SPs with low-energy electrons.

Coherent spin wave excitation with radio-frequency spin–orbit torque

Spin waves, collective perturbations of magnetic moments, are both fundamental probes for magnetic physics and promising candidates for energy-efficient signal processing and computation. Traditionally, coherent propagating spin waves have been generated by radio frequency (RF) inductive Oersted fields from current-carrying electrodes. An alternative mechanism, spin–orbit torque (SOT), offers more localized excitation through interfacial spin accumulation but has been mostly limited to DC to kHz frequencies. SOT driven by RF currents, with potentially enhanced pumping efficiency and unique spin dynamics, remains largely unexplored, especially in magnetic insulators. Here, we conduct a comprehensive theoretical and computational investigation into the generation of coherent spin waves via RF-SOT in the prototypical yttrium iron garnet. We characterize the excitation of forward volume, backward volume, and surface modes in both linear and nonlinear regimes, employing single and interdigitated electrode configurations. We reveal and explain several unique and surprising features of RF-SOT compared to inductive excitation, including higher efficiency, distinct mode selectivity, and directional symmetry, a ∼3π/4 phase offset, reduced anharmonic distortion in the nonlinear regime, and the absence of second harmonic generation. These insights position RF-SOT as a promising new mechanism for future magnonic and spintronic applications.

THE NARROW-BAND FILTER BASED ON A MAGNETOPHOTONIC CRYSTAL INVOLVING LAYERS WITH HYPERBOLIC DISPERSION LAWS

Subject and Purpose. Narrow-band filters are among the basic components of modern communication systems, instruments for spectros- copy, high-sensitivity sensors, etc. Photonic crystal structures open up broad possibilities for creating compact-sized, narrow-band filters in the optical and terahertz ranges. Tuning of spectral characteristics of photonic crystal filters is usually carried out through introduction of certain elements into their structure that are sensitive to external electric and magnetic fields. This work has been aimed at investigating electrodynamic characteristics of one-dimensional magnetophotonic crystals with structural layers characterized by "hyperbolic" disper- sion, and suggesting a multichannel, narrow-band filter on their base. Methods and Methodology. The dispersion equation for excitations in an infinite magnetophotonic crystal has been obtained within the framework of the Floquet-Bloch theory, with the use of fundamental solutions of Hill’s equation. The transfer matrix approach has been used to obtain an analytical expression for the transmission coefficient. Results. The band diagram of the one-dimensional magnetophotonic crystal has been analyzed for the case where one of the layers on the structure’s spatial period is characterized by a hyperbolic dispersion law. The areas of existence of surface wave regimes have been found for such layers for the case of normal incidence of the wave upon the finite-seized magnetophotonic crystal. Frequency dependences of the transmission coefficient are characterized by a set of high-Q resonant peaks relating to Fabry-Perot resonances in a periodic struc- ture of finite length. Conclusions. Application of a finite-seized, one-dimensional magnetophotonic crystal is considered as of a means forachieving mul- tichannel optical filtering and formation of a frequency comb. Expressions for the dispersion equation and transmission coefficient have been obtained within the framework of the Floquet-Bloch theory and with the use of the transfer matrix. The feasibility of surface mode excitation has been shown for gyrotropic layers of the periodic structure characterized by a hyperbolic dispersion law, for the case of nor- mal incidence upon the magnetophotonic crystal. The spectral response of the filter contains narrow-band peaks with a high transmission efficiency. By increasing the number of the structure’s periods it is possible to form a frequency comb, which effect can be useful for appli- cations in metrology and modern optical communication systems.

Modeling of very high frequency large-electrode capacitively coupled plasmas with a fully electromagnetic particle-in-cell code

Phenomena taking place in capacitively coupled plasmas with large electrodes and driven at very high frequencies are studied numerically utilizing a novel energy- and charge-conserving implicit fully electromagnetic particle-in-cell (PIC)/Monte Carlo code ECCOPIC2M. The code is verified with three model problems and is validated with results obtained in an earlier experimental work (Sawada et al 2014 Japan. J. Appl. Phys. 53 03DB01). The code shows a good agreement with the experimental data in four cases with various collisionality and absorbed power. It is demonstrated that under the considered parameters, the discharge produces radially uniform ion energy distribution functions for the ions hitting both electrodes. In contrast, ion fluxes exhibit a strong radial nonuniformity, which, however, can be different at the powered and grounded electrodes at increased pressure. It is found that this nonuniformity stems from the nonuniformity of the ionization source, which is in turn shaped by mechanisms leading to the generation of energetic electrons. The mechanisms are caused by the interaction of electrons with the surface waves of two axial electric field symmetry types with respect to the reactor midplane. The asymmetric modes dominate electron heating in the radial direction and produce energetic electrons via the relatively inefficient Ohmic heating mechanism. In the axial direction, the electron energization occurs mainly through an efficient collisionless mechanism caused by the interaction of electrons in the vicinity of an expanding sheath with the sheath motion, which is affected by the excitation of the surface modes of both types. The generation of energetic electron populations as a result of such mechanisms is shown directly. Although some aspects of the underlying physics were demonstrated in the previous literature with other models, the PIC method is advantageous for the predictive modeling due to a complex interplay between the surface mode excitations and the nonlocal physics of the corresponding type of plasma discharges operated at low pressures, which is hard to reproduce in other models realistically.

Collective excitation of Bose–Einstein condensate of 23Na via high-partial wave Feshbach resonance

We experimentally observe the collective excitation (called surface-mode excitation) of Bose–Einstein condensate of 23Na by ramping the external magnetic field across the high-partial wave magnetic Feshbach resonance corresponding to vary the atomic interaction. We check the collective surface mode excitation of state for the three d-wave and three g-wave Feshbach resonances below 600 G and find that only two d-wave resonances present the strong excitation, another d-wave resonance only creates a weak excitation, and all g-wave resonances do not, which reflects the strength of these magnetic Feshbach resonances. For the collective excitation, the excitation of surface modes along the axial weak-confinement and radial strong-confinement of optical dipole trap shows different characteristics. We also study the lifetime of the collective oscillation by measuring the damping rate of the oscillation amplitude, which is caused by the mechanisms of dephasing effect and collisional relaxation. This excitation method gives us a new tool for investigating the properties of ultracold quantum gases without changing the trap frequencies.

Near-field radiative heat transfer management by subwavelength plasmonic crystals

Engineering the heat flux between two surfaces kept at different temperatures relies on the ability to tailor the dispersion of modes sustained by the system. Metasurfaces made of ordered arrays of subwavelength spherical nanoparticles have an optical response that depends not only on materials but also on their geometrical parameters. Our system is modeled by using an effective medium approximation allowing the homogenization of individual metasurfaces and replacing them with anisotropic layers. Excitation or suppression of surface and hyperbolic modes can be controlled by means of different degrees of freedom offered by the metasurfaces. By leveraging this flexibility, we theoretically show that the near-field radiative heat transfer between two such metasurfaces can be effectively geometrically tuned.

Mid-infrared sensor based on resonance excitation of graphene plasmon polariton-coupled Bloch surface modes at the interface of anisotropically truncated one-dimensional ternary photonic crystal

In this article, we have theoretically demonstrated the sensor performance in mid-IR frequency domain by resonance excitation of Bloch surface modes through energy coupling with graphene surface plasmon polaritons in truncated one-dimensional (1D) ternary photonic crystal (TPC) under Kretschmann configuration coupling technique. The involvement of optical anisotropy via graphene monolayer enhances light-matter interaction thus enabling excellent control over the resonance wavelength (RW) of the excited Bloch surface modes (EBSMs). For incident transverse magnetic-polarized electromagnetic waves, the band structure, reflection spectra and field profile are computed using 2 × 2 transfer matrix method. The control over the RW has been established by examining the effect of variation of different model parameters like thickness of truncation layer, refractive index of external medium, temperature, chemical potential and the periodicity of TPC. The variations in periodicity result in the expansion of the range of confinement of EBSMs in photonic band gaps from 19 THz to 65 THz . We proved that the variation in temperature between 300 ∘ K and 500 ∘ K confines the EBSMs with high coupling efficiency (CE). Also, at room temperature the best achievable quality factor (QF) is 1421 and a high Q.F. of 8170 is attained at 260 ∘ K .

Excitation of Airborne Acoustic Surface Modes Driven by a Turbulent Flow

This experiment demonstrates the generation of trapped acoustic surface waves excited by a turbulent flow source through the coupling of pressure fluctuations at the interface between an acoustic metamaterial and a flow environment. The turbulent flow, which behaves as a stochastic pressure source, was interfaced with an acoustic metasurface waveguide stationed in a quiescent environment via a single Kevlar-covered cavity, which ensured no significant disturbance to the flow. The metasurface waveguide produced an acoustic surface mode through evanescent diffractive coupling of the pressure field. This acoustic mode was trapped at the quiescent surface, with its mode dispersion determined by the surface geometry. The results of two different metasurface geometries are discussed: 1) a slotted cavity array, and 2) a meander connected cavity array, with each demonstrating a different trapped surface wave characteristic. Fourier transform and correlation analyses of spatially resolved temporal acoustic signals, measured close to the metamaterial surface, were used to construct the frequency- and wavevector-dependent acoustic mode dispersion. The results demonstrate that the flow can be used to excite acoustic surface modes and that their mode dispersion may be tailored toward realizing novel control of turbulent flow through acoustic-flow interactions.

Mid-infrared Biosensor Based on Bloch Surface Mode Excitation in Truncated One-Dimensional Ternary Photonic Crystal Under Kretschmann Configuration

We have theoretically investigated the performance of mid-infrared (mid-IR) sensitive biosensor constructed by truncated one-dimensional (1D) ternary photonic crystal (TPC) on the prism base under Kretschmann configuration coupling technique. The control over resonance wavelength (RW) of the excited Bloch surface modes (EBSMs) for the case of incident transverse magnetic polarized electromagnetic waves (EMWs) has been shown. The band structures and the reflection spectra of the considered model are computed using transfer matrix method (TMM). The variation in different design parameters of the theoretically constructed biosensor, such as thickness of the truncation layer, refractive index of the sensing medium, and number of unit cells in photonic crystal (PC), allows us to have good control over the resonance wavelengths in fixed frequency regime of the PBGs. The excitation of BSMs is characterized by a dip in the reflectance spectra. The measured sensitivity of the sensor is $${10}^{-3}$$ orders of magnitude sensitive to the change in $${d}_{{c}}$$ and $${d}_{{t}}$$ values.

Magneto-Optical Control of Radiation in Photonic Crystal Structures via the Excitation of Surface Modes

It is shown that the intensity of magneto-optical effects is enhanced by structures containing photonic crystal layers and an iron garnet ferromagnetic dielectric as they support the excitation of quasi-surface Tamm modes at the boundary between the two. Such structures have high Q-factor resonances and can enhance the transverse magneto-optical Kerr effect. Up to 100% magneto-optical modulation can be achieved due to near-surface localization of the electromagnetic field.

Seismic metasurfaces on porous layered media: Surface resonators and fluid-solid interaction effects on the propagation of Rayleigh waves

Seismic surface wave mitigation using metamaterials is a growing research field propelled by intrinsic theoretical value and possible application prospects. Up to date, the complexity of site conditions found in engineering practice, which can include layered stratigraphy and variable water table level, has been discarded in the development of analytical frameworks to favor the derivation of simple, yet effective, closed-form dispersion laws. This work provides a further step towards the analytical study of “seismic metasurfaces” in real site conditions considering the propagation of Rayleigh waves through a layered porous substrate equipped with local resonators. To this aim, we combine classical elasticity theory, Biot’s poroelasticity and an effective medium approach to describe the metasurface dynamics and its coupling with the poroelastic substrate. The developed framework naturally includes simpler configurations like seismic metasurfaces atop homogeneous dry or saturated soils. Apart from known phenomena like wave-resonance hybridization and surface wave band gaps, we predict the existence of an extended frequency range where surface waves are attenuated due to energy leakage in the form of slow pressure waves, as a result of the fluid-solid interaction. Besides, we demonstrate that the surface wave band gap and the related surface-to-shear wave conversion is robust to variations in the water table level. Conversely, when the dry and saturated layers have different material parameters, for example, due to different porosity ratios, the surface-to-shear wave conversion can be accompanied by the excitation of higher-order surface modes, which remain channeled below the metasurface. These analytical findings, augmented and confirmed by numerical simulations, evidence the importance of accounting for fluid-solid interaction in the dynamics of seismic metasurfaces.

Coaxial and surface mode excitation by an ICRF antenna in large machines like DEMO and ITER

A study of radio frequency (RF) field excitation in the edge of the plasma of DEMO is performed by means of the semi-analytic coupling code ANTITER II. The modeling uses the designed antenna and a reference low coupling density profile of ITER. The results show the existence of coaxial modes propagating between the wall and the Alfvén resonance region where surface modes are excited leading to large standing wave patterns all around the machine. The excitation of these modes can be strongly reduced for a strap current distribution of the antenna array which fulfills the two conditions and (Ii: current of strap i, Si: toroidal strap position). These conditions are satisfied by the triple strap antenna of AUG that has allowed successful ICRH operation in a W coated machine. They are also achieved for the (0ππ0) phasing of the four columns of two poloidal triplets of radiating straps constituting the ITER antenna array. Ways to further optimize the cancellation of these edge modes are also investigated.

Generation of orbital angular momentum modes via holographic leaky-wave metasurfaces

Recently, electromagnetic (EM) waves carrying orbital angular momentum (OAM) has received considerable attention in increasingly many different realms, such as communication systems, super-resolution imaging, optical communications and quantum state manipulation. In this paper, two different kinds of two dimensional (2-D) holographic leaky-wave metasurfaces with a single OAM mode at a single frequency (18 GHz) are introduced through designs and experiments. The classic leaky-wave and a microwave holography theorem are combined to construct the holographic leaky-wave metasurfaces. The leaky wave metasurfaces-based holographic concept are implemented with isotropic artificial surface impedances and made of hexagonal metallic patches. By varying the size of the metallic patches, the effective impedances may be realized. The monopole launchers are utilized for the excitation of TM surface mode, whereby their wave functions can be approximated by the Hankel function of the second kind. The objective waves represented by the desired beams carrying different orbital angular momentum modes. Electromagnetic full-wave simulations and experimental measurements have been performed to substantiate the proposed method.

Excitation of Low-Frequency Surface Modes in the Plasma Layer Under the Action of Two-Frequency Laser Radiation

We construct the theory of generation of low-frequency surface eigenmodes in a rarefied plasma layer under the action of two-frequency laser radiation. We show that a significant increase in the energy of the plasma-layer low-frequency eigenmode occurs under resonance conditions, where the frequency of the mode coincides with the difference in laser frequencies. We establish that under the resonance conditions the energy flux density carried by the low-frequency eigenmode can be comparable to the laser-radiation intensity.

Tunable wave localization for Tamm modes in graphene-based photonic crystals

In this work, we demonstrate the existence of TE polarized surface Bloch modes localized at the boundary of semi-infinite graphene-based photonic crystal and homogeneous dielectric. Imposing an alternate low-high doping level on graphene layers along the multilayer, it is possible to open bandgaps where mode localization can be achieved for TE polarization. The surface modes reported here exist in the low terahertz regime with a corresponding long decaying length and close to allowed band. Due to the tunable gap characteristics of the periodic structure proposed, an on-off switch for TE polarized surface Bloch modes can be achieved. We proved that TE polarized surface mode excitation can be achieved by attenuated total reflection technique in the Otto configuration, making them suitable for experimental probing. Fermi levels considered in this work lie in the range 0.2eV<μ<0.8eV, which is experimentally achievable in electrical gated configurations.

Experimental demonstration of broadband light trapping by exciting surface modes of an all-dielectric taper

We design an all-dielectric taper and then excite its surface modes by illuminating a plane wave upon the taper to achieve broadband light trapping spanning from 20 to 100 GHz. Via Lewin’s theory, such excitation of surface modes could be analogous to “trapped rainbow”, i.e., activation of negative Goos-Hänchen effect within a negative refractive waveguide. To further reinforce this statement, the corresponding power flow distributions within the all-dielectric taper are recorded in finite-integration time domain simulation and suggest that a chromatic incident pulse is indeed trapped at different critical thicknesses of the taper, a character of the negative refractive waveguide. Finally, the transmittance is measured and compared to the simulated ones. The two followed the similar trend.

Near-Field Thermal Radiation between Nanostructures of Natural Anisotropic Material

While nanostructures exhibit remarkable optical and thermal properties, including super-Planckian emission beyond the black-body limit, they are usually made of isotropic materials; anisotropic nanostructures are rarely considered, despite their potential for applications. Investigating near-field thermal radiation between gratings of natural anisotropic graphite, the authors find that nanopatterning enhances heat flux dramatically, due to the excitation of anisotropic surface modes not possessed by plain graphite. This deepened understanding could open routes to more efficient thermal management and thermophotovoltaic energy conversion.

Polarization-Field Influence on Light-Ion Channeling in Carbon Nanotubes

The polarization potential of the interaction between fast charged particles and multilayer-nanotube walls, which arises due to the excitation of surface modes of electromagnetic oscillations, is estimated. A nanotube is interpreted as a set of concentric cylindrical layers with specified dielectric properties. Formulas for calculating the polarization-field potential are derived as applied to a charged particle moving parallel to the multilayer nanotube axis. Numerical calculations are performed in the single-mode approximation of a dielectric function. The polarization forces arising in a multilayer nanotube are compared with those corresponding to the single-layer one. It is demonstrated that, under certain conditions, existing external layers can sufficiently affect the polarization forces acting on charged-particle channeling in a nanotube. At the same time, model calculations indicate that outer layers exert an insignificant influence on the polarization losses of the channeling-particle energy.

Tamm-plasmon polaritons in one-dimensional photonic quasi-crystals.

We present an investigation to ascertain the existence of Tamm-plasmon-polariton-like modes in one-dimensional (1D) quasi-periodic photonic systems. Photonic bandgap formation in quasi-crystals is essentially a consequence of long-range periodicity exhibited by multilayers and, thus, it can be explained using the dispersion relation in the Brillouin zone. Defining a "Zak"-like topological phase in 1D quasi-crystals, we propose a recipe to ascertain the existence of Tamm-like photonic surface modes in a metal-terminated quasi-crystal lattice. Additionally, we also explore the conditions of efficient excitation of such surface modes along with their dispersion characteristics.

Modeling of Resonant Surface Wave Excitation in a Large CCP Reactor

Low-pressure plasmas produced in capacitively coupled plasma (CCP) reactors are used extensively in the plasma processing industry. Although the physics of small scale CCP reactors operated under low pressure is fairly well understood, larger scale CCP reactors constructed to meet the industrial needs often exhibit surprising behavior in experiments, which affects discharge parameters of importance to the industrial applications. This can be explained by the resonant excitation of surface modes that a CCP reactor can support. Limitations of the drift-diffusion approximation in description of the surface mode phenomena in low-pressure plasmas are discussed. It is shown that a particular class of the surface modes can be adequately modeled using the electrostatic approximation if the finite electron inertia is taken into account. Continuing a previous work [1] , the present study demonstrates how such modes are triggered and how they affect the radial plasma density profile uniformity. This is done by using a self-consistent 2d3v electrostatic implicit PIC/MCC code. It is shown that in highly collisional plasmas, the nonuniformities caused by the modes disappear due to the large mode damping.