Localized Surface Modes Research Articles

Акустические эффекты на волноводных микроструктурах в кремнии

Basic waveguide structures in anisotropic monocrystalline silicon medium are investigated for the existence of localized acoustic modes. The geometries of the structures were determined by technological methods and the practice of anisotropic etching, which guarantees reproducible high-quality structures in terms of geometric and crystallographic parameters. The acoustic effects were modeled by semi-analytical finite element method in the problems on eigenvalues and on the response to a harmonic traction source. Edge and surface localized modes were found. The spectral composition of the localized modes was interpreted on the basis of the surface acoustic wave slowness curves. Pseudomodes, highly localized leakage waves, were detected in a number of considered structures. The obtained results allow to evaluate promising geometrical configurations for further detailed study of the complete scope of dispersion properties of both single waveguides and their finite set.

Relative humidity sensor based on a silver nano-grating made from DVD discs.

A relative humidity sensor based on a silver nano-grating was proposed. By stripping and cleaning commercially available CD and DVD discs, polycarbonate plates with different grating periods are obtained. These plates as templates are coated with a layer of sputtered silver film to form silver nano-gratings, which exhibit refractive index sensing sensitivities of 517nm/RIU and 742.9nm/RIU, respectively. The finite-difference time-domain simulation results conform the excited surface plasmon polariton modes and localized surface plasmon modes on the nano-grating. By spin coating a layer of humidity-sensitive porous silica with optimized thickness, the silver nano-grating shows a relative humidity detection sensitivity of 0.23nm/%RH.

First-principles investigation of a symmetry incompatible adsorbate-substrate system: PF3 on Cu(001)

The structural relaxation of the adsorption of PF3 on Cu(001), resulting in a c(4 × 2) overlayer, was studied using first-principles Density Functional Theory (DFT) calculations in the generalized gradient approximation (GGA). Unlike previous experimental results, we found no tilting of the molecules on the surface, with the C3v axis of PF3 perfectly perpendicular to the surface. Then using Density Functional Perturbation Theory (DFPT), we calculated the phonon spectra of the Cu (001): PF3 c(4 × 2) at two high symmetry points in the surface Brillouin zone, Γ¯ and M¯. By plotting the projected density of states, we found modes localized on the surface and determined how they changed upon adsorption. Our results indicate considerable mixing with bulk phonon modes at certain back-folded q-points of the bare-surface Brillouin zone. These modes have dominant vertical vibration on the topmost layer. Modes with surface covered atoms that move with the molecule as a unit can undergo softening up to 1.5 meV. However, modes with dominant motion on the second layer remain unperturbed. In addition, we studied molecular external modes (i.e. modes that arise from free motions of the PF3 molecule) that are in the range of bulk and surface modes of Cu. Depending on the symmetry of the underlying structure, PF3 vibrations can remain isolated and induce a localized surface mode or mix with bulk modes and become a resonance.

Plasmonic Tamm states in periodic stubbed MIM waveguides: analytical and numerical study

We investigate both analytically and numerically the existence of localized surface modes, the so-called plasmonic Tamm states (PTSs), in a new and versatile platform based on a periodic array of metal-insulator-metal (MIM) stubs grafted along a MIM waveguide. By considering a semi-infinite structure in which we modify the length of the segment at the surface, we show the existence of surface states inside the bandgaps of the periodic structure and investigate the dependence of the localized modes as a function of the geometrical parameters and the boundary conditions applied at the surface. Three types of surface boundary conditions are considered, namely, two limiting cases of zero surface impedance (or perfect electric conductor), infinite surface impedance (or perfect magnetic conductor), and a third case where the structure is in contact with a real metal. In the latter case, we show that the existence of the interface state can be demonstrated based on topological arguments using the Zak phase. We also demonstrate that if a finite size comb-crystal is vertically grafted along a horizontal waveguide, the PTSs can be detected from the dips in the amplitudes of transmission and reflection coefficients as well as from the peaks in their delay times and the local density of states (LDOS). Our theoretical study is first performed analytically with the help of a Green’s function method, which allows the calculation of the dispersion relations of the bulk and surface modes and the LDOS, as well as the transmission and reflection coefficients of the plasmonic comb-like structure. Then, these results are confirmed by a numerical simulation utilizing a 2D finite element method. Besides providing a deep physical analysis of the PTSs, our work demonstrates the capability of the analytical method as a predictive approach in more complex structures. The proposed designs in this paper can be useful to realize highly sensitive plasmonic nanosensors.

Surface localized phonon modes at the Si(553)-Au nanowire system

The vibrational properties of the Si(553)-Au surface are studied by Raman spectroscopy and ab initio calculations. A multitude of surface localized phonon modes with wave number below 200 ${\mathrm{cm}}^{\ensuremath{-}1}$ is experimentally observed, along with two modes at about 400 ${\mathrm{cm}}^{\ensuremath{-}1}$. Atomistic models within density functional theory allow to assign the low-energy spectral features to vibrations within the Au chain, while the Raman signatures at higher energies are mostly localized at the Si step edge. The Raman activity of nominally silent modes associated with a large charge transfer between Au chain and Si step edge states is explained by scattering at charge density fluctuations. Temperature-dependent measurements reveal specific mode shifts that are discussed in terms of a recently proposed order-disorder phase transition. The presence of model-specific displacement patterns allows us to identify the structural models compatible with the measured spectra at low and at room temperature.

Acoustic Tamm states in slender tubes

We present an analytical and numerical study of the possibility of existence of surface localized modes, the so-called Tamm states, in a one-dimensional (1D) comb-like phononic crystal (PnC). The structure is made out of periodic array of stubs of lengths d2 grafted along a waveguide and separated from each other by a tube of length d1. We show the existence of surface modes for the semi-infinite structure. In particular, when one considers two complementary semi-infinite systems obtained by cutting the infinite one into two parts, we obtain one surface mode per gap induced by the surface of the two complementary systems. Furthermore, we demonstrate that these surface modes can be detected from the maxima and minima of the transmission spectrum, when the finite structure is grafted vertically along a homogeneous acoustic waveguide. That means that one can observe experimentally this type of modes for acoustic waves in slender tubes. These results may find many practical applications in noise control and highly sensitive PnC sensors.

Surface modes in plasmonic stubbed structures

We present an analytical and numerical study about the existence of surface localized modes, known as Tamm states, in a one-dimensional (1D) comb-like plasmonic band gap structure. Surface plasmon polaritons (SPPs) waveguides with coupled resonators have been widely studied in recent years, because of their potential applications in highly integrated optical circuits. The system studied here is composed of an infinite 1D waveguide, along which stubs of length d1 are grafted periodically with spacing period d2. The analytical study has been performed by means of the Green’s function method which allows the calculation of the dispersion relations of the bulk, surface states of the plasmonic structure and the transmission coefficient. The band structure, as well as the transmission spectrum exhibit passbands separated by stopbands. The surface modes inside the gaps of the semi-infinite structure can be introduced by a defect at its surface. The analytical results are confirmed by numerical simulation using finite element method via Comsol Multiphysics software. These structures can be used to realize highly sensitive plasmonic sensors.

Graphene-Based One-Dimensional Terahertz Phononic Crystal: Band Structures and Surface Modes.

In this paper, we provide a theoretical and numerical study of the acoustic properties of infinite and semi-infinite superlattices made out of graphene-semiconductor bilayers. In addition to the band structure, we emphasize the existence and behavior of localized and resonant acoustic modes associated with the free surface of such structures. These modes are polarized in the sagittal plane, defined by the incident wavevector and the normal to the layers. The surface modes are obtained from the peaks of the density of states, either inside the bulk bands or inside the minigaps of the superlattice. In these structures, the two directions of vibrations (longitudinal and transverse) are coupled giving rise to two bulk bands associated with the two polarizations of the waves. The creation of the free surface of the superlattice induces true surface localized modes inside the terahertz acoustic forbidden gaps, but also pseudo-surface modes which appear as well-defined resonances inside the allowed bands of the superlattice. Despite the low thickness of the graphene layer, and though graphene is a gapless material, when it is inserted periodically in a semiconductor, it allows the opening of wide gaps for all values of the wave vector (parallel to the interfaces). Numerical illustrations of the band structures and surface modes are given for graphene-Si superlattices, and the surface layer can be either Si or graphene. These surface acoustic modes can be used to realize liquid or bio-sensors graphene-based phononic crystal operating in the THz frequency domain.

Metal-insulator-metal nanoresonators – strongly confined modes for high surface sensitivity

Abstract Photonic and plasmonic refractive index sensors are able to detect increasingly smaller refractive index changes and concentrations of clinically relevant substances. They typically exploit optical resonances and aim to maximise the field overlap with the analyte in order to achieve high sensitivity. Correspondingly, they operate on the basis of maximizing the bulk sensitivity, which favours spatially extended modes. We note that this strategy, counter-intuitively, is not necessarily suitable for detecting biomolecules and one should focus on the surface sensitivity instead. Here, we show that by confining light tightly in metal-insulator-metal (MIM) nanoresonators, the surface sensitivity is significantly increased despite a clear decrease in bulk sensitivity. In particular, we experimentally show the operation of third order MIM resonators which support both extended surface plasmon polariton (SPP) modes and localized MIM modes. We are able to demonstrate that the MIM mode has a sensitivity of 55 nm/RIU to a 10 nm layer, which is approximately twice as high as that of the SPP mode. Overall, our work emphasizes the importance of the surface sensitivity over the more commonly used bulk sensitivity and it shows a novel approach for improving it. These insights are highly relevant for the design of next generation optical biosensors.

DFT and thermodynamics calculations of surface cation release in LiCoO2

While complex metal oxides (CMOs) such as LiCoO2 (LCO) are currently used in multiple electronic devices, their environmental impacts are not well understood. In this work, we apply density functional theory (DFT) and thermodynamics modeling to study LCO surface transformations. We performed Raman studies on bulk LCO, and compared experimental and computational results. Full vibrational analysis of the model LCO surfaces show localized surface modes that are distinct from bulk, varying in Li and OH surface terminations. Central to this study are calculations to assess the dependence of the DFT + thermodynamics methodology on computational parameters, such as the choice of the exchange-correlation functional, and model geometry, specifically varying slab thickness and supercell dimensions. We discuss how the results can be used to establish upper- and lower-bounds for favorable surface cation vacancy formation under varying pH conditions. The model predicts that at a pH of 7, up to 16% of surface Co will undergo dissolution. We go on to discuss how these model results relate to experimental dissolution studies. We also extrapolate how our results can provide useful insights to guide the (re)design of CMOs with tailored ion release behavior.

Polarization hybridization of surface waves on anisotropic metasurface

In this work we conduct numerical and experimental characterization of mixed TE-TM polarized surface waves on an anisotropic metasurface in the super high frequency range (from 2 to 8 GHz). We investigate the evolution of equal frequency contours for eigenmodes. The drastic changes of the equal frequency contours shapes, called topological transitions, are observed. Finally, we experimentally demonstrate the polarization hybridization of two localized surface modes.

Anderson localization of light in dimension d−1

Localization of electromagnetic waves in disordered potentials is prevented by polarization terms, so only light scattering systems of dimensions $d=1$ and $2$ with scalar properties exhibit light localization. We here show that this result breaks down due to the existence of surface modes, which possess lower-dimensional scattering properties. In particular, vectorial waves in $3D$ ($2D$) presents surface localized modes with scaling consistent with localization $2D$ ($1D$)

A high-performance refractive index sensor based on a triangular-lattice photonic crystal with defect and surface modes

The refractive index sensing capabilities of a 2D triangular-lattice photonic crystal (PhC) with defective and surface modes are studied. These are composed of silicon columns arranged in a liquid environment. Through changing the rods’ shape on the PhC’s surface from a circle to an ellipse and optimizing the sizes of two adjacent ellipses, the surface localized mode can be generated. Combining the central defective mode with surface modes, an ultra-small three-working-wavelength PhC refractive index sensor is proposed. The shift of the resonant wavelengths influenced by the slight alteration of the protein’s thickness attached by the special rods is studied, and it is found that a minimum alteration of the protein’s thickness of about 4 nm can be distinguished, and a maximum refractive index sensitivity of 0.516 is obtained. Three kinds of analytes can be detected in real-time and at the same time, which are bounded around the input/output surfaces and the central rods through biological specific binding.

Extracting torsional band gaps and transient waves in phononic crystal beams: Method and validation

Due to an absence of proper measuring techniques, torsional waves/vibrations in one-dimensional phononic crystals (PCs) or elastic metamaterials are usually theoretically or numerically studied without experimental validations. This work proposes a measurement method capable of extracting torsional band gaps and transient torsional waves from the total responses of PC and metamaterial beams. Specifically, a pair of point-wise self-demodulated fiber Bragg grating (SFBG) displacement sensors, which can be arranged at points very close to each other on a thin PC beam, is utilized along with the proposed extraction method. The SFBGs are symmetrically set up at two locations about the central line on the surface of the PC beam. The torsional band gaps and the transient torsional waves are extracted from the differential responses of the two SFBGs, in which the common bending waves are cancelled out. In addition to the torsional behaviors, the bending waves can also be extracted from the summation responses of the two SFBGs. We first examine the dynamic linearity of the SFBGs. Then, steady-state detection of the torsional and bending band gaps of a PC beam is performed. Finally, a cantilever PC beam subjected to an eccentric steel ball impact, whose loading history is recorded by a pair of polyvinylidene fluoride (PVDF) films, is investigated to demonstrate the extraction of the torsional/bending bandgaps and waves from the early short time transient responses. The feasibility of the proposed torsional (or bending) extraction method is justified by the method of reverberation-ray matrix (MRRM), which is capable of providing “pure” torsional (or bending) behaviors of the PC beam. With the proposed extraction method, we also observe surface localized modes in the transmission of the PC beam, which are strongly related to the arrangement of the constituent components with different impedances in the unit cells. We believe that the proposed extraction method for torsional wave, along with the SFBG sensing technique and the MRRM, can provide future researchers with measurement and analysis method to validate their designs of torsional phononic crystals or metamaterials.

Large-scale, broadband absorber based on three-dimensional aluminum nanospike arrays substrate for surface plasmon induced hot electrons photodetection

Performance of plamson induced hot electrons-based photodetectors largely relies on the photon absorption capability. To improve the optical absorption, many perfect absorbers based on the periodic metallic nanostructures have been designed and fabricated through low-throughput, costly and time-consuming lithographic processes, which seriously limit the future potential applications of plasmonic hot electrons optoelectronics devices. Here, a large-scale, broadband absorber consisting of ITO film, ZnO layer, Au film and Al nanospike array substrate was designed and fabricated for hot electrons-based photodetection. The new designed absorber’s absorptivity can be up to 70% in the broad wavelength range from 400 nm to 800 nm (even up to 90% in the wavelength range from 400–550 nm) and most of the absorption comes from the Au film, which is effective for the generation of hot electrons. The enhanced broadband absorption is ascribed to the surface plasmon polariton mode and localized surface plasmon resonance mode supported by the nanospike arrays. The influence of geometry and material parameters on the optical absorption properties is also specifically investigated through numerical simulation. The efficient and broadband absorption of a nanospikes device results in a much larger photocurrent compared with that of a planar reference device. Our approach, which is compatible with large-scale manufacturing, paves the way for the practical implementation of hot electrons-based optoelectronic devices.

Surface vibrations in the T4 and H3 Pb phases on Si(111)

We present here a combined experimental Raman spectroscopy and ab initio theoretical study of the vibrational modes of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ reconstructed SIC phase of Pb on Si(111) and discuss their relation to the atomic surface structure. The Raman response of the surface localized vibrational modes, in particular, is identified in the low-frequency spectral range (down to $15\phantom{\rule{3.33333pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$. We demonstrate that Raman spectroscopy is a very powerful approach to test atomic structures of surfaces and a valuable complement to standard surface analytics. While the calculated spectra of H3 and T4 are too similar to allow a discrimination of these phases, the good overall agreement to the measured Raman spectra enables a classification of the observed vibrational modes.

Metasurfaces Atop Metamaterials: Surface Morphology Induces Linear Dichroism in Gyroid Optical Metamaterials

Optical metamaterials offer the tantalizing possibility of creating extraordinary optical properties through the careful design and arrangement of subwavelength structural units. Gyroid-structured optical metamaterials possess a chiral, cubic, and triply periodic bulk morphology that exhibits a redshifted effective plasma frequency. They also exhibit a strong linear dichroism, the origin of which is not yet understood. Here, the interaction of light with gold gyroid optical metamaterials is studied and a strong correlation between the surface morphology and its linear dichroism is found. The termination of the gyroid surface breaks the cubic symmetry of the bulk lattice and gives rise to the observed wavelength- and polarization-dependent reflection. The results show that light couples into both localized and propagating plasmon modes associated with anisotropic surface protrusions and the gaps between such protrusions. The localized surface modes give rise to the anisotropic optical response, creating the linear dichroism. Simulated reflection spectra are highly sensitive to minute details of these surface terminations, down to the nanometer level, and can be understood with analogy to the optical properties of a 2D anisotropic metasurface atop a 3D isotropic metamaterial. This pronounced sensitivity to the subwavelength surface morphology has significant consequences for both the design and application of optical metamaterials.

Calculations of surface spin dynamics associated with biquadratic exchange and uniaxial anisotropy field in (S > 1) heisenberg ferromagnet by means of matching procedure.

Calculations within framework of both non-interacting spin-wave theory and random phase approximation have been carried out to investigate elementary magnetic excitations by scanning the occurrence of localized modes associated with surface spin dynamics. Mean field approximation and single spin-wave function theory in the context of semi-infinite Heisenberg ferromagnet are exploited. We exploit the mathematical formalism of matching method which starting from secular equation of spin motion established for the bulk of semi-infinite structure will make it possible to describe the symmetry properties of propagating (evanescent) precessional fields in parallel (perpendicular) direction to surface terrace. The localized states associated with surface are obtained by matching the properties of quantized propagating modes with evanescent modes occurring in vicinity of surface region. This mathematical procedure allows treating the continuity equations for spin precessional field amplitude operating in the border bulk media-surface region from their special suitable symmetry properties related to those induced by the broken translational symmetry arising from the existence of surface. The solutions are described considering the presence of uniaxial anisotropy field and biquadratic exchange as well as magnetic anisotropy field. The focus is on the existence, the nature as well as on the form and stability of localized surface modes. Also, obtained solutions are discussed qualitatively by observing the response of energy branches to the variation of strength of biquadratic exchange term and surface anisotropy field as well as magnetic exchange parameters occurring on the surface domain. Numerical examples of the localized modes energies associated with the surface are shown and they are found to exhibit various effects due to the strength of biquadratic exchange and magnetic anisotropy field under certain circumstances. The analytical formulation exposed in this work and its methodology based solely on the symmetry properties as well as the qualitative agreement describing our results with those obtained in the literature provide a considerable contribution to the surface spin dynamics calculations under the influence of biquadratic coupling and uniaxial anisotropy term. Moreover, the results obtained through matching procedure make it possible to remove an ambiguity concerning the existing disparities in analysis and interpretation of the main results already reported in literature.

The influence of the two-dimensional sinusoidal gratings on the near-field radiative heat flux between two doped silicon films

The near-field radiative heat flux between two two-dimensional sinusoidal grating surfaces made of doped-silicon is investigated by the finite difference time domain (FDTD) method. The parameter Λ, which is defined as the ratio of the amplitude to the period of the two-dimensional sinusoidal grating surface, is used to characterize the surface feature. When the amplitude is much smaller than the period, the surface effect is not significant to modify the near-field radiative heat flux between two smooth films. However, when the amplitude is comparable with the period, the near-field radiative heat flux between two smooth films can be enhanced remarkably. The coupling of the localized surface modes between the may be responsible for the variation of the near-field radiative heat flux when the parameter Λ is changed. Moreover, the effect of the lateral displacement of the films on the near-field radiative heat flux is also studied. The results indicate that the near-field radiative heat flux will achieve the maximum value when the peaks of the sinusoidal gratings are aligned. This work will be valuable to research on contactless thermal management in nanoscale.

Surface modes in plasmonic Bragg fibers with negative average permittivity.

We investigate surface modes in plasmonic Bragg fibers composed of nanostructured coaxial cylindrical metal-dielectric multilayers. We demonstrate that the existence of surface modes is determined by the sign of the spatially averaged permittivity of the plasmonic Bragg fiber, ε¯. Specifically, localized surface modes occur at the interface between the cylindrical core with ε¯<0 and the outermost uniform dielectric medium, which is similar to the topologically protected plasmonic surface modes at the interface between two different one-dimensional planar metal-dielectric lattices with opposite signs of the averaged permittivity. Moreover, when increasing the number of dielectric-metal rings, the propagation constant of surface modes with different azimuthal mode numbers is approaching that of surface plasmon polaritons formed at the corresponding planar metal/dielectric interface. Robustness of such surface modes of plasmonic Bragg fibers is demonstrated as well.