Surface Resonance Modes Research Articles

Theoretical computation of the phononic vibrational dynamics of Au3Pd, AuPd, and AuPd3 nanostructures at the [AunPdm]/Au(110) bimetallic ordered surface alloy systems

In this work we develop a calculation for the surface phononic dynamical properties of the three ordered bimetallic surface alloy nanostructures [AunPdm]/Au(110) where [AunPdm] = Au3Pd, AuPd, and AuPd3, for the appropriate concentrations of palladium atoms in the proportions 25 %, 50 %, and 75 %, respectively, at the surface boundaries. They are ordered equilibrium structures, which have been characterized, by Atanasov and Hou (2009). Using the phase field matching theory (PFMT), and the Green's function formalism, the surface phonon dispersion curves are computed, in the harmonic approximation, along the high-symmetry directions ΓX, XS, SY and YΓ of the Brillouin zone of the corresponding surface alloy nanostructure. The local vibration densities of states (LDOS) are also calculated for these systems. The results reveal that the number and the dynamic behavior of the surface phonon and resonance modes, are very sensitive to the concentration of the palladium adsorbate atoms in the [AunPdm] nanostructures. The number and characteristics of these modes are modified at the surface and shifted to higher energies as the concentration increases. These phononic results can greatly contribute to our understanding of the dynamic effects underlying potential technological applications of these surface alloy nanostructures.

Near-Field Radiative Heat Transfer between Disordered Multilayer Systems

Near-field radiative heat transfer (NFRHT) research is an important research project after a major breakthrough in nanotechnology. Based on the multilayer structure, we find that due to the existence of inherent losses, the decoupling of hyperbolic modes (HMs) after changing the filling ratio leads to suppression of heat flow near the surface mode resonance frequency. It complements the physical landscape of enhancement of near-field radiative heat transfer by HMs and more surface states supported by multiple surfaces. More importantly, considering the difficulty of accurate preparation at the nanoscale, we introduce the disorder factor to describe the magnitude of the random variation of the layer thickness of the multilayer structure and then explore the effect on heat transfer when the layer thickness is slightly different from the exact value expected. We find that the near-field radiative heat flux decreases gradually as the disorder increases because of interlayer energy localization. However, the reduction in heat transfer does not exceed an order of magnitude, although the disorder is already very large. At the same time, the regulation effect of the disorder on NFRHT is close to that of the same degree of filling ratio, which highlights the importance of disordered systems. This work qualitatively describes the effect of disorder on heat transfer and provides instructive data for the fabrication of NFRHT devices.

Spectral tuning of Bloch Surface Wave resonances by light-controlled optical anisotropy

Abstract Fostered by the recent advancements in photonic technologies, the need for all-optical dynamic control on complex photonic elements is emerging as more and more relevant, especially in integrated photonics and metasurface-based flat-optics. In this framework, optically-induced anisotropy has been proposed as powerful mean enabling tuning functionalities in several planar architectures. Here, we design and fabricate an anisotropic two-dimensional bull’s eye cavity inscribed within an optically-active polymeric film spun on a one-dimensional photonic crystal sustaining Bloch surface waves (BSW). Thanks to the cavity morphology, two surface resonant modes with substantially orthogonal polarizations can be coupled within the cavity from free-space illumination. We demonstrate that a dynamic control on the resonant mode energies can be easily operated by modulating the orientation of the optically-induced birefringence on the surface, via a polarized external laser beam. Overall, reversible blue- and red-shifts of the resonant BSWs are observed within a spectral range of about 2 nm, with a moderate laser power illumination. The polymeric structure is constituted by a novel blend of an azopolymer and a thermally-sensitive resist, which allows a precise patterning via thermal scanning probe lithography, while providing a significant structural integrity against photo-fluidization or mass-flow effects commonly occurring in irradiated azopolymers. The proposed approach based on tailored birefringence opens up new pathways to finely control the optical coupling of localized surface modes to/from free-space radiation, particularly in hybrid organic–inorganic devices.

Shear Modes in a 2D Polar Metal.

Low-frequency shear and breathing modes are important Raman signatures of two-dimensional (2D) materials, providing information on the number of layers and insights into interlayer interactions. We elucidate the nature of low-frequency modes in a 2D polar metal-2D Ga covalently bonded to a SiC substrate, using a first-principles Green's function-based approach. The low-frequency Raman modes are dominated by surface resonance modes, consisting primarily of out-of-phase shear modes in Ga, coupled to SiC phonons. Breathing modes are strongly coupled to the substrate and do not give rise to peaks in the phonon spectra. The highest-frequency shear mode blue-shifts significantly with increasing thickness, reflecting both an increase in the number of Ga layers and an increase in the effective interlayer force constant. The surface resonance modes evolve into localized 2D Ga modes as the phonon momentum increases. The predicted low-frequency modes are consistent with Raman measurements on 2D polar Ga.

Controlling resonant surface modes by arbitrary light induced optical anisotropies

In this work the sensitivity of Bloch Surface Waves to laser-induced anisotropy of azo-polymeric thin layers is expe rimentally shown . The nanoscale reshaping of the films via thermal-Scanning Probe Lithography allows to couple light to circular photonic nanocavities, tailoring on-demand resonant BSW confined within the nanocavity.

Surface-Mode Resonance Coupling Effect and High-Temperature Sensing Characteristics in Hollow-Core Photonic Bandgap Fibers

Surface-Mode Resonance Coupling Effect and High-Temperature Sensing Characteristics in Hollow-Core Photonic Bandgap Fibers

Degenerate Coupled Mode Division and Superposition Under Symmetry Breaking

Resonances in micrometer metal cavity structures are very important for the interactions between materials and light. Three similar cavities connected with waveguides are investigated through the finite difference time domain (FDTD) method and the coupled mode theory. Two fundamental surface resonance modes are demonstrated in the two simple cavities separately. Then two simple cavities are combined to form the third cavity. The fundamental resonant modes couple positively or oppositely to form coupled-mode resonances in the combined cavity. When the combined cavity structures are symmetric, the coupled-mode resonances lead to two transmission peaks. While the symmetry is broken with tens of nanometers displacements, the transmission peaks convert to dips. It is believed the Q value variation of coupled-mode resonances plays a key role in the conversion. When the structure is symmetric, the coupled-mode resonances in the upper and lower parts of the cavity have the same Q value and are degenerate. The superposition of them leads transmission peaks. While the symmetry is broken, the Q values of resonances in the upper and lower part of the cavity are different, leading to the degenerate coupled mode division. The superposition of the different Q-factor modes leads to the dips. The sensitive variation to the symmetry of structures can be used to control light-material interactions, optical switch, and improve the sensitivity of sensor devices.

Thermal rectification and spin-spin coupling of nonreciprocal localized and surface modes

We study the rectification of near-field radiative heat transfer between two InSb nano-particles due to the presence of non-reciprocal surface modes in a nearby InSb sample when an external magnetic field is applied and its dependence on the magnetic field strength. We reveal the spin-spin coupling mechanism of the localized particle resonances and the surface mode resonances which is substantiated by the directional heat flux in the given setup. We discuss further the interplay of the frequency shift, the propagation length, and local density of states on the strength and directionality of the rectification as well as the non-reciprocal heating effect of the nanoparticles.

Large scale compatible fabrication of gold capped titanium dioxide nanoantennas using a shadowing effect for photoelectrochemical water splitting

In this paper, a visible light driven plasmonic based photoelectrochemical water splitting (PEC-WS) cell is designed with an elegant two-step fabrication route. First, titanium dioxide (TiO2) nanowires (NWs) were synthesized using the hydrothermal method. Then, angled deposition was used to selectively coat the tips of the NWs yielding Au-capped TiO2 NWs with multiple sizes and shapes. The provided randomness leads to a multi-resonant system where the superposition of these resonance modes leads to an overall broadband absorption. The excited localized surface resonance (LSPR) modes contribute to the performance enhancement of the cell via near field effects and a hot electron injection mechanism. Moreover, these nanotips can trigger the formation of Fabry-Pérot (FP) cavity modes. The combination of the above-mentioned mechanisms leads to a high performance visible light driven plasmonic cell. At an applied potential of 1.23 V vs RHE, a photocurrent value as high as 82 μA/cm2 is acquired for the plasmonic based photoanode. The proposed design strategy is a large scale compatible route with no material restriction. Therefore, vast variety of semiconductor-metal pairs can be fabricated to obtain highly efficient water splitting cell for hydrogen generation.

Avoiding imaging artifacts from resonant modes in metamaterial superlenses

Superlenses are imaging components that can overcome the diffraction limit associated with conventional dielectric lenses. An ideal superlens is a flat infinite slab of homogeneous double-negative material with $\ensuremath{\varepsilon}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}\ensuremath{-}1$ embedded in air, which achieves perfect imaging by restoring all the spatial frequency components of an object to the image plane. It has been shown that any deviation from these homogeneous material parameter values limits the resolution of the lens and introduces a surface mode resonance with fields that dominate the image. While material loss can suppress the resonant mode, loss also reduces spatial resolution. In this paper, we investigate resonant modes arising in metamaterial superlenses of infinite and finite extent and show how they cause imaging artifacts and reduce imaging fidelity. We choose a well-studied periodic structure consisting of an array of magnetodielectric cylinders for the lens medium under test. We demonstrate that the presence of these artifacts can lead to erroneous interpretation of the standard two-source resolution test for lenses. We show that artifacts can be mitigated by introducing point defects into the array, which move the resonant modes to higher spatial frequencies and, in the case of finite lenses, suppress their amplitudes through radiation losses. This strategy enables more robust and reliable subwavelength imaging performance, improves the spatial resolution of the metamaterial lens, and reduces the deleterious effects of material losses.

Unidirectional distributed acoustic reflection transducers for quantum applications

Recent significant advances in coupling superconducting qubits to acoustic wave resonators have led to demonstrations of quantum control of surface and bulk acoustic resonant modes as well as Wigner tomography of quantum states in these modes. These advances were achieved through the efficient coupling afforded by piezoelectric materials combined with GHz-frequency acoustic Fabry-Perot cavities. Quantum control of “itinerant” surface acoustic waves appears in reach but is challenging due to the limitations of conventional transducers in the appropriate gigahertz-frequency band. In particular, gigahertz-frequency unidirectional transducers would provide an important addition to the desired quantum toolbox, promising unit efficiency with directional control over the surface acoustic wave emission pattern. Here, we report the design, fabrication, and experimental characterization of unidirectional distributed acoustic reflection transducers demonstrating a high transduction frequency of 4.8 GHz with a peak directivity larger than 25 dB and a directivity greater than 15 dB over a bandwidth of 17 MHz. A numerical model reproduces the main features of the transducer response quite well, with ten adjustable parameters (most of which are constrained by geometric and physical considerations). This represents a significant step toward quantum control of itinerant quantum acoustic waves.

Development of a ring cavity-based fibre optic sensor for MR-compatible medical sensing applications

Advances in robotic systems and rapid developments in minimally invasive surgery (MIS) have made their use possible in the operating room due to many distinct advantages offered by MIS over conventional surgical procedures. The constant increase of medical examinations, surgeries and surgical interventions using intraoperative guidance with magnetic resonance imaging (MRI) has promoted research on new sensors to be applied in this scenario. However, due to the challenging environment under MRI system limits the applicability of materials and traditional electronic sensors. Optical fibres are small in size, chemically inert, immune to electromagnetic interference and offer the real-time in vivo multi-parameter measurement capability. Herein, we report a ring cavity-based fibre optic sensor design using MR-compatible polymer material. Optical fibres were used to excite surface resonance modes (SRM) of the ring cavity-based sensor. The paper reports initial investigations on ring cavity based MR-compatible fibre optic sensor design. Computational simulations were carried out to study the effect of structural and material parameters on the sensor design. Ring cavity was developed using polymethyl methacrylate. Developed ring cavity will be used to develop the MR-compatible fibre optic sensor for medical applications.

Surface anisotropy and spin wave resonance in submicron amorphous wires

Abstract The influence of surface magnetic anisotropy on ferromagnetic resonance (FMR) in submicron wires is investigated both theoretically and experimentally. An analytical formula for resonance fields of radial spin waves in a long axially magnetized circular cylinder is obtained. Three types of surface anisotropy with the easy direction along one of the cylindrical coordinates are considered. For sufficiently strong surface anisotropy with hard direction parallel to the cylinder axis the surface spin wave mode is observed. The theoretical results are verified by FMR measurements at six microwave frequencies from 9 to 69 GHz on glass-covered amorphous FeCrSiB wires with diameters from 541 to 1032 nm. From the bulk spin wave resonances the exchange stiffness constant about 6.5 10−7 erg/cm is obtained. The frequency dependence of the surface mode resonance field indicates that a perpendicular surface anisotropy with anisotropy constant Ks = 6.2 erg/cm2 is present at the metal/glass interface.

Volume and surface mode coupling experiments in periodic surface structures for use in mm-THz high power radiation sources

Planar Periodic Surface Lattice (PSL) structures of different configurations have been designed, fabricated and measured in the 140-220 GHz frequency band. Surface mode resonances are observed in ‘mesh’ PSL structures. We establish that, when mounted on suitable metal-backed dielectric substrates, PSLs exhibit ‘mode-locked’ coherent cavity eigenmodes formed from coupled volume and surface modes. The ‘proof-of-principle’ coupling of volume and surface modes and concept of mode selection in a large cavity, which can lead to the innovation of high power mm-THz radiation sources, is demonstrated. Evidence of coupled eigenmode formation in a 0.64 mm planar PSL measured at 325-500 GHz is presented, verifying the scalability of this work. Excellent agreement between numerical modelling and experiment is reported.

Water wave scattering by an array of rectangular breakwaters on a step bottom topography

Water wave scattering by an array of identical rectangular breakwaters is investigated in the presence of a heterogenous bottom boundary in the cases of both infinite and semi-infinite fluid domains. Boundary value problem is formulated by employing small amplitude water wave theory and is reduced to a system of linear algebraic equations using matching eigenfunction expansion method. These equations are solved numerically to perceive the complete solution of the problems under present interest. The reflection and transmission coefficients are computed in different modes, and the energy relation is used to check the accuracy of computed results. Effects of different parameters on the total reflected and transmitted wave energy are computed and analyzed. Further, the surface motion is computed to see the wave phenomena in the vicinity of the breakwaters. The unknown coefficients appearing in velocity potentials are estimated, the frequency of resonance mode is resolved, and resonance surface modes are plotted in the case of the semi infinite fluid domain. This study is useful for the breakwater designer to install breakwaters near an uneven bottom topography.

Interferometric investigation of the gas-state monolayer of mono-rhamnolipid adsorbing at an oil/water interface

We present a study on biosurfactant uptake at the interface of a millimeter-sized drop of squalene in water, by a recently developed differential interferometric technique. The technique allows detecting capillary waves, with amplitudes of the order of 10−9 m, excited on the surface of the drop by an electric field in the order of 5 V/cm. By studying the resonant surface modes of the drop it is possible to assess the interfacial properties as a function of the surfactant concentration in the water bulk. The technique allows to follow the adsorption process at extremely low surfactant concentration, not accessible by other methods, investigating the gas state in the π-A diagrams of 2D amphiphilic monolayers.

A coherent description of thermal radiative devices and its application on the near-field negative electroluminescent cooling

Using the transimissivity between two thermal reservoirs and the generalized Planck distributions, we describe the devices that use radiative energy transfer between thermal reservoirs in a unified formalism. Four types of devices are distinguished. For power generators that use the temperature difference between reservoirs, photovoltaic (PV) and thermoradiative (TR) devices respectively use the low-temperature photovoltaic cell and high-temperature thermoradiative cell to generate electricity. For active cooling, the electroluminescent (EL) cooling devices apply a forward bias voltage on the object we want to cool, whereas the negative EL cooling devices apply a reverse bias voltage to the heat sink. The relationship among these four devices is explicated. The performance of the negative EL cooling is analyzed, both in the Shockley-Queisser (blackbody spectrum and radiative recombination) framework and the near-field enhancement. The “impedance match” condition derived for PV systems is applied to the negative EL devices. One advantageous feature of the negative EL cooling is that it does not apply the voltage to the target object which we want to cool, and the near-field enhancement can apply to various target materials that support the surface resonant modes.

Bias-free and antenna-coupled CW terahertz array emitter with anomalous Schottky barriers

We present a new bias-free antenna-coupled CW terahertz photomixer with interdigitated electrodes. Each finger pair is made of metal/semiconductor/metal (MSM) electrodes with dissimilar Schottky barriers. The two dissimilar metals in each MSM have a barrier height of difference (ΔϕB) and a finite lateral spacing (s). In the proposed teraheretz emitter, not only is the optical absorption and hence photogeneration enhanced by the surface resonant modes, but also the high built-in field increases the terahertz current. Furthermore, by having a large radiation resistance of the antenna integrated to the array electrodes, based on our simulations, it is possible to achieve the maximum power of 35 μW from a dipole antenna coupled to the proposed electrode array with 10 μm ×10 μm active area and pitch of Λ=800 nm. This is more than 196 times larger terahertz power than the highest terahertz power radiated from the array emitters of the same area that contains the bias-free antennaless array of far-field emitters with double pitch size. We have also investigated the limits of thermal breakdown and efficiency degradation of the proposed emitter that play an important role due to the reduction of the active area. Such terahertz sources can pave the way to various biomedical applications such as endoscopic imaging without a need for hazardous external circuitry for biasing, reducing patient health risk.

Thermoradiative device enhanced by near-field coupled structures

Thermoradiative (TR) technology was recently proposed as a new mechanism to efficiently convert low-temperature waste heat into electric energy. It has been shown that near-field coupling between a TR cell and a heat sink is an effective way in boosting the performance of the TR device. The enhancement becomes more significant when surface resonant modes are supported by the heat sink. Surface plasmon polaritons on metal surfaces, and surface phonon polaritons on Lorentz materials, are two major types of materials investigated in the near-field enhancement. However, to significantly boost the performance, the resonant mode energy needs to match the bandgap energy of the TR cell, which is often difficult to realize with natural material selections. Typical metals have plasma frequency much higher than the bandgap of TR cell. In this paper, we show that new resonant modes with energy close to the bandgap of TR cell can be created with nanostructured gratings on the heat sink. This enables use to build the heat sink using commonly available metallic materials, whose typical surface resonant frequency is much higher than the TR bandgap.

Diffraction problem of a floating breakwater with an array of small ports

In this article, we have studied the linear water wave scattering by an infinitely extended fixed floating cylinder of rectangular cross section which is placed in front of a periodic array of identical fixed small bodies. The velocity potentials that appear in the boundary value problem are expressed in the explicit form in terms of infinite Fourier double series using the method of separation of variables. By matching the velocity and pressure at the interfaces and using the orthogonality property of eigenfunctions, a system of algebraic equations of unknowns are obtained. These are solved numerically to determine the velocity potential completely. The existence of resonance modes in the open water region is demonstrated by analyzing coefficients which appear in the velocity potential and the resonance surface modes are plotted for different physical parameters. This study can be used in the design of multiple offshore structures.