Non-dispersive Surface Research Articles

Observation of Higher-Order Nodal-Line Semimetal in Phononic Crystals.

Higher-order topological insulators and semimetals, which generalize the conventional bulk-boundary correspondence, have attracted extensive research interest. Among them, higher-order Weyl semimetals feature twofold linear crossing points in three-dimensional momentum space, 2D Fermi-arc surface states, and 1D hinge states. Higher-order nodal-point semimetals possessing Weyl points or Dirac points have been implemented. However, higher-order nodal-line or nodal-surface semimetals remain to be further explored in experiments in spite of many previous theoretical efforts. In this work, we realize a second-order nodal-line semimetal in 3D phononic crystals. The bulk nodal lines, 2D drumhead surface states guaranteed by Zak phases, and 1D flat hinge states attributed to k_{z}-dependent quadrupole moments are observed in simulations and experiments. Our findings of nondispersive surface and hinge states may promote applications in acoustic sensing and energy harvesting.

Numerical Study of Free-Surface Electro-Hydrodynamic Wave Turbulence

Direct numerical simulation of threedimensional chaotic motion of a dielectric liquid with a free surface under the action of external horizontal electric field is carried out. The numerical model takes into account the effects of surface tension, viscosity, and external isotropic random forcing acting at large scales. A transition from dispersive capillary wave turbulence to quasi-isotropic nondispersive EHD surface turbulence with increase of the external electric field strength is numerically observed for the first time. At the regime of developed EHD wave turbulence, the total electrical energy is found to be much greater than the energy of capillary waves, i.e., electrohydrodynamic effects play a dominant role. At the same time, anisotropic effects are detected that lead to the generation of capillary wave packets traveling perpendicular to the external field direction. Despite the revealed anisotropy, the calculated spectrum of EHD wave turbulence is in very good agreement with the analytical spectrum obtained on the basis of dimensional analysis of weak turbulence spectra.

Plasmonic Gold Nanohole Arrays for Surface-Enhanced Sum Frequency Generation Detection.

Nobel metal nanohole arrays have been used extensively in chemical and biological systems because of their fascinating optical properties. Gold nanohole arrays (Au NHAs) were prepared as surface plasmon polariton (SPP) generators for the surface-enhanced sum-frequency generation (SFG) detection of 4-Mercaptobenzonitrile (4-MBN). The angle-resolved reflectance spectra revealed that the Au NHAs have three angle-dependent SPP modes and two non-dispersive localized surface plasmon resonance (LSPR) modes under different structural orientation angles (sample surface orientation). An enhancement factor of ~30 was achieved when the SPP and LSPR modes of the Au NHAs were tuned to match the incident visible (VIS) and output SFG, respectively. This multi-mode matching strategy provided flexible controls and selective spectral windows for surface-enhanced measurements, and was especially useful in nonlinear spectroscopy where more than one light beam was involved. The structural orientation- and power-dependent performance demonstrated the potential of plasmonic NHAs in SFG and other nonlinear sensing applications, and provided a promising surface molecular analysis development platform.

Non-dispersive wavepackets generated in layered structures by a scanning laser source

In this article, we describe the use of a continuous-wave laser scanning method to generate non-dispersive surface acoustic wavepackets, which propagate along the sample surface without any waveform change. To achieve this goal, a coated linear elastic film on a non-linear substrate allows for careful balancing of dispersion and non-linearity effects. The scanning speed of the laser source and the thickness of the coated film were parametrically investigated to determine the optimal scanning speed for the generation of ultrasound for a given thickness of the coated film. In the first step, four different combinations of scanning speeds and the thickness of the coated film are presented to illustrate the generation of the narrowband ultrasound. The purpose of the scanning laser source is to effectively generate large amplitude ultrasound that takes the material into the nonlinear range. Further optimization through a careful matching combination of the scanning speed and the thickness of the coated film, whereby the dispersion effect was compensated entirely by the non-linearity effect, was used to generate non-dispersive ultrasonic wavepackets, which subsequently propagate with little distortion. The main findings of the simulations indicate that non-dispersive surface acoustic wavepackets for coated systems can be generated via the scanning laser source approach for specific values of scanning speed and thickness of the coated film.

Design of Chemical Surface Treatment for Laser-Textured Metal Alloys to Achieve Extreme Wetting Behavior.

Extreme wetting activities of laser-textured metal alloys have received significant interest due to their superior performance in a wide range of commercial applications and fundamental research studies. Fundamentally, extreme wettability of structured metal alloys depends on both the surface structure and surface chemistry. However, compared with the generation of physical topology on the surface, the role of surface chemistry is less explored for the laser texturing processes of metal alloys to tune the wettability. This work introduces a systematic design approach to modify the surface chemistry of laser textured metal alloys to achieve various extreme wettabilities, including superhydrophobicity/superoleophobicity, superhydrophilicity/superoleophilicity, and coexistence of superoleophobicity and superhydrophilicity. Microscale trenches are first created on the aluminum alloy 6061 surfaces by nanosecond pulse laser surface texturing. Subsequently, the textured surface is immersion-treated in several chemical solutions to attach target functional groups on the surface to achieve the final extreme wettability. Anchoring fluorinated groups (-CF2- and -CF3) with very low dispersive and nondispersive surface energy leads to superoleophobicity and superhydrophobicity, resulting in repelling both water and diiodomethane. Attachment of the polar nitrile (-C≡N) group with very high nondispersive and high dispersive surface energy achieves superhydrophilicity and superoleophilicity by drawing water and diiodomethane molecules in the laser-textured capillaries. At last, anchoring fluorinated groups (-CF2- and -CF3) and polar sodium carboxylate (-COONa) together leads to very low dispersive and very high nondispersive surface energy components. It results in the coexistence of superoleophobicity and superhydrophilicity, where the treated surface attracts water but repels diiodomethane.

Casimir-Polder potential in the presence of a Fock state

Atom-surface forces using excited states have a host of compelling applications, including repulsive and lateral forces. However, such states can be fragile and difficult to prepare. Here we report an explicit normal-mode based calculation of the Casimir-Polder potential between a ground-state atom and a non-dispersive surface in the presence of an external quantised field. The potential we derive shares some features with that of excited-state Casimir-Polder forces even though we consider a ground-state atom. Our work provides a physically transparent and intuitive picture of driven Casimir-Polder potentials, as well as expanding on previous investigations by providing analytic results that fully include retardation, as well as being applicable for any choice of mutual alignment of the atom's dipole moment, the external field, and the surface normal.

Synthesis of low surface-energy polyepichlorohydrin triazoles thin film

In this investigation, a new polymer with low surface energy was synthesized by grafting a triazole group onto polyepichlorohydrin (PECH) rubber that contained no halogens. The chlorine on PECH was first replaced by an azide group, and this attached azide was then converted to a triazole group with alkyl chains using the azide-alkyne Huisgen cycloaddition reaction. Analyses confirmed the structure of final product, PECH-triazole polymer. The grafting reactions increased the surface roughness. The static contact angles of water or CH2I2 droplets on the PECH-azole film were 101.7° and 71.3°, respectively. The advancing and receding contact angles for water on PECH-azide were 119.8° and 13.7°, respectively. The PECH-triazole polymer has omniphobic properties with rose petal characteristics. The PECH-triazole has low dispersive surface energy (21 mN/m) and negligible non-dispersive surface energy, giving a wetting envelope that is similar to the one of PTFE polymer. X-ray photoelectron spectroscopy and transmission infrared spectroscopy suggested that the interactions of the N atoms on the triazole ring and the O atoms on the PECH backbone constrained the orientation of CH2 groups and reduced the surface energy of the thin film.

Infrared broadband metasurface absorber for reducing the thermal mass of a microbolometer

We demonstrate an infrared broadband metasurface absorber that is suitable for increasing the response speed of a microbolometer by reducing its thermal mass. A large fraction of holes are made in a periodic pattern on a thin lossy metal layer characterised with a non-dispersive effective surface impedance. This can be used as a non-resonant metasurface that can be integrated with a Salisbury screen absorber to construct an absorbing membrane for a microbolometer that can significantly reduce the thermal mass while maintaining high infrared broadband absorption in the long wavelength infrared (LWIR) band. The non-dispersive effective surface impedance can be matched to the free space by optimising the surface resistance of the thin lossy metal layer depending on the size of the patterned holes by using a dc approximation method. In experiments a high broadband absorption was maintained even when the fill factor of the absorbing area was reduced to 28% (hole area: 72%), and it was theoretically maintained even when the fill factor of the absorbing area was reduced to 19% (hole area: 81%). Therefore, a metasurface with a non-dispersive effective surface impedance is a promising solution for reducing the thermal mass of infrared microbolometer pixels.

Effects of crack-dilatancy on Rayleigh waves in fluid-saturated porous media

The study considers the propagation of surface waves on the stress-free surface of a porous solid saturated with non-viscous fluid. The surface pores have the option of being either sealed or fully-opened. With the presence of dilatant cracks, the interior of the porous solid is characterised through three different crack-regimes, based on the connections between embedded cracks. Secular equations are derived in closed form for the propagation of Rayleigh waves in the porous media with sealed or fullyopened surface pores. The velocity of non-dispersive surface waves varies significantly with the density of cracks present. However, aspect (thickness to radius) ratio of (circular) cracks may not have much effect on the velocity of Rayleigh waves. The opening of surface pores may be an important reason for a faster propagation of Rayleigh waves in any realistic elastic medium. Finally, the dilatancy due to the growth of cracks up to their interconnection or drainage may be able to affect the velocity of Rayleigh waves quite significantly.Keywords: Rayleigh waves, phase velocity, dilatancy, porous solid, cracksInternational Journal of Engineering, Science and Technology Vol. 6, No. 1, 2014, pp. 77-87

Adsorption energy of stoichiometric molecules and surface energy at morphologically important facets of a Ca(OH)2 crystal

Abstract Non-dispersive (polar) contribution to adsorption energy of stoichiometric molecules and surface energy of morphologically important {0 0 0 1} and { 1 ¯ 0 1 0 } type facets of a Ca(OH)2 crystal are evaluated on the basis of calculation of the corresponding surface-associated Madelung constants. At the nano-size level, this contribution both of the adsorption and surface energies of indicated types of facets have been obtained to depend predominantly on the characteristic size r1 of a {0 0 0 1} type facet due to much stronger bonding of ions within individual (0 0 0 1) basal planes in comparison with that between ions belonging to adjacent (0 0 0 1) planes. The non-dispersive component of the surface energy of Ca(OH)2 crystal is highly anisotropic, with much higher value calculated for the { 1 ¯ 0 1 0 } type facets. Both for {0 0 0 1} and { 1 ¯ 0 1 0 } facets, the non-dispersive surface energy in the edge and near-edge area substantially exceeds that at center of the facet, regardless of the crystal sizes. In the size range of r1 ≥ 40 nm, out of the marginal area the surface energy of a facet is not practically influenced by the size effect and may be characterized with a sufficient accuracy by corresponding value that is achieved in the limit of large sizes, r1 → ∞. At small sizes r1 ≤ 4 nm, the distribution of the surface energy on each of {0 0 0 1} and { 1 ¯ 0 1 0 } type facets is strongly inhomogeneous and is very sensitive to variation of the characteristic size r1. The non-dispersive adsorption energy of stoichiometric Ca(OH)2 molecules on the {0 0 0 1} and { 1 ¯ 0 1 0 } facets with variation of the size r1 follows the same trend as the corresponding non-dispersive surface energy. Analytical expressions are obtained for the total (sum of non-dispersive and dispersive components) surface and total adsorption energies that incorporate the size effect resulting predominantly from the non-dispersive component. The proposed theoretical approach may be applied to analysis of the same energy characteristics of the materials that are isostructural with the Ca(OH)2: alkaline earth hydroxides – Mg(OH)2, Sr(OH)2, and Ba(OH)2, as well as some other compounds with the brucite structure.

Electroelastic gap waves between dissimilar piezoelectric materials in different classes of symmetry

Shear surface waves guided by a gap between dissimilar piezoelectric half-spaces that belong to different classes of material symmetry are systematically investigated. It is shown that at most two surface waves can be guided by the structure. Equations for calculating surface wave velocities of propagation and corresponding existence conditions are obtained in closed form. For some special cases, surface wave velocities are derived explicitly. It is shown that despite the presence of a characteristic length in the problem (that is, the gap width) a non-dispersive surface wave independent of the wavelength and gap width can be guided by the structure. Electroelastic wave components and existence conditions of this non-dispersive wave are obtained. It is demonstrated that both the gap width and the dielectric permittivity of the gap have significant influences on the dynamic behaviour of the structure so that even small gaps cannot always be simplified and modelled as permeable, impermeable or absorbent with infinitesimal width.

Dispersive Rayleigh Type Waves in Layered Systems Consisting of Piezoelectric Crystals Bismuth Silicate and Bismuth Germanate

The lowest-order modes of the dispersive six-partial waves of Rayleigh type (RTW6) have been numerically obtained for two layered systems consisting of a layer on a substrate in [100] propagation direction of (001)-cut for both cubic crystals of class 23. Dispersion relations are shown for both a layer of Bi 12 SiO 20 on a substrate of Bi 12 GeO 20 and vice versa. Dispersion relations show one mode in each case with clear maximum and minimum, at which it is analytically shown that the group velocity is equal to the phase velocity. It was concluded that in corresponding highly-symmetric cases, the obtained non- dispersive six-partial surface waves in the treated layered systems are a new non-dispersive type, termed Rayleigh-Zakharenko type (RZTW6) non-dispersive six-partial surface waves. These can exist in layered systems consisting of a layer on a substrate. The possibility of the existence in layered systems of non-dispersive waves of both the nine-partial Zakharenko type (ZTW9) for centrosymmetrical crystals, and the thirteen-partial Zakharenko type waves (ZTW13) for non-centrosymmetrical crystals is suggested. In addition, it is shown that dependence of the phase velocity V ph = V ph (kh 1 , kh 2 ,..., kh m ) in a multi-layered system can be reduced to dependence V ph = V ph (kh) for a layered system consisting of a layer on a substrate. Thus, both systems can be studied in the same way.

Reflectivity behavior of opals of gold nanoparticle coated spheres

The light reflectance in three-dimensional metallodielectric photonic crystals assembled from polyelectrolyte-coated latex spheres infiltrated with gold nanoparticles has been studied. Strong deviation of the optical reflectance of Au opals from bare opals has been observed, including flattening of the diffraction resonance dispersion and nondispersive surface plasmon bands.

Non-dispersive and dispersive electromagnetoacoustic SH surface modes in piezoelectric media

This paper deals with a detailed analysis of the electromagnetic problem for the propagation of shear horizontal (SH) surface waves in a non-conducting piezoelectric half-space. The compatibility equations are solved in both cases of grounded surfaces and surfaces matched with an external potential. It is shown that a unique quasi-acoustical mode exists in non-dispersive media and the corresponding solution is worked out in a closed form having recourse to a method of complex analysis. Dispersive half-spaces are considered accounting for a one-resonance model and the dispersion equation is numerically solved for a matched surface. Two types of admissible surface waves are found in different frequency ranges. One mode generalises the well-known Bleustein–Gulyaev wave, while the second one is a quasi-electromagnetic mode occurring above the characteristic frequency of the model. Results are compared with those obtained by the quasi-static approximation.

Excitation of remarkably nondispersive surface plasmons on a nondiffracting, dual-pitch metal grating

A nondiffracting metallic lamellar grating formed from three equally spaced grooves per repeat period, with one being slightly shallower than the other two is examined at microwave frequencies. When filled with a slightly lossy dielectric, this structure supports a remarkably nondispersive surface plasmon polariton mode, which exhibits strong selective absorption of incident power. Measured reflectivities show excellent agreement with the results predicted by a rigorous coupled wave theory.

Properties of a Monolithic Electroacoustic Device Geometry Using GaAs Resonant Tunnelling Structures

We present experimental and theoretical work on a monolithic electroacoustic system which exploits the interaction between a 1 GHz surface acoustic wave and electrons in a GaAs-AlGaAs resonant tunnelling structure. Non-annealed Ohmic contacts using in situ grown Al are employed to obtain stable dc operation. The microwave response of the system is explained, with features attributed to charge accumulation in the spacer layer adjacent to the quantum well. The influence of screening of the surface acoustic wave potential by the highly-doped substrate is also investigated, showing the need for a wide depletion layer. Potential applications for this system in signal processing and power microwave devices are discussed, along with a suggested mechanism for a non-dispersive surface acoustic wave transducer which relies on the nonlinear properties of the system.

Adhesion at partially restructured polymer surfaces

The controlled modification of the surface structure of styrene-4-vinyl pyridene (SVP) and of ethylene-acrylic acid (EAA) copolymers was achieved by the timed exposure of the polymers to non-orienting (n-alkane) and orienting (water) fluids. The degree of surface modification was monitored by periodic measurements of non-dispersive surface energies from static contact angle data. Assemblies of rigid PVC/SVP, of poly(methyl methacrylate) (PMMA)/EAA and of polypropylene (PP)/EAA were prepared with the SVP and EAA in various stages of restructuring. Bond characteristics of these assemblies were evaluated from single lap-shear measurements. Bond strengths responded strongly to the orientational states of the copolymer adhesives. In PVC/SVP and PMMA/EAA, significant increases in bond strength were observed when orientational changes raised the nondispersive contribution, γ nd, to the surface energies of these copolymers. Surprisingly, full increments in bond strengths were realized at only about 25–40% of the total increment in γ nd. Surface restructuring had little effect on adhesion with the non-polar PP substrate. Residual bond strengths, following controlled aging, varied directly with the initial bond strength. The residual bond strengths may be used to specify ideal restructuring events in the adhesives which would render the assemblies fully resistant to the chosen aging process. Acid/base interaction concepts have been used to rationalize the observations. These suggest that only relatively few polar elements need to migrate into the surface region of the adhesives to maximize initial bond strength, but larger numbers are needed to ensure adequate resistance to aging.

Observation of highly dispersive surface states on GaN(0001)1×1

The electronic structure of n-type, Si-doped, wurtzite GaN(0001)1\ifmmode\times\else\texttimes\fi{}1 surfaces has been studied using synchrotron radiation excited angle-resolved photoemission. The GaN thin films were grown by metal-organic chemical-vapor deposition on SiC. Two previously unobserved surface bands were measured and fully characterized. One of the states is highly nonlocalized, dispersing throughout much of the valence band along the $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{K}\ensuremath{-}\overline{M}$and $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{M}$ directions of the 1\ifmmode\times\else\texttimes\fi{}1 surface Brillouin zone. The identification of these states as surface bands was confirmed both by their lack of dispersion perpendicular to the surface, and by the sensitivity of the states to hydrogen adsorption. The symmetry properties of the states were determined using the linear polarization of the incident synchrotron radiation. These states are quite distant from the localized nondispersive surface state previously observed on GaN.

Surface tensions and anchoring transitions of nematic liquid crystals on gradually oxidized substrates

Intermediate anchorings of nematic liquid crystals onto substrates are now intensively studied essentially because of their potential display applications. Here, we use the active oxygen gas arising from a plasma reactor to realize progressive chemical modifications of glass substrates covered with silane. In this manner, the dispersive and nondispersive surface tensions of the substrate are gradually modified involving a continuous change in the differential wetting properties of the substrate with both parts of the liquid crystal molecules, and in the anchoring energy. Intermediate anchorings with tilted angles are thus obtained in the vicinity of an anchoring transition from the pseudohomeotropic to the planar alignment, where symmetry is broken.

Surface and Interaction Characteristics of Liquid Crystal Polymers and their Miscibility with Thermoplastic Hosts

Inverse gas chromatographic measurements have shown that the surface of thermo-tropic liquid crystal polymers, (TLCP), are capable of mainly dispersion force (L/W) interactions, with only slight contributions from non-dispersion (acid-base) forces. Structural moieties capable of non-dispersive interactions are primarily oriented into the TLCP bulk, perhaps accounting for the self-assembly effects in these polymers. Moreover, nondispersive surface forces were found to be thermolabile, so that above characteristic temperatures only L/W forces remain. Blends of TLCP with polycarbonate and polyetherimide hosts prepared below the characteristic temperatures displayed morphology typical of immiscible blends. When prepared at, and quenched from, temperatures above the pertinent characteristic value, electron micrographs showed evidence of adhesion at matrix/TLCP contacts, simulating the behavior of more miscible systems.