Two-dimensional Periodic Arrangement Research Articles

Optical anapoles excited by UV-A illumination.

We report the excitation of optical anapole states at ultraviolet (UV) wavelengths. Numerical simulations indicate that TiO2 nano-rectangles with varying length-to-width ratios can support such modes within the 350-380 nm range. We further propose a two-dimensional periodic arrangement of these nano-rectangles deposited atop a fused silica substrate. Understanding and manipulating optical anapole states in the ultraviolet spectrum is crucial for advancing next-generation photonic devices and enhancing nonlinear optical processes, such as generation of highly energetic vacuum ultraviolet light through third-harmonic generation.

Aperiodic Two-Dimensional Acoustic Black Holes for Broadband Vibration Attenuation in a Strip

PurposeAcoustic black holes (ABHs) are promising for vibration control in lightweight structures as proven for one- or two-dimensional periodic arrangements. Here, we explored the effects of spatial disorder and heterogeneous designs of ABHs to broaden an intrinsically limited attenuation bandwidth of periodic counterparts.MethodWe proposed several strategies to introduce non-periodic arrangements and/or different ABH profiles by solving a maximization problem for the attenuation bandwidth of a plate strip decorated by five ABHs. These strategies allow for finding appropriate dimensions and positions of the ABHs by analyzing a small design subset and are verified experimentally.ResultsThe identified periodic heterogeneous ABHs enable greatly extending the attenuation bandwidth, while disordered identical ABHs allow for increasing the attenuation intensity as compared to the corresponding periodic configurations. The mechanisms underlying the wave attenuation enhancement were clarified by tracing the evolution of the wave transmission and structural vibration modes at each design step. We have found that the broadened wave attenuation attributes to the activation of strongly localized modes at broadband frequencies in aperiodic scenarios. These abundant modes are multi-frequency local resonances in ABHs that are sensitive to both the ABH profile and their spatial arrangement.ConclusionWe prove that relaxing the periodicity requirement on multiple two-dimensional ABHs can extend the vibration attenuation to broadband regimes below the ABH characteristic frequency, numerically and experimentally. Aperiodic designs of ABHs thus enlarge the design space by enabling a broadband wave mitigation with attenuation intensity comparable to that of periodic counterparts without increasing the structural mass.

Enhanced chiroptical activity with slotted high refractive index dielectric nanodisks

In this paper we employ finite difference time-domain (FDTD) calculations to demonstrate the enhancement of chiroptical activity in high refractive index dielectric slotted nanodisks. The chiral enhancement mechanism consists of pairing the electric and magnetic modes of the slit cavity with the resonances of the rest of the disk, i.e., the weak chiroptical activity ($\ensuremath{\sim}{10}^{\ensuremath{-}1}$) inside the chiral gap region (which is chiral itself) is enhanced through the interaction with the electric and magnetic resonances in the nanodisk. These results are extended to a two-dimensional periodic arrangement of slotted nanosdisks forming a metasurface. The chiral electromagnetic near field at the slotted region can be harnessed for increasing the asymmetry in energy absorption of circularly polarized light (CPL). This chiral enhancement makes this novel metasurface useful for sensing or to control the optical forces that can be exerted on chiral molecules placed in those slotted regions. The latter can be relevant to the pharmaceutical industry since distinct enantiomers of racemic drugs frequently exhibit different biological activities.

Calculation of band structures of a phononic crystal within a waveguide in 3D with cubic inclusions using a Periodic Green’s Function Method

The Phononic Crystals have generated a growing scientific interest as a means to control the dispersion of waves in various technological applications such as telecommunications. In particular, Phononic Crystal Waveguides are composed of periodic distributions of dispersers immersed in a propagation medium and, designed by an arrangement with dimensions and periods comparable to the wavelength. These crystals have properties that give them the ability to guide acoustic waves efficiently. In this paper, we present a numerical Boundary Element Method, which requires the use of a Periodic Green’s Function. This method allows to calculate the band structure of phononic crystals in two- and three-dimensions. In particular, the band structure is calculated for a waveguide formed by two flat, and parallel plates that involve a two-dimensional periodic arrangement of cubic inclusions. All surfaces involved are considered acoustic hard surfaces. The system considered, in addition to being a waveguide is in itself a phononic crystal, so that this type of systems present an alternative to manufacture to phononic crystal that can act as a phononic crystal and as an acoustic waveguide. These properties present some interest from a technological point of view.

Hybrid method of plane-wave and cylindrical-wave expansions for distributed Bragg-reflector pillars: formalism and its application to topological photonics

A hybrid computational method of plane-wave and cylindrical-wave expansions for distributed Bragg-reflector (DBR) pillars is proposed. The plane-wave expansion is employed to represent the one-dimensional periodic structure of the DBR. The cylindrical-wave expansion is employed to describe the scattering by circular pillars with the DBR structure inside. This formalism enables us to calculate the radiation fields, $t$-matrices, scattering cross sections, photonic band structures, and quality factors of the DBR pillars. Furthermore, optical properties of arrayed DBR pillars are also investigated with the aid of the multiple-scattering method. Using this formalism, we demonstrate explicitly that high $Q$ photonic band modes including the so-called bound states in continuum are obtained both in isolated and arrayed DBR pillars. We also present a novel formation of gapless Dirac-cone surface states in a three-dimensional photonic crystal composed of a two-dimensional periodic arrangement of core-shell DBR pillars.

A broadband cross-polarization conversion anisotropic metasurface based on multiple plasmon resonances

A compact broadband cross-polarization conversion metasurface functioning in the microwave regime is realized and experimentally demonstrated. The metasurface consists of a two-dimensional periodic arrangement of anisotropic doubleslit split-ring-resonator-based unit cells printed on top of a dielectric substrate, backed by metallic cladding. The proposed metasurface converts an x- or y-polarized wave into its orthogonal polarization over a fractional bandwidth of 100% from 5– 15 GHz, both for normal as well as oblique incidence. Moreover, the sub-wavelength unit-cell size, thin dielectric substrate, and unique unit-cell design collectively make the response of the metasurface same for both polarizations and insensitive to the incidence angle. The designed structure is fabricated and tested. The measurement and simulation results are found to be consistent with each other.

Reflection and transmission properties of a metasurface composed of resonant loaded wire dipoles

A considered metasurface is assumed to consist of a two-dimensional periodic arrangement of inductively loaded wires. The effects of incident angles and inductive loads upon equivalent surface impedance, reflection, and transmission properties of this metasurface have been investigated using numerical simulations. It is shown that at a particular frequency and angle, a metasurface can be cloaked for the incoming incident wave with nearly zero reflection. It is further studied that this approximate zero reflection is independent of the inductances of the inductively loaded wire dipoles.

Two-dimensional periodic arrangement of one-dimensional polymerized C60 evidenced by high-resolution cryo-transmission electron microscopy

We have observed an electron-beam (EB) irradiated C60 single crystal (SC) film, using high-resolution cryo-transmission electron microscopy (cryo-HRTEM) measured at a low temperature of 80 K, and obtained the lattice images of one-dimensional (1D) C60 polymer. The intermolecular distance between adjacent C60 molecules of the 1D polymer was obtained to be 0.92 nm, which is in good agreement with the previous result of 0.93 nm estimated from the electron diffraction (ED) patterns obtained at room temperature. Although the previous work indicated that the 1D C60 polymer is explained by a mixture of body-centered orthorhombic (BCO) and hexagonal-closed pack (HCP) models, the present cryo-HRTEM images and their fast Fourier-transformed patterns clearly exclude the BCO model, and demonstrate that the 1D C60 polymer structure can be explained only by the HCP-monoclinic model.

Self-Assembled p-Carborane Analogue of p-Mercaptobenzoic Acid on Au{111}

The p-carborane cluster analogue of p-mercaptobenzoic acid, 1-HS-12-COOH-1,12-C2B10H10, has been synthesized and characterized using nuclear magnetic resonance spectroscopy, single-crystal X-ray diffraction analysis, quantum-chemical calculations, and scanning tunneling microscopy. The single-crystal structure and selected packing aspects are discussed and presented in comparison with the two-dimensional periodic arrangements. Scanning tunneling micrographs, recorded under ambient conditions, are used to compare pure monolayers of 1-HS-1,12-C2B10H11 to coadsorbed monolayers of both the parental precursor and carboxyl-functionalized p-carboranethiolate on Au{111}. Monolayers of both constituents are further characterized by X-ray photoelectron spectroscopy, which shows good agreement between the stoichiometry of each pure monolayer and the nominal stoichiometries of the respective molecules. Results indicate that most of the molecules of both derivatives adsorb as thiolates but that a small fraction of eac...

Transfer matrix modeling and experimental validation of cellular porous material with resonant inclusions.

Porous materials are widely used for improving sound absorption and sound transmission loss of vibrating structures. However, their efficiency is limited to medium and high frequencies of sound. A solution for improving their low frequency behavior while keeping an acceptable thickness is to embed resonant structures such as Helmholtz resonators (HRs). This work investigates the absorption and transmission acoustic performances of a cellular porous material with a two-dimensional periodic arrangement of HR inclusions. A low frequency model of a resonant periodic unit cell based on the parallel transfer matrix method is presented. The model is validated by comparison with impedance tube measurements and simulations based on both the finite element method and a homogenization based model. At the HR resonance frequency (i) the transmission loss is greatly improved and (ii) the sound absorption of the foam can be either decreased or improved depending on the HR tuning frequency and on the thickness and properties of the host foam. Finally, the diffuse field sound absorption and diffuse field sound transmission loss performance of a 2.6 m(2) resonant cellular material are measured. It is shown that the improvements observed at the Helmholtz resonant frequency on a single cell are confirmed at a larger scale.

Interplay of hydrogen bonding and molecule–substrate interaction in self-assembled adlayer structures of a hydroxyphenyl-substituted porphyrin

The formation of hydrogen-bonded organic nano-structures and the role of the substrate lattice thereby were investigated by scanning tunneling microscopy. The self-organization of 5,10,15,20-tetra(p-hydroxyphenyl)porphyrin (H2THPP) molecules leads to two molecular arrangements on Au(111). One of these is characterized by pair-wise hydrogen bonding between hydroxyl groups and a low packing density which enables a rotation of individual molecules in the structure. A different interaction with stronger chain-like hydrogen bonding and additional interactions of phenyl groups was observed for the second structure. The influence of the substrate on the epitaxial behavior is demonstrated by the adsorption of H2THPP on the highly anisotropic Ag(110) substrate. There, several balances between the occupation of favorable adsorption positions and the number of hydrogen bonds per molecule were found. The molecules form molecular chains on Ag(110) and also assemble into two-dimensional periodic arrangements of differently sized close-packed blocks similar to the second type of supramolecular ordering found on Au(111). Dispersion corrected Density Functional Theory calculations were applied to understand the adsorption and complex epitaxy of these molecules. It is shown that the azimuthal orientation of the saddle-shape deformed molecule plays an important role not only for the intermolecular but also for the molecule–substrate interaction.

Clock field in arrayed magnetic logic gates with a magnetic force microscope tip

A magnetic logic gate (MLG), which is based on magnetic quantum dot cellular automata (MQCA), is capable of NAND/NOR logic operations. By arranging MLGs in a two-dimensional periodic array, a highly functional circuit can be created. However, NAND/NOR gates are difficult to form into a two-dimensional periodic arrangement. Here, we propose NOT/AND/ORs gate based on MLGs, which can be arranged in a two-dimensional periodic array. To demonstrate logic operations, we performed numerical simulations based on the macro-spin model. To execute logic operation in the arrayed structure, we used the stray field from a magnetic force microscope tip.

Plasmonic bandgap formation in two-dimensional periodic arrangements of gold patches with subwavelength gaps

We report on the formation of plasmonic bandgaps in two-dimensional periodic arrangements of gold patches. Orthogonal arrays of subwavelength slits with different periodicities have been studied by means of a three-dimensional finite-difference time-domain (FDTD) code, changing incident polarization and geometrical parameters. Spectral response of gold patches having different a form factor and surrounded by different media have been also investigated and compared in order to give a full description of bandgap shifts paving the way for the design of polarization-sensitive devices.

Directional dependence of the second harmonic response in two-dimensional nonlinear photonic crystals

A two-dimensional periodic arrangement of alternating ferroelectric domains in LiNbO3, with asymmetric domain duty cycle has been used to demonstrate that counterpropagating beams along the polar axis can generate disparate second harmonic patterns, which are nonsymmetrical with respect to the source and the point of observation. These findings provide alternative routes to generate dissimilar light-matter interaction processes in two-dimensional structures assembled onto polar surfaces, including metals for plasmonics or biological compounds.

1D and 2D phononic crystal sensors

Acoustic band gap materials, so-called phononic crystals, provide a new sensor platform for determination of material properties in small cavities. The sensor employs specific transmission windows within the band gap to determine properties of a component that builds the phononic crystal. The frequency where transmission takes place is correlated to material properties and permits the determination of several parameters of practical interest like concentration of an analyte. The capability of the concept will be demonstrated with a one-dimensional arrangement of solid plates and liquid filled cavities and a two-dimensional periodic arrangement of liquid filled holes in a solid matrix.

Phononic crystals for liquid sensor applications

Acoustic band gap materials, so-called phononic crystals, are introduced as a new platform for sensing material properties in small cavities. The sensor employs specific transmission windows within the band gap to determine properties of one component that builds the phononic crystal. The dependence of the frequency where transmission takes place is correlated to material properties, specifically to the sound velocity of a liquid. This value is related to several parameters of practical interest like the concentration of one component in a mixture or conversion rate in a microreactor. The capability of the concept will be demonstrated with a one-dimensional arrangement of solid plates and liquid-filled cavities and a two-dimensional periodic arrangement of liquid-filled holes in a solid matrix. The properties of 1D phononic crystals will be analysed in terms of the effective acoustic impedance and the resulting transmission behaviour and experimentally verified. The transmission properties of the 2D phononic crystal will be modelled with the layer multiple-scattering theory. Similar features which can be employed for sensing purposes will be discussed.

Mesoporous CeO 2: Synthesis by nanocasting, characterisation and catalytic properties

Mesoporous CeO 2 was synthesised by using CMK-3 carbon as a structure matrix. Nitrogen physisorption, powder X-ray diffraction, transmission electron microscopy (TEM), selected-area electron diffraction (SAED), energy-dispersive X-ray (EDX), X-ray absorption near-edge structure (XANES), and thermal (TG/MS) analysis were used for their characterisation. Methanol decomposition to hydrogen, CO, and methane was used as a catalytic test reaction. The obtained products exhibit uniform pores with a diameter of ca. 5 nm in a two-dimensional hexagonal periodic arrangement, as well as interparticle porosity, broadly distributed around ca. 35 nm; the specific surface area is 148 m 2 g −1. The pore walls are polycrystalline. The polycrystalline nature and high surface-to-volume ratio of the products is reflected in an increased signal intensity in X-ray absorption spectroscopy. The synthesis of CeO 2 from Ce(NO 3) 3 within the pores of the carbon matrix and the subsequent thermal combustion of the carbon is monitored by thermal analysis. Catalytic tests reveal that the activity of the mesoporous products in methanol decomposition are substantially higher than for a non-porous sample.

Elastic wave scattering by two-dimensional periodical array of cylinders

We extend the multiple-scattering theory (MST) for elastic wave scattering and propagating in two-dimensional composite. The formalism for the band structure calculation is presented by taking into account the full vector character of the elastic wave. As a demonstration of application of the formalism we calculate the band structure of elastic wave propagating in a two-dimensional periodic arrangement of cylinders. The results manifest that the MST shows great promise in complementing the plane-wave (PW) approach for the study of elastic wave.

Molecular arrangements in Langmuir–Blodgett films of 2-dodecyl-, 2-pentadecyl- and 2-octadecyl-7,7,8,8-tetracyanoquinodimethanes

Atomic force microscopy (AFM) on Langmuir–Blodgett (LB) films of alkyl-tetracyanoquinodimethanes (alkyl-TCNQs) have been carried out. The two-dimensional periodic molecular arrangements are observed on the surface of each LB films. The observed periodic structure due to the two-dimensional molecular arrangements in the LB films of C 12- ( a 0=0.83±0.02 nm, b 0=0.47±0.01 nm, and γ=95±2°) and C 18TCNQ ( a 0=0.83±0.02 nm, b 0=0.48±0.01 nm, and γ=94±2°) are almost identical with each other, while that of the C 15TCNQ ( a 0=0.78±0.03 nm, b 0=0.47±0.01 nm, and γ=101±2°) is different from those films. Since the alkyl-TCNQ LB films consists of bimolecular layers, the observed images are due to the two-dimensional arrangements of the TCNQ chromophores. Thus, the difference in the way of subcell packing of the alkyl chains must affect the packing of the TCNQ chromophores, and the difference of the molecular arrangements seems to depend on an even or odd number of the length of the alkyl chain. These results agree with the result of infrared spectroscopy: the alkyl chains in the LB films of C 12- and C 18TCNQ crystallize with orthorhombic subcell packing, while those of C 15TCNQ are in the hexagonal or pseudohexagonal subcell packing.

Highly oriented thin film of a mixed-stack charge-transfer complex

Doping of 5,10-dimethyl-5,10-dihydrophenazine ((Me) 2P) to a 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (C 18TCNQ) LB film has been examined in a vapor phase, and it is found that the mixed-stack charge-transfer (CT) complex of (Me) 2P and C 18TCNQ in a quasi-ionic phase is formed by the doping. The (Me) 2P-C 18TCNQ LB film is found to consist of many platelet-like microcrystal domains with a well-ordered layered structure of thickness 3.31 nm. In the high-resolution atomic force microscopy measurements, a two-dimensional periodic molecular arrangement is observed. The period is 0.96 nm × 0.83 nm and the angle between the axes is 59°. By polarized electronic absorption spectroscopy it is confirmed that the molecular arrangement consists of the alternatively stacked array of (Me) 2P and C 18TCNQ. The doping in the vapor phase is very effective for the preparation of highly crystallized and oriented thin films of mixed-stack CT complexes.