Array Of Spirals Research Articles

Enhancement of Deep-Subwavelength Band Gaps in Flat Spiral-Based Phononic Metamaterials Using the Trampoline Phenomena

Abstract Elastic and acoustic metamaterials can sculpt dispersion of waves through resonances. In turn, resonances can give rise to negative effective properties, usually localized around the resonance frequencies, which support band gaps at subwavelength frequencies (i.e., below the Bragg-scattering limit). However, the band gaps width correlates strongly with the resonators’ mass and volume, which limits their functionality in applications. Trampoline phenomena have been numerically and experimentally shown to broaden the operational frequency ranges of two-dimensional, pillar-based metamaterials through perforation. In this work, we demonstrate trampoline phenomena in lightweight and planar lattices consisting of arrays of Archimedean spirals in unit cells. Spiral-based metamaterials have been shown to support different band gap opening mechanisms, namely, Bragg-scattering, local resonances and inertia amplification. Here, we numerically analyze and experimentally realize trampoline phenomena in planar metasurfaces for different lattice tessellations. Finally, we carry out a comparative study between trampoline pillars and spirals and show that trampoline spirals outperform the pillars in lightweight, compactness and operational bandwidth.

Switchable polar spirals in tricolor oxide superlattices

There are increasing evidences that ferroelectric states at the nanoscale can exhibit fascinating topological structures including polar vortices and skyrmions, akin to those observed in the ferromagnetic systems. Here we report the discovery of a new type of polar topological structure, an ordered array of nanoscale spirals, in the PbTiO3/BiFeO3/SrTiO3 tricolor ferroelectric superlattice system obtained via phase-field simulations. This polar spiral structure is composed of fine ordered semi-vortex arrays with vortex cores forming a wavy distribution. It is demonstrated that the tricolor system has an ultrahigh Curie temperature of ∼1000 K and a temperature of ∼800 K for the phase transformation from spiral structure to in-plane orthorhombic domain structure, demonstrating a great thermal stability. The periodicity phase-diagram is constructed, showing multiple phases from inplane domain, polar spiral, and polar vortex to flux-closure with increasing ferroelectric layer thickness. Moreover, the spiral structure has a net in-plane polarization that could be switched by an experimentally-feasible irrotational in-plane field. The switching process involves a metastable vortex state and is fully reversible. This discovery could open up a new routine to design novel multiferroic topological structures with enhanced stability and tunability towards potential future applications in next-generation electronics.

Resolving new ultrastructural features of cytokinetic abscission with soft-X-ray cryo-tomography.

Mammalian cytokinetic abscission is mediated by the ESCRT membrane fission machinery. While much has been clarified on the topology and kinetics of abscission through high-resolution microscopy, key questions regarding the mechanism of abscission remain open. Here we apply cryogenic soft-X-ray tomography to elucidate new ultrastructural details in the intercellular membrane bridge connecting cells undergoing abscission. In particular, we resolve defined ring-like structures inside the midbody dark zone that have been inaccessible to EM, and identify membrane extrusions at the abscission sites. In cells at late stages of abscission we resolve a complex array of helical spirals, extending the structural information obtained by EM. Our results highlight the advantages of soft-X-ray tomography and emphasize the importance of using complementary approaches for characterizing cellular structures. Notably, by providing new structural data from intact cells we present a realistic view on the topology of abscission and suggest new mechanistic models for ESCRT mediated abscission.

Estimating the Bandwidth of Spiral Antenna Arrays

An analytical method to estimate the bandwidth of planar phased array of spiral antennas is presented. Three main parameters are considered: circular polarization, reflection coefficient, and sidelobe levels. An application of the method for a planar array of asymmetrical Archimedean spirals is presented for a scan angle of $\theta {= 30^ \circ} $ . The estimated bandwidth and the bandwidth obtained with the simulation results differ in 3.5%.

Superconducting RF Metamaterials Made With Magnetically Active Planar Spirals

Superconducting metamaterials combine the advantages of low-loss, large inductance (with the addition of kinetic inductance), and extreme tunability compared to their normal metal counterparts. Therefore, they allow realization of compact designs operating at low frequencies. We have recently developed radio frequency (RF) metamaterials with a high loaded quality factor and an electrical size as small as $\sim$$\lambda$658, ($\lambda$ is the free space wavelength) by using Nb thin films. The RF metamaterial is composed of truly planar spirals patterned with lithographic techniques. Linear transmission characteristics of these metamaterials show robust Lorentzian resonant peaks in the sub- 100 MHz frequency range below the $T_c$ of Nb. Though Nb is a non-magnetic material, the circulating currents in the spirals generated by RF signals produce a strong magnetic response, which can be tuned sensitively either by temperature or magnetic field thanks to the superconducting nature of the design. We have also observed strong nonlinearity and meta-stable jumps in the transmission data with increasing RF input power until the Nb is driven into the normal state. We discuss the factors modifying the induced magnetic response from single and 1-D arrays of spirals in the light of numerical simulations.

Stacked Spirals for Biosensor Telemetry

This paper presents an inductive power transmission coil for biosensor-based telemetric implants. Using stacked spirals reduces the consumed space and the self-resonant frequency (SRF) of the spiral and the required power transmission frequency for the implanted device. A four-layer 15 mm × 15 mm spiral coil of seven turns is simulated in CST Microwave Studio (TM), constructed and tested in hardware with comparable results. Measurements also include the receipt of power inductively from an energized array of spirals powered by a Class-E transmitter at 27 MHz, and rectified to 1 V for biosensor applications.

Compact Coplanar Waveguide Spiral Antenna With Circular Polarization for Wideband Applications

This letter presents a compact, uniplanar, circularly polarized, coplanar waveguide (CPW) spiral antenna for wideband applications. The antenna is directly fed by a 50-Ω CPW from the outside edge of the spiral, thus a balun for matching is not required. This feed provides the capability to have an entire uniplanar array of spirals. It is found that a thick substrate of high dielectric constant absorbs most of the radiated power and results in a unidirectional radiation pattern, which is desirable in many applications. Therefore, this antenna structure is fabricated on a substrate of high dielectric constant, resulting in miniaturization and capability for integrated system applications. It is experimentally verified that the proposed spiral antenna has stable radiation pattern, and its axial ratio is less than 3 dB over the frequency range of 11.4-17.5 GHz.

Coherent acoustic vibrations in silicon submicron spiral arrays

Mechanical properties of complex silicon submicron structures have been studied both experimentally and theoretically using time resolved ultrafast spectroscopy and finite element analysis. Periodic and random arrays of single-turned silicon submircron spirals were grown using the oblique angle deposition technique. Resonant vibrational modes of the submicron spirals were coherently excited by femtosecond laser pulses. Excitation of multiple harmonics of the resonant vibrations has been observed, and the mode patterns of the excited vibrations in the submicron spirals have been calculated.

Modeling of Spirals with Equal Dielectric, Magnetic, and Chiral Susceptibilities

In this article, we study spiral particles with optimized design parameters, which can make possible the realization of media with equal dielectric, magnetic, and chiral susceptibilities. Two different spiral structures are investigated: the canonical spiral (which consists of a split loop with two straight-wire sections, orthogonal to the loop plane and connected to the edges of the gap) and the true helix (which is obtained by bending a wire with a constant pitch angle). The transmission and reflection coefficients of arrays of spiral particles are obtained under plane wave excitation by numerical simulation. The properties of slabs formed by periodic chiral and racemic arrays of spirals are investigated. Good agreement is found between the presented results and the previously reported theoretical and experimental studies.

Slicing It All Ways: Mathematical Models for Tonal Induction, Approximation, and Segmentation Using the Spiral Array

This paper presents the spiral array model and its associated algorithms for tonal induction, approximation, and segmentation. The spiral array is a geometric model for tonality that clusters perceptually similar tonal entities. The model summarizes music information as interior points inside an array of spirals. Distances in the spiral array space are used to quantify tonal similarity. The paper traces the evolution, and presents general forms, of the existing algorithms for key finding, pitch spelling, and segmentation, and proposes a new O(n) algorithm, Argus, for tonal segmentation. The proposed algorithm computes a value that quantifies the discrepancy between the local contexts in the future and past at each point in time. Discrepancy values exceeding control thresholds are shown to mark the segmentation boundaries of the test set that concur with expert analyses. A number of window sizes and threshold settings are investigated. The algorithm is demonstrated using Edward MacDowell’s To A Wild Rose and tested on Franz Schubert’s Allegretto from Moment Musical D780 No. 6 and Thema from Impromptu D935 No. 4. The algorithm accurately locates tonal boundaries in all three case studies.

Oxidation of Structured Silicon Thin Films for Inverse Silicon Square Spiral Photonic Crystal Fabrication

ABSTRACTPhotonic Band Gap (PBG) crystals are artificially engineered structures of periodically alternating dielectric materials that enable manipulation of light in unique ways. As proposed by O. Toader and S. John, large bandwidth PBG crystals based on the diamond lattice can be constructed by fabricating a two-dimensional periodic array of silicon square spirals. Furthermore, an improved photonic crystal can be constructed by fabricating the inverse of this structure, such that void square spirals are situated in a silicon backfill. This structure yields a theoretical maximum PBG of 24%, while the direct silicon square spiral structure yields a theoretical maximum of 16%. [1]The direct silicon square spiral structure has previously been successfully fabricated using the glancing angle deposition (GLAD) process [2]. The GLAD fabrication process uses advanced substrate motion and oblique incidence deposition to engineer precise nanostructures such as helices, chevrons, or polygonal spirals. These nanostructures are grown with lateral and longitudinal periodicity using a pre-patterned substrate consisting of a tetragonal array of relief structures.Upon successful fabrication of the direct silicon square spiral structure, it was proposed that fabrication of the inverse structure could be completed using a backfill process with a silica template [2]. A periodic array of silica square spirals would be fabricated using the GLAD technique. The template would then be filled with amorphous silicon using low-pressure chemical vapour deposition (LPCVD). To reveal the inverse silicon structure, the silica template would be removed by means of chemical etching with an isotropic chemical etchant. Creation of a suitable template for the inversion of silicon square spirals is an important step in the realization of the inverse silicon square spiral PBG crystal. However, we have seen empirically that silica square spiral films grown directly via the GLAD technique are structurally inferior to silicon films. We present an intermediate step for the fabrication of a suitable template, whereby a silicon structure is fabricated with appropriate geometries and subsequently oxidized. The procedure has been optimized to ensure a thorough oxidization of the silicon posts, while minimally distorting the structure of the square spiral template.

Artificial Uniaxial Bianisotropic Media at Oblique Incidence of Electromagnetic Waves

The response of uniaxial bianisotropic slabs to obliquely incident electromagnetic waves is studied, and the exact solution of this boundary-value problem is derived. This includes the calculation of all characteristics of reflected and transmitted waves. The study is relevant to recent experimental studies of arrays of parallel spirals. The present theory gives an analytical model of such artificial bianisotropic media.

Reflection and transmission by a uniaxially bi-anisotropic slab under normal incidence of plane waves

In this paper plane-wave reflection and transmission phenomena in slabs of uniaxial bi-anisotropic media are theoretically considered. The axis direction is in the interface plane and normal incidence of the plane wave is assumed. Chiral reciprocal slabs and a special class of non-reciprocal media are covered. The study is relevant to recent experimental studies of arrays of parallel spirals. The present theory gives an analytical model of such artificial bi-anisotropic media.

Reflection of electromagnetic waves from dense arrays of thin long conductive spirals

Complex three-dimensional grids formed by parallel conductive spirals are considered using a homogenization procedure. Spirals are replaced by straight wires with periodically inserted loops. Averaged electric and magnetic currents are determined which allows to calculate the reflection and transmission coefficients. Numerical examples for co‐ and cross‐polarized reflection and transmission coefficients are given.

Two-dimensional micromechanical bimorph arrays for detection of thermal radiation

We demonstrate that two-dimensional arrays of micromechanical bimorphs can be used as thermal sensors to image infrared (IR) radiation. A density of 100 pixels per mm2 is achieved by coiling a bimorph beam into the shape of a flat spiral. Temperature variations of a given spiral are converted to modulations of visible light by illuminating the spiral array with a visible source. The optical properties of the spiral resemble a Fresnel zone plate when light reflected off neighboring rings of the spiral is focused. When a spiral is heated through the absorption of IR radiation, thermally induced bending of the bimorph degrades the focusing efficiency by distorting the spiral. This reduces the optical intensity at the focal point. Arrays of spirals can be monitored with a commercial CCD camera. At 40 Hz, the temperature resolution and noise equivalent power of a 75 μm diam spiral are 50 μK/√Hz and 20 nW/√Hz, respectively, and the thermal response time is 270 μs.