High Contrast Material Research Articles

Restoring Real-Space Images of the Structure of Muscle and Other Biological Specimens from Conventional X-Ray Diffraction Patterns

The X-ray diffraction pattern of an object is the Fourier transform of its electron density distribution. Applying an inverse Fourier transformation to the diffraction pattern should in principle result in the restoration of the real-space structure of the object at different magnifications. However, the phase information is lost when the diffraction pattern is recorded on a detector, and the real-space structure is not restored without phase information. This is called the phase problem and has been the greatest obstacle in implementing X-ray diffraction techniques in wide fields. Recently, however, computer-based techniques have been developed to restore the real-space images of objects by recover or preserve phase information. These are collectively called coherent diffractive imaging (CDI). These techniques have been successfully implemented for high-contrast materials such as metal nanoparticles, but they have been less successful for low-contrast biological specimens. Also, it is generally believed that highly coherent X-ray beams are required such as the ones generated by X-ray free electron laser facilities. Here we show the results of implementation of the CDI techniques to diffraction patterns obtained in conventional storage-ring synchrotron radiation facilities. The materials include low-density biological specimens such as muscle and eukaryotic flagella. The successful implementation of the CDI techniques will greatly expand the usefulness of X-ray diffraction techniques.

The lowest vibration spectra of multi-component structures with contrast material properties

The paper is concerned with the lowest vibration modes of multi-component rods and cylinders with alternating high contrast material properties of the components. It is demonstrated that these modes correspond to almost rigid body motions of the “stiffer” components. A general perturbation scheme is developed. At leading order polynomial frequency equations are derived, along with linear algebraic equations for the associated eigenforms. In addition, for multi-component rods, higher order corrections are also obtained. Examples of three- and five-component structures are presented, illustrating the efficiency of the proposed approach.

Achieving tunable dual-emissive and high-contrast mechanochromic materials by manipulating steric hindrance effects

A rational strategy for the design and synthesis of tunable dual-emissive and single-component white-light-emitting materials by manipulating steric hindrance effects.

Broadband c-Si metasurfaces with polarization control at visible wavelengths: applications to 3D stereoscopic holography.

Visual arts and entertainment related industries are continuously looking at promising innovative technologies to improve users' experience with state-of-the-art visualization platforms. This requires further developments on pixel resolution and device miniaturization which can be achieved, for instance, with high contrast materials, such as crystalline silicon (c-Si). Here, a new broadband stereoscopic hologram metasurface is introduced, where independent phase control is achieved for two orthogonal polarizations in the visible spectrum. The holograms are fabricated with a birefringent metasurface consisting of elliptical c-Si nanoposts on Sapphire substrate. Two holograms are combined on the same metasurface (one for each polarization) where each is encoded with four phase levels. The theoretical bandwidth is 110 nm with a signal to noise ratio (SNR) >15 dB. The stereoscopic view is obtained with a pair of cross-polarized filters in front of the observers' eyes. The measured transmission and diffraction efficiencies are about 70% and 15%, respectively, at 532 nm (the design wavelength). The metasurfaces are also investigated at 444.9 nm and 635 nm to experimentally assess their bandwidth performance. The stereoscopic effect is surprisingly good at 444.9 nm (and less so at 635 nm) with transmission and diffraction efficiencies around 70% and 18%, respectively.

The effect of molecular symmetry on the mechanofluorochromic properties of 4H-pyran derivatives

Two series of D-π-A 4H-pyran derivatives using 1H-indene-1,3(2H)-dione as an electron-withdrawing, namely asymmetric AP-1-AP-3 and symmetric SP-1-SP-3, were synthesized to investigate the effect of the molecular symmetry on their mechanofluorochromic (MFC) activities. All these compounds with twisted molecular conformations exhibit obvious aggregation-induced emission properties caused by the restriction of intramolecular rotation in the aggregated sate. Although AP-1 and SP-1 are MFC-inactive, methyl-substituted AP-2 and trifluoromethyl-substituted AP-3 with low symmetry display reversible MFC activities, whereas the corresponding compounds SP-2 and SP-3 with high symmetry have no obvious solid-state emission color changes upon grinding. The results indicate the effect of the introduction of methyl and trifluoromethyl groups on the MFC properties of these compounds is controlled by the molecular symmetry. The MFC properties of AP-2 and AP-3 are ascribed to the phase transition from a crystalline state to another and an amorphous state, respectively, as revealed by X-ray diffraction and differential scanning calorimetry experiments. Our results indicate that the adjustment of the molecular symmetry can be regarded as a way of constructing a new and/or high-contrast MFC-active material from a particular MFC-inactive molecule.

Estimates for elliptic systems in a narrow region arising from composite materials

In this paper, we establish the pointwise upper and lower bounds of the gradients of solutions to a class of elliptic systems, including linear systems of elasticity, in a general narrow region and in all dimensions. This problem arises from the study of damage analysis of high-contrast composite materials. Our results show that the damage may initiate from the narrowest place.

Bandgaps in two-dimensional high-contrast periodic elastic beam lattice materials

We consider elastic waves in a two-dimensional periodic lattice network of Timoshenko-type beams. We show that for general configurations involving certain highly-contrasting components a high-contrast modification of the homogenization theory is capable of accounting for bandgaps, explicitly relating those to low resonant frequencies of the “soft” components. An explicit example of a square-periodic network of beams with a single isolated resonant beam within a periodicity cell is considered in detail.

Bipyridine-based aggregation-induced phosphorescent emission (AIPE)-active gold(I) complex with reversible phosphorescent mechanochromism and self-assembly characteristics

Mechanochromic luminescent materials have received a great deal of attention due to their potential applications in sensors and optoelectronic devices. Meanwhile, the aggregation-induced emission active materials commonly exhibit bright luminescence upon aggregate formation, which is extremely important for preparation of high-contrast mechanochromism materials. On the other hand, it is well-known that pyridine-type ligands are one of the most versatile ligands. Unfortunately, the number of pyridine gold complexes is quite limited, not to mention pyridine-containing gold(I) complexes with aggregation-induced emission or mechanochromic phenomenon. In this work, a new bipyridine-based aggregation-induced phosphorescent emission active gold(I) complex is reported. The novel luminogen exhibits reversible mechanochromism phenomenon with phosphorescent changes between weak green and strong yellow emissions. Moreover, the luminogen shows a significative self-assembly behavior. This work will be beneficial for the exploitation of force-responsive phosphorescent sensor materials with remarkable self-assembly behavior.

High-performance axicon lenses based on high-contrast, multilayer gratings

Axicon lenses are versatile optical elements that can convert Gaussian beams to Bessel-like beams. In this letter, we demonstrate that axicons operating with high efficiencies and at large angles can be produced using high-contrast, multilayer gratings made from silicon. Efficient beam deflection of incident monochromatic light is enabled by higher-order optical modes in the silicon structure. Compared to diffractive devices made from low-contrast materials such as silicon dioxide, our multilayer devices have a relatively low spatial profile, reducing shadowing effects and enabling high efficiencies at large deflection angles. In addition, the feature sizes of these structures are relatively large, making the fabrication of near-infrared devices accessible with conventional optical lithography. Experimental lenses with deflection angles as large as 40° display field profiles that agree well with theory. Our concept can be used to design optical elements that produce higher-order Bessel-like beams, and the combination of high-contrast materials with multilayer architectures will more generally enable new classes of diffractive photonic structures.

Band-pass filters based on photonic crystal

Multilayer photonic crystal structures with bleaching layers are being investigated. In order to calculate the characteristics of ultra-wideband filters on their basis, T-lines lossless model was used. Amplitude-frequency characteristics for the synthesized filters of 5th, 11th and 17th orders are given. It is proved that by a significant increase in filter N order, the difference between the connection coefficients of central resonators’ layers’ becomes negligible. This makes it possible to develop 27-order filter, in which almost half of the layers are realized by periodic interchange of only two identical high-contrast materials. The investigated band-pass filters, including the ones on a glass substrate, have high frequency-selective properties at a relative bandwidth of 80%.

Analysis of a thin, penetrable, and non‐uniformly loaded cylindrical reflector illuminated by a complex line source

A thin, penetrable, and cylindrical reflector is illuminated by the incident field of a complex source point. The scattered field inside the reflector is not considered and its effect is modelled through a thin layer generalised boundary condition (GBC). The authors formulate the structure as an electromagnetic boundary value problem and two resultant coupled singular integral equation system of equations are solved by using regularisation techniques. The GBC provides us to simulate the thin layer better than the resistive model which is applicable only for very thin sheets. Hence, the more reliable data can be obtained for high-contrast and low-loss dielectric material. The scattering and absorption characteristics of the front-fed and offset reflectors are obtained depending on system parameters. Also, the effects of the edge loading are examined for both E- and H-polarisations. The convergence and the accuracy of the formulation are verified in reasonable computational running time.

Photonic ring resonator filters for astronomical OH suppression.

Ring resonators provide a means of filtering specific wavelengths from a waveguide, and optionally dropping the filtered wavelengths into a second waveguide. Both of these features are potentially useful for astronomical instruments. In this paper we focus on their use as notch filters to remove the signal from atmospheric OH emission lines from astronomical spectra. We derive the design requirements for ring resonators for OH suppression from theory and finite difference time domain simulations. We find that rings with small radii (< 10 μm) are required to provide an adequate free spectral range, leading to high index contrast materials such as Si and Si3N4. Critically coupled rings with high self-coupling coefficients should provide the necessary Q factors, suppression depth, and throughput for efficient OH suppression, but will require post-inscription tuning of the coupling and the resonant wavelengths. The overall prospects for the use of ring resonators in astronomical instruments is promising, provided efficient fibre-chip coupling can be achieved.

Rotational magneto-acousto-electric tomography (MAET): theory and experimental validation

We present a novel two-dimensional (2D) MAET scanner, with a rotating object of interest and two fixed pairs of electrodes. Such an acquisition scheme, with our novel reconstruction techniques, recovers the boundaries of the regions of constant conductivity uniformly well, regardless of their orientation. We also present a general image reconstruction algorithm for the 2D MAET in a circular chamber with point-like electrodes immersed into the saline surrounding the object. An alternative linearized reconstruction procedure is developed, suitable for recovering the material interfaces (boundaries) when a non-ideal piezoelectric transducer is used for acoustic excitation. The work of the scanner and the linearized reconstruction algorithm is demonstrated using several phantoms made of high-contrast materials and a biological sample.

Efficient Silicon Metasurfaces for Visible Light

Dielectric metasurfaces require high refractive index contrast materials for optimum performance. This requirement imposes a severe restraint; devices have either been demonstrated at wavelengths of 700nm and above using high-index semiconductors such as silicon, or they use lower index dielectric materials such as TiO$_{2}$ or Si$_{3}$N$_{4}$ and operate in the visible wavelength regime. Here, we show that the high refractive index of silicon can be exploited at wavelengths as short as 532 nm by demonstrating a silicon metasurface with a transmission efficiency of 47% at this wavelength. The metasurface consists of a graded array of silicon posts arranged in a square lattice on a quartz substrate. We show full 2{\pi} phase control and we experimentally demonstrate polarization-independent beam deflection at 532nm wavelength. The crystalline silicon is placed on a quartz substrate by a bespoke layer transfer technique and we note that an efficiency >70% may be achieved for a further optimized structure in the same material. Our results open a new way for realizing efficient metasurfaces based on silicon in the visible wavelength regime.

Dispersion of elastic waves in laminated glass

Elastic sandwich-type structures with high-contrast material and geometrical properties have numerous applications in modern engineering, including, in particular, laminated glass, photovoltaic panels, precipitator plates in gas filters, etc. Multi-parametric modelling of such structures assumes taking into consideration various types of contrast in stiffness, density and thickness. The present contribution is concerned with analysis of low-frequency dispersion of elastic waves in case of an antisymmetric motion of a three-layered symmetric plate, modelling laminated glass. The conditions on material and geometrical parameters, leading to the lowest non-zero thickness shear resonance frequency tending to zero, are formulated. In this case the dispersion relation possesses two low-frequency modes instead of a single fundamental low-frequency mode, which is typical for a homogeneous plate. A two-mode uniform asymptotic approximation is constructed, along with local approximations for the fundamental mode and the first shear harmonic.

High contrast homogenisation in nonlinear elasticity under small loads

We study the homogenisation of geometrically nonlinear elastic composites with high contrast. The composites we analyse consist of a perforated matrix material, which we call the “stiff” material, and a “soft” material that fills the remaining pores. We assume that the pores are of size [Formula: see text] and are periodically distributed with period ε. We also assume that the stiffness of the soft material degenerates with rate [Formula: see text], [Formula: see text], so that the contrast between the two materials becomes infinite as [Formula: see text]. We study the homogenisation limit [Formula: see text] in a low energy regime, where the displacement of the stiff component is infinitesimally small. We derive an effective two-scale model, which, depending on the scaling of the energy, is either a quadratic functional or a partially quadratic functional that still allows for large strains in the soft inclusions. In the latter case, averaging out the small scale-term justifies a single-scale model for high-contrast materials, which features a non-linear and non-monotone effect describing a coupling between microscopic and the effective macroscopic displacements.

Numerical modeling of mullions in the Taili high deformation zone, North China: Implications for the rheology of granitic rocks

This paper presents a combined field measurement and finite element modeling analysis of the mullions occurring on the contact of two granitic rocks with different grain size in the Taili High-Strain Deformation Zone (THDZ), West Liaoning of North China. All of the field data are located in the plot zone of the modeling results. Numerical modeling results indicate that: (1) The inter-angle between the tangent lines cross the cusp point and the ratio R of amplitude and width of mullions are the most effective parameters to describe the geometric shape and evolution of mullions, as well as useful indicators of the rheology of rocks. (2) The competence contrast controls the growth rate of mullions under shortening. It determines the possible ratio R of final mullions. Moreover, decreasing of the cusp angle in high competence contrast materials is faster than that in low competence contrast model. (3) The initial disturbance is an essential factor for the generation of mullions. Those contacts with higher initial disturbance will develop into mullions more easily and have a high growth rate during the same shortening deformation regime. (4) The rheology and deformation behavior of the granitic rocks in the study area are primarily controlled by the grain sizes of quartz and feldspar. The effective viscosity ratio of biotite adamellite and granitic gneisses is about 0.01–0.5. The deformation mechanisms of these granitic rocks should be dominated by a grain-size-sensitive diffusion creep.

Indium phosphide all air-gap Fabry-Pérot filters for near-infrared spectroscopic applications

Food quality can be characterized by noninvasive techniques such as spectroscopy in the Near Infrared wavelength range. For example, 930 -1450 nm wavelength range can be used to detect diseases and differentiate between meat samples. Miniaturization of such NIR spectrometers is useful for quick and mobile characterization of food samples. Spectrometers can be miniaturized, without compromising the spectral resolution, using Fabry-Pérot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. The most commonly used mirrors in the design of FP filters are Distributed Bragg Reflections (DBRs) consisting of alternating high and low refractive index material pairs, due to their high reflectivity compared to metal mirrors. However, DBRs have high reflectivity for a selected range of wavelengths known as the stopband of the DBR. This range is usually much smaller than the sensitivity range of the spectrometer detector. Therefore, a bandpass filter is usually required to restrict wavelengths outside the stopband of the FP DBRs. Such bandpass filters are difficult to design and implement. Alternatively, high index contrast materials must be can be used to broaden the stopband width of the FP DBRs. In this work, Indium phosphide all air-gap filters are proposed in conjunction with InGaAs based detectors. The designed filter has a wide stopband covering the entire InGaAs detector sensitivity range. The filter can be tuned in the 950-1450 nm with single mode operation. The designed filter can hence be used for noninvasive meat quality control.

Digital design of multimaterial photonic particles

Scattering of light from dielectric particles whose size is on the order of an optical wavelength underlies a plethora of visual phenomena in nature and is a foundation for optical coatings and paints. Tailoring the internal nanoscale geometry of such "photonic particles" allows tuning their optical scattering characteristics beyond those afforded by their constitutive materials-however, flexible yet scalable processing approaches to produce such particles are lacking. Here, we show that a thermally induced in-fiber fluid instability permits the "digital design" of multimaterial photonic particles: the precise allocation of high refractive-index contrast materials at independently addressable radial and azimuthal coordinates within its 3D architecture. Exploiting this unique capability in all-dielectric systems, we tune the scattering cross-section of equisized particles via radial structuring and induce polarization-sensitive scattering from spherical particles with broken internal rotational symmetry. The scalability of this fabrication strategy promises a generation of optical coatings in which sophisticated functionality is realized at the level of the individual particles.

Proton computed tomography images with algebraic reconstruction

A prototype of proton Computed Tomography (pCT) system for hadron-therapy has been manufactured and tested in a 175MeV proton beam with a non-homogeneous phantom designed to simulate high-contrast material. BI-SART reconstruction algorithms have been implemented with GPU parallelism, taking into account of most likely paths of protons in matter. Reconstructed tomography images with density resolutions r.m.s. down to ~1% and spatial resolutions <1mm, achieved within processing times of ~15′ for a 512×512 pixels image prove that this technique will be beneficial if used instead of X-CT in hadron-therapy.