Asymmetric Crystals Research Articles

Observation of multifunctional robust topological states based on asymmetric C4 photonic crystals

Recent advancements in high-order topological insulators have heralded new opportunities for the innovation and utilization of optical devices. This paper presents a composite asymmetric C4 photonic crystal to achieve multifunctional, robust topological states. Through detailed analysis of the fine changes in the topological bandgap induced by distortion parameters, we facilitate the realization of topological edge states in wavelength division multiplexing applications. We utilize both trivial and nontrivial properties of the topological bandgap to precisely manipulate zero-dimensional angular states, one-dimensional topological boundary states, and two-dimensional body states. Through simulations and experimental results, our advanced asymmetric C4 photonic crystal structure demonstrates superior robustness for the transmission of topological edge states. Our research paves the way for the deployment of more robust topological boundary state transmission systems and advances the application potential of higher-order topological states.

Multi-Frequency Asymmetric Absorption-Transmission Metastructures-Photonic Crystals and Their Application as a Refractive Index Sensor.

In this paper, a kind of metastructure-photonic crystal (MPC) with multi-frequency asymmetric absorption-transmission properties is proposed. It is composed of various dielectric layers arranged in a periodically tilting pattern. When electromagnetic waves (EMWs) enter from the opposite direction, MPC shows an obvious asymmetry. EMWs are absorbed at 13.71 GHz, 14.37 GHz, and 17.10 GHz in forward incidence, with maximum absorptions of 0.919, 0.917, and 0.956, respectively. In the case of backward incidence, transmission above 0.877 is achieved. Additionally, the MPC is utilized for refractive index (RI) sensing, allowing for wide RI range detection. The refractive index unit is denoted as RIU. The RI detection range is 1.4~3.0, with the corresponding absorption peak variation range being 17.054~17.194 GHz, and a sensitivity of 86 MHz/RIU. By adjusting the number of MPC cycles and tilt angle, the sensing performance and operating frequency band can be tailored to meet various operational requirements. This MPC-based RI sensor is simple to fabricate and has the potential to be used in the development of high-performance and compact sensing devices.

Increased spatial coherence length from an asymmetric crystal reflection at grazing exit.

Coherent X-ray imaging is an active field at synchrotron sources. The images rely on the available coherent flux over a limited field of view. At many synchrotron beamlines a double-crystal monochromator (DCM) is employed in a standard nondispersive arrangement. For coherent diffraction imaging it is advantageous to increase the available field of view by increasing the spatial coherence length (SCL) of a beam exiting such a DCM. Here, Talbot interferometry data together with ray-tracing simulations for a (+ - - +) four-reflection experimental arrangement are presented, wherein the first two reflections are in the DCM and the final fourth reflection is asymmetric at grazing exit. Analyses of the interferometry data combined with the simulations show that compared with the beam exiting the DCM a gain of 76% in the SCL was achieved, albeit with a factor of 20 reduction in flux density, which may not be a severe penalty at a synchrotron beamline. Previous efforts reported in the literature to increase the SCL that employed asymmetric crystal diffraction at grazing incidence are also discussed. A much reduced SCL is found presently in simulations wherein the same asymmetric crystal is set for grazing incidence instead of grazing exit. In addition, the present study is compared and contrasted with two other means of increasing the SCL. These are (i) focusing the beam onto an aperture to act as a secondary source, and (ii) allowing the beam to propagate in vacuum an additional distance along the beamline.

Asymmetric dual-core liquid crystal channel-based tunable mode converter

In this work, a higher order-to-fundamental mode converter is reported and analyzed based on an asymmetric dual channel waveguide (ADC-WG) on silicon. In the reported structure, one of the two waveguides is infiltrated with nematic liquid crystal (NLC) material to add temperature tunability while the other one is a solid BK7 waveguide. The modal characteristics are obtained using the full vectorial finite difference method (FVFDM). In addition, the structural parameters and optical characteristics of the employed materials are investigated to achieve good wavelength selectivity with a short device length (LD). Thus, a compact mode converter that can work at different wavelengths including the telecommunication wavelength i.e., 1.55 μm with LD ~ 482.31 μm and a low crosstalk of − 19.86 dB is presented. To prove the thermal tunability of the suggested mode converter, its operation is tested through a temperature range between 20 and 35 °C and the results show that the mode conversion process is achieved at each temperature with different phase matching wavelengths (λPMW) but with quite similar coupling length (LC). The proposed device can therefore be effectively utilized in integrated photonic circuits.

Effect of angle of incidence on defect modes of symmetric and asymmetric one-dimensional photonic crystals

The impact of the angle of incidence on defect modes within a one-dimensional photonic crystal (PC) containing a central defect has been thoroughly examined. We have considered two distinct types of photonic crystals, each featuring a solitary defect: symmetric PCs and asymmetric PCs. Our findings reveal that in the case of asymmetric PCs, a defect mode emerges precisely at the central wavelength. Conversely, for symmetric PCs, two distinct defect modes appear in the proximity of the central wavelength. By widening the defect layer, these two defect modes can be merged into a single central defect mode. However, it's worth noting that the intensities of these merged defect modes differ from those observed in the asymmetric case. We have analyzed the propagation characteristics of the proposed structure using the transfer matrix method.

Microstructure evolution of porous calcium hexaluminate compacts during first heating

Porous calcium hexaluminate (CaAl12O19 or CA6) structures show long-term stability at high temperatures, high refractoriness, low thermal conductivity, and excellent chemical resistance, thus being Taylor-made for thermal insulation. Previous works attained porous structures from in situ formation of CA6 combining CaO and Al2O3 sources in solid-state reactive sintering. Although the approach is time-and-energy-saving regarding the production of large parts of complex shapes, it faces considerable difficulties related to the expansive formation of calcium aluminates. To overcome such drawbacks, pre-formed porous CA6 aggregates improve the system's dimensional stability before sintering. Despite the straightforward processing, few studies have investigated such materials systemically. This article addresses the combination of pre-formed porous CA6 aggregates and organic and inorganic binders for the production of porous structures by two shaping processes, namely, uniaxial pressing and direct casting of aqueous suspensions. After drying, the samples' microstructure and physical properties evolution were investigated up to sintering (1100-1500 °C) through total porosity, Young's modulus, compression strength, pore diameter, and thermal conductivity measurements, dilatometric analyses, and scanning electron microscopy. Reference samples of coarse calcined alumina were tested under the same conditions to highlight the impacts of CA6 particles' morphology. Compared to them, the CA6-containing samples showed almost no variation in total porosity and average pore size levels during thermal treatments, although their strength and rigidity increased significantly after sintering. Their microstructure remained practically unchanged after drying, showing clusters of large asymmetrical CA6 crystals bonded to each other by their edges, and surrounding a large fraction of 1.8–2.2 μm pores. According to the results, although water content, processing method, and compacting levels are important parameters, particles' microstructure, ratio of intra-particle pores, and asymmetrical shape are key variables for the development of physical properties. Such characteristics strongly contributed to their densification resistance and lower thermal conductivity after exposure to high temperatures.

Asymmetrical liquid crystals synthesis for effective sensing: Fluorescence investigations

Currently, a new set of asymmetrical dimeric molecules with a Schiff-base at one end and a chalcone alkali substitution by di-ether are made and their liquid crystal and fluorescence properties are investigated. The proposed chemical structure was confirmed by routine analytical methods using FT-IR and NMR spectroscopy. Additionally, polarized optical microscopy (POM); was used to assess the pointed compounds' liquid crystalline characteristics, while differential scanning calorimetry (DSC); was employed to confirm the compounds' transitions. The results report the length of the alkali chain at one end of the molecule is critical to the creation of the mesophase type. Compounds with medium-length chains exhibit nematic mesophases with moderate transition temperatures. The fluorescence of compounds was analyzed, with blue-emitting results indicating the presence of blue-light-emitting features. The 10a had higher ɛ; as well as red-shifted λmax; and Fmax; compared to the others and has blue light emission properties. Eventually; the expanding range of possible uses to include OLED material and fluorescent probes for sensing applications.

Low-threshold bistable absorption in asymmetrical one-dimensional photonic crystals containing Weyl semimetal defects

Low-threshold bistable absorption in asymmetrical one-dimensional photonic crystals containing Weyl semimetal defects

Large-view x-ray imaging for medical applications using the world’s only vertically polarized synchrotron radiation beam and a single asymmetric Si crystal

Objective. X-ray microangiography provides detailed information on the internal structure and function of a biological subject. Its ability to evaluate the microvasculature of small animals is useful for acquiring basic and clinical medical knowledge. The following three conditions are necessary to attain detailed knowledge of biological functions: (1) high temporal resolution with sufficient x-ray intensity, (2) high spatial resolution, and (3) a wide field of view. Because synchrotron radiation microangiography systems provide high sapatial resolution and high temporal resolution as a result of their high x-ray intensity, such systems have been developed at various synchrotron radiation facilities, starting with the photon factory, leading to numerous medical discoveries. However, the three aforementioned functions are incompatible with the use of synchrotron radiation because the x-ray intensity decreases when a wide field of view is obtained. To overcome these problems, we developed a new x-ray optical system for microangiography in rats using synchrotron radiation x-rays. Approach. Instead of using monochromatic synchrotron radiation x-rays with a conventional double-crystal monochromator, we used white synchrotron radiation x-rays and an asymmetric Si crystal to simultaneously monochromatize the beam and widen the field of view. Main results. The intensity profile and spatial resolution of the x-ray images were then evaluated. The proposed x-ray optics increased the x-ray intensity and beam width by factors of 1.3 and 2.7, respectively, compared with those of conventional monochromatic x-rays. In addition, in vivo studies on microangiography in rats were performed to confirm that the images had sufficient intensity, spatial resolution, and field of view. One of a series of images taken at 50 ms frame−1 was shown as an example. Significance. This x-ray optics provides sufficient x-ray intensity, high spatial resolution, and a wide field of view. This technique is expected providing new insights into the evaluation of the vascular system.

Symmetrical anisotropy enables dynamic diffraction control in photonics.

Despite the steady advancements in nanofabrication made over the past decade that had prompted a plethora of intriguing applications across various fields, achieving compatibility between miniaturized photonic devices and electronic dimensions remains unachievable due to the inherent diffraction limit of photonic devices. Herein, we present an approach based on anisotropic scaling of the shapes of photonic crystals (PhCs) to overcome the diffraction limit and achieve controlled diffraction limit along the ΓX direction. Thus, we demonstrate that scaling the direction perpendicular to the wave's propagation (y-direction) by 1/2 and 1/4 significantly improves the diffraction limit by two and four orders of magnitude, respectively. This approach opens up possibilities for high-frequency wave guiding in a cermet configuration, which was previously unachievable. Furthermore, we illustrate the existence of a quasi-bound state in the continuum (QBICs) in asymmetric dimer network-type photonic crystals (PhCs).

Proton-decoupled deuterium NMR study of an asymmetric liquid crystal dimer having two nematic phases.

Proton-decoupled deuterium NMR spectra were obtained for an asymmetric liquid crystal dimer 1-(4-cyanobiphenyl-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl)hexane (CB6OCB) containing a single -CD_{2}- group. The sample has two nematic liquid crystal phases: a twist-bend nematic, N_{TB}, at the lowest temperature followed by a uniaxial nematic, N_{U}, on increasing the temperature. Proton decoupling reduces the linewidths of the peaks in the deuterium spectrum from kHz to ∼100Hz, enabling quadrupolar splittings, Δν, to be obtained with enhanced precision as well as the dipolar coupling between deuterium nuclei within the CD_{2} group, hence enhancing the information content.

Near-IR unidirectional absorption in a tunable asymmetric one-dimensional photonic crystal with VO2 defect layers

In this paper, a dynamically modulated Near-IR asymmetric composite photonic crystal (PC) is proposed, which constitutes by a one-dimensional PC (1-D PC) with vanadium dioxide (VO2) phase transition defect layers. By combining asymmetric composite PC with VO2 phase material, which will undergo the semiconductor-metal transition (SMT) under thermal stimulation, to realize the controllable unidirectional multi-channel absorber under temperature control. Based on a relatively simple 1-D stacked thin film model, the model is investigated and optimized in terms of the structure, number of periods, and the thickness of defect layers, with the result of 20 nm for VO2 defect layers and seven circles for the post-defect period. By using the pre-defect period number of 3, an average absorbance of 0.19 can be achieved when VO2 is in the semiconductor phase at low temperature. With the rise in temperature, VO2 transitions to metal phase, where the structure absorption reaches 0.99. In addition, changing the per-defect period number to 5, the average absorption at semiconductor and metal VO2 is 0.73 and 0.10, respectively. The differential absorption around the SMT enables the tunability of single photonic devices. During the simulation, the effects of electric field and incidence angle on the structure are also analyzed. Meanwhile, the Bruggeman approximation effective medium theory is introduced in this work, and the changes of the absorption during the phase transition from semiconductor to metal in the VO2 defect layers are also given. These characteristics are applicable to controllable multispectral absorbers, infrared detectors, limiter, and optical switchers.

A Deep Learning Regression Model for Photonic Crystal Fiber Sensor with XAI Feature Selection and Analysis.

A Deep Learning Multi-output regression model is employed to correctly model the relationships between optical design parameters of an asymmetric Twin Elliptical Core Photonic Crystal Fiber (TEC-PCF) and its sensing performances. TEC-PCF acts as a biosensor to detect the blood glucose level taking hemoglobin components into account. Since asymmetric TEC-PCF uses a dual elliptical core, four super modes have to be evaluated to analyze the sensing performance in terms of effective index difference, transmission spectrum, coupling length, and sensor sensitivity. The dataset used in this work is of the optical design parameters of the sensor and Finite Element Method (FEM) results with effective indices of four super modes obtained from the COMSOL Multiphysics by varying hemoglobin concentration to 120 g/L, 140 g/L, and 160 g/L. Gretel.ai's free open-source synthetic data library is used to augment the dataset to make the training more efficient. Explainable AI (XAI) feature analysis using Shapley Additive Explanations (SHAP) framework is used for two purposes: feature selection and to know the feature's effect on prediction. The former led to the development of an optimal model with much fewer computational demands and the latter made the model interpretable. The proposed model can predict the accurate super modes when given input specifications with wavelength ranging from 1.27-1.5 μm and for various glucose concentrations under the influence of hemoglobin much faster compared to the other numerical simulations which are computationally expensive. Computation time taken by the proposed Artificial Neural Network (ANN) model, the proposed model with XAI and FEM is also being compared.

Solving the crystal structure of heterocyclic Poly(arylene benzimidazole) via model compound single crystal and theoretical calculations

Heterocyclic aramid containing benzimidazole units has demonstrated to be one of the strongest fibers. Due to positional isomerism of asymmetric benzimidazole and conformational isomerism of phenylamide, solving the crystal structure of poly(arylene benzimidazole) (PABI) aramid was a prominent challenge. Herein, we first reported the crystal structure of PABI aramid. First, the model compound: N-(4-(5-benzamido-1H-benzo[d] imidazole-2-yl) phenyl) benzamide (PBA) was cultured into single crystal: the molecules took a perfect ross-tenon-mortise interlocking conformation, in which the hydrogen bonding made a giant three-dimensional network. The PABI crystals with PBA crystal stacking feature were subsequently constructed and optimized through theoretical quantum calculation, disclosing a monoclinic unit cell with the parameters (a = 8.56 Å; b = 5.05 Å; and c = 36.78 Å; and β = 91.4°) and Pn space group. Their plausibility and validity were verified, since the simulated XRD and 2D-WAXD patterns reproduced the experimental XRD patterns. Our strategy of linking model compound PBA single crystals to obtain higher PABI crystals, paved the way to solving asymmetrical heterocyclic macromolecules crystals composed of easily isomerized chemical structures.

Design and analysis of optical biosensor based on symmetric and asymmetric structured one-dimensional photonic crystal for tuberculosis detection

We looked into three distinct topologies, using a one-dimensional photonic crystal with a defect layer in two symmetrical and one asymmetrical configurations as an optical biosensor application for detection in tuberculosis (TB). In order to investigate the optical transmission characteristics of such structures, we used a well-known transfer matrix approach. A shift in resonant peak is estimated, when blood of healthy or TB infected human is infiltrated in defect layer. The impact of several parameters, including the thickness of the defect layer, the number of unit cells, and the angle of incidence for all three configurations, has been studied. The sensitivity, quality factor, figure of merit, and detection limit for all structures have been studied and it is found that asymmetric structure is the best suitable structure for optical sensor application in the present case.

Dissipation-induced acoustic nonreciprocity

Linear acoustic transmission through a stationary elastic medium possesses Rayleigh reciprocal symmetry with respect to pressure produced by a source at point A(B) and measured at point B(A), pA(rB) = pB(rA). In ideal (inviscid) fluids, pressure plays the role of the scalar potential for velocity, v∼∇p. The Rayleigh reciprocity theorem is formulated for velocity potentials but not for their gradients. Any function of the absolute value of velocity, v(r), lacks reciprocal symmetry. In particular, acoustic intencity I = pv is not symmetric, IA(rB) ≠ IB(rA). Dynamics of ideal fluid is time-reversible, and lack of reciprocal symmetry is attributed to asymmetry in scattering. However, in a viscous environment dissipation breaks T symmetry making fluid dynamics irreversible. We report that acoustic transmission is not only irreversible but also becomes nonreciprocal due to different amount of dissipated energy for forward and backward propagation, provided that mirror symmetry (P symmetry) is broken. Since dissipation occurs within a narrow viscous layer at solid–fluid interface, the amount of nonreciprocity in transmission strongly depends on the quality of the surface of scatterers. Experiments performed with scatterers of different surface quality show how surface roughness affects the level of nonreciprocity. In a series of experiments with asymmetric phononic crystal, Tesla valve, and acoustic cavity, we demonstrate dissipation-induced nonreciprocity in transmission. [This work is supported by the NSF under EFRI Grant No. 1741677.]

Leaning induced layer undulated tilted smectic phase of asymmetric bent-core liquid crystals.

We report the synthesis and experimental studies on the liquid crystalline phase of a homologous series of compounds comprised of strongly asymmetric bent-core banana shaped molecules. Our x-ray diffraction studies clearly indicate that the compounds exhibit a frustrated tilted smectic phase with layer undulated structure. The low value of the dielectric constant as well as switching current measurements indicate the absence of polarization in this layer undulated phase. In spite of the absence of polarization, a planar aligned sample can be transformed irreversibly to a higher birefringent texture on the application of a high electric field. The zero field texture can only be retrieved by heating the sample to the isotropic phase and cooling it subsequently to the mesophase. We propose a double tilted smectic structure having layer undulation to account for the experimental observations with the layer undulation arising due to the leaning of the molecules in the layer.

Unidirectional transmission analysis of all-optical routers with asymmetric structured photonic crystals based on FDTD

The unidirectional transmission behavior of photonic crystal (PC) consisting of defect pairs is investigated by the finite-difference time-domain (FDTD) method. By selecting the appropriate size proportion of the constitutional PC defects and changing the power intensity of the incident light, the transmission contrast as well as the maximum transmission can be significantly improved. The simulation results are supported by the coupled-mode theory(CMT). This conclusion provides a reference for the design of an all-optical router with low insertion loss.

Transverse Photonic Crystal Mode Engineering for Broad-Area Semiconductor Lasers With Narrow-Divergence Angles

Broad-area semiconductor lasers, coupled with asymmetric transverse photonic crystals (TPCs), are designed by the effective index method and the transfer matrix method. Due to the propagation constant of the fundamental mode lying in the forbidden band of the TPCs, but those of the high-order modes lying in the allowed band of the TPCs, the fundamental mode is mainly concentrated in the active ridge waveguide, while all the high-order modes extend into the lossy TPCs on both sides of the active ridge waveguide. Therefore, the fundamental mode possesses larger optical confinement factor than those of the high-order modes, and reaches the lasing threshold more easily, which is demonstrated by the single-lobe horizontal far-field pattern of the TPC broad-area laser under an injection current of 0.3 A near the threshold. The proposed TPC broad-area laser provides a new strategy of designing broad-area semiconductor lasers with narrow divergence angles.

Intense d-p Hybridization in Nb3O15 Tripolymer Induced the Largest Second Harmonic Generation Response and Birefringence in Germanates.

We herein report two asymmetric germanate crystals, KNbGe3O9 and K3Nb3Ge2O13, with different structures and optical properties derived from divergent polymerized forms of GeO4 and NbO6 groups. Remarkably, K3Nb3Ge2O13 achieved a rare combination of the strongest second harmonic generation (SHG) response of 17.5×KDP@1064nm and the largest birefringence of 0.196@546nm in germanates. It features unique [Nb3O12]∞ tubular chains constructed by circular Nb3O15 tripolymers. Theoretical calculations reveal that the d-p interactions in the Nb3O15 group are responsible for outstanding optical properties. This work emphasizes the significance of the polymerizable functional units in obtaining high-performance nonlinear optical (NLO) crystals.