Lattice Of Circular Holes Research Articles

Dual-wavelength chiral metasurfaces based on quasi-bound states in the continuum

In this paper, we numerically demonstrate a dual-wavelength chiral metasurface with the chirality only existing at two resonant wavelengths, making use of the excitation of two quasi-bound states in the continuum (quasi-BICs). This chiral metasurface is constructed on the basis of photonic crystal slabs with square lattice of circular holes by introducing rectangular holes as perturbations. The two quasi-BICs supported by chiral metasurface are orthogonal to each other, thus the chirality at the two resonant wavelengths can be controlled independently by changing the geometry of the perturbations. Furthermore, a chirality-switchable metasurface is demonstrated taking advantage of the sensitivity of quasi-BICs to the variation of out-of-plane symmetry caused by changing refractive index of surroundings. The proposed chiral metasurface could find potential applications in nonlinear optics, sensing, imaging and near-to-eye displays.

Modulation of out-of-plane reflected waves by using acoustic metasurfaces with tapered corrugated holes

In this paper, modulation of reflected wavefront out of the incident plane by a tunable acoustic metasurface is investigated based on the fully generalized Snell’s law in the three-dimensional space. The metasurface is constructed by a square lattice of circular holes with gradient annular bumps. The phase shift is tuned by changing the volume of water filled in the holes. The acoustic wave steering out of the incident plane and the out-of-plane acoustic focusing with the oblique incidence at the subwavelength scale are demonstrated numerically by selecting suitable distributions of water depth. The numerical results show that the wavefront of the reflected wave can be manipulated over a wide frequency range; and the gradient design of the unit cells can suppress the parasitic reflection. The present work is relevant to the practical design of novel acoustic devices.

Anisotropy of the effective elastic modulus of a steel plate with a lattice of circular holes

Plates with a doubly periodic commensurate lattice of circular holes are studied. The plates are cut from a corrosion-resistant steel sheet along and across the rolling direction. Uniaxial tensile tests are performed, and the effective elastic modulus is determined for longitudinal and transverse plates with various lattice orientations with respect to the longitudinal axis of a plate. The experimental and calculated results agree well.

Slow light engineering in a photonic crystal slab waveguide through optofluidic infiltration and geometric modulation

In this paper, a new type of flat-band slow light structure with high group index (n g) and large normalized delay-bandwidth product (NDBP) in a silicon on insulator (SOI) based photonic crystal (PC) slab waveguide with a triangular lattice of circular holes is demonstrated. The dispersion engineering is performed by infiltrating optical fluids with different refractive indices n f in the first row and shifting the second row of air holes adjacent to the PC waveguide (PCW) in the longitudinal direction. In the optimized case, a high NDBP of 0.32 with a group index of 54.55 and a bandwidth of 9.13 nm could be obtained. Furthermore, an ultra-low group velocity dispersion (GVD) in the range of 10–20 s2/m is achieved in all of the structures. These results are obtained by numerical simulations based on three-dimensional (3D) plane wave expansion (PWE) method.

Fine-tunable micro-patterns in Chitosan-based scaffolds: fabrication process and material properties

Fine-tunable micro-patterns in Chitosan-based scaffolds: fabrication process and material properties

Strength analysis of an elastic plane containing a square lattice of circular holes under mechanical loading

The strength characteristics of an elastic plane weakened by a square lattice of circular holes are considered. The stress concentrations in three distinct lattices under the conditions of uniaxial tension/compression in various directions are studied. The minimum and maximum values of the stress concentrations are calculated, and the stress fields in various lattices are considered. We show that under the compression conditions fracture can occur inside the material rather than on the hole boundaries. It is demonstrated that in dense lattices the stress concentrations exhibit power-law dependence on the structure parameter equal to the ratio of the length of the interval between the holes to the hole radius.

Strength of an Elastic Plane with a Square Lattice of Circular Holes under External Loading

Strength characteristics of an elastic plane weakened by round holes arranged in a quadratic order are considered. The stress concentrations in the three different hole lattices are studied in conditions of the uniaxial tension/compression with different directions (0°÷45°, it is enough, because the lattices are symmetrical). The minimal and maximal concentrations values are calculated; the stress fields are considered in different lattices. It is shown that in the particular conditions of fracture at compression can occur in the inner zone of the material rather than at the hole contours. It is demonstrated that in dense lattices a power-like type of the concentrations dependencies on the structural parameter takes place; the parameter is a ratio of the holes interval to the hole radius. Next, the modes of initiation of fracture in elastic perforated plates (lattices) under external uniaxial compression are considered. Special attention is paid to the internal mode of initiation (i.e., to the case of fracture initiation inside the material, and not on the contours of the holes). The parametric region of the internal mode is studied, it depends on the angle of the applied external loads and the structural lattice parameter; the size of the region (extreme values) is calculated. A particular point of the region is found; it is shown that at this point there are three possibilities of fracture initiation: two positions at the hole contour, and one position inside of material. Keywords: elasticity, perforated plates, lattice of circular holes, fracture initiation, uniaxial compression.

Coupled-Wave Theory for Square-Lattice Photonic Crystal Lasers With TE Polarization

We present a coupled-wave analysis for square-lattice photonic crystal lasers with transverse electric polarization. A model consisting of eight plane waves coupled by Bragg diffraction is used to describe two-dimensional optical coupling. The resonant frequencies and threshold criteria for the modes of oscillation have been determined for the case of index periodicity with a lattice of circular holes. The spatial intensity distributions of these resonant modes have also been calculated. For the fundamental modes, we have investigated how the intensity distribution varies as a function of coupling strength. The dependence of the threshold gain of these modes on hole size has also been elucidated. This semianalytical approach provides a comprehensive understanding of square-lattice photonic crystal lasers and allows more effective optimization of their cavity design.

Elastic-instability triggered pattern formation

Recent experiments have exploited elastic instabilities in membranes to create complex patterns. However, the rational design of such structures poses many challenges, as they are products of nonlinear elastic behavior. We pose a simple model for determining the orientational order of such patterns using only linear elasticity theory which correctly predicts the outcomes of several experiments. Each element of the pattern is modeled by a "dislocation dipole" located at a point on a lattice, which then interacts elastically with all other dipoles in the system. We explicitly consider a membrane with a square lattice of circular holes under uniform compression and examine the changes in morphology as it is allowed to relax in a specified direction.

Modelling air-bridge photonic crystal defects by guided mode expansion

Abstract A guided mode expansion treatment is presented for theoretical analysis of air-bridge photonic crystal cavity structures. The optimum choice of waveguide refractive index for the basis states is discussed. The method’s inherent speed is demonstrated with the optimisation of an L3 defect in a hexagonal lattice of circular holes.

Etched distributed Bragg reflectors as three-dimensional photonic crystals: photonic bands and density of states.

The photonic band dispersion and density of states (DOS) are calculated for the three-dimensional (3D) hexagonal structure corresponding to a distributed Bragg reflector patterned with a 2D triangular lattice of circular holes. Results for the Si/SiO(2) and GaAs/Al(x)Ga(1-x)As systems determine the optimal parameters for which a gap in the 2D plane occurs and overlaps the 1D gap of the multilayer. The DOS is considerably reduced in correspondence with the overlap of 2D and 1D gaps. Also, the local density of states (i.e., the DOS weighted with the squared electric field at a given point) has strong variations depending on the position. Both results imply substantial changes of spontaneous emission rates and patterns for a local emitter embedded in the structure and make this system attractive for the fabrication of a 3D photonic crystal with controlled radiative properties.

An optical drill for the fabrication of photonic crystals

An optical drill is proposed for the nanofabrication of three-dimensional (3D) (diamond-like) lattices in positive photoresists, thereby opening a way towards 3D photonic crystals at optical wavelengths. Basically, an ensemble of coherent light beams is used to produce a holographic pattern in a photoresist of strongly nonlinear response. Multiple exposures are then performed for different phase shifts of one (several) of the beams. By doing this, not only are atomic bonds simulated with the spatial shift of the holographic pattern, but also the interconnections allow the resist development after exposure. An experimental demonstration is shown in a 2D case where three diffracted beams originating from the same UV Ar+ laser are combined to create a triangular lattice of circular holes. Results are given for various phase shifts of one of the diffracted beams and for different exposure times of the photoresist. The extension to the four laser beam case necessary for the fabrication of 3D structures is discussed.

Nucleation of superconductivity in a thin film with a lattice of circular holes

We investigate experimentally the nucleation of superconductivity in a thin Aluminium film with a square lattice of microholes in a uniform perpendicular magnetic field H. It is shown that in a non-zero magnetic field, this system has an elevated critical temperature T c * (H) in comparison to the reference film without holes. This effect can be considered as a generalisation of the well known surface superconductivity effect for the case of a finite radius of the surface and a multiply connected geometry of the sample. Quantization of the fluxoid around each hole leads to oscillations in the T c * (H) dependence with a period approximately corresponding to one flux quantum through a hole. Also another type of oscillation with a smaller period which is equal to one flux quantum per unit cell was observed. We discuss the second type of oscillation in terms of an interaction between the holes. We believe that our results can be useful in the analysis of high-Tc superconductors with columnar defects because, as it is shown here, a comparison of the Tc(H) dependence before and after irradiation can give some special information on the properties of the amorphous tracks produced by highenergy ions. While many experimental parameters are different in our system and in HTSC we present arguments why this analogy should be correct.

Nucleation of superconductivity in a thin film with an array of holes

The results of an investigation of the surface superconductivity effect generalised for the case of a finite radius of the superconductor boundary and a multiply connected geometry of the sample are presented. It is demonstrated experimentally that in perpendicular non-zero magnetic field the critical temperature of the aluminum film with the lattice of circular holes is higher than that one of the uniform film and exhibits oscillations with magnetic field with the period approximately corresponding to one flux quantum per a hole. Another type of oscillations of the critical temperature with a smaller period (one flux per a unit cell) is observed at the same time. We discuss them in terms of the interaction between the holes. Our results can be applied for an analysis of the high-Tc materials with columnar defects because a comparison of the critical temperature versus magnetic field dependence before and after the irradiation can give some information of the properties of the amorphous tracks produced by high-energy ions.