Hexagonal Hole Arrays Research Articles

Investigation of magnetic hysteresis in biased Ta/Pt/Co/FeMn/Ta antidots: Influence of structural dimensions

There exists a controversy in the literature concerning the values of coercive and bias fields in antidots magnetic structures formed by a hexagonal network of nanoholes. The coercive fields (HC) and the exchange bias fields (∣HEXC∣) for antidots (deposited on ultrathin anodic aluminum oxide, namely, AAO) are either increased or diminished by comparison with the same magnetic nanostructures grown on continuous substrates (namely, CML). We propose to elucidate these debates by showing the importance of the easy axis of the magnetization, the direction of the applied magnetic field, the thicknesses of the layers, and the 3D-topology of nanoholes, as well as the magnetic and thermal history of the magnetic measurements. Here, biased Ta(5 nm)/Pt(5 nm)/Co(0.6 nm)/Fe50Mn50(X)/Ta(5 nm) antidots are investigated by extraordinary Hall effect measurements at 5 K, where X varies in the (0–5.5) nm range. The substrate consists in a hexagonal array of holes, described by the pair of (p,d) values, respectively, the period as the distance from center to center of two consecutive holes and the hole diameter. The dimensions of antidots are (p≈100 and d≈40 nm) for X=(2–5.5) nm, (p≈150 and d≈60 nm) for X=3.5 nm, and (p≈100 and d≈60 nm) for X=0. A continuous stack using Si/SiO2(100 nm) is used for comparison. HC and ∣HEXC∣ gradually increase when X is enhanced for both substrates, with nevertheless a weak decrease at high X for the continuous system. Perpendicular magnetic anisotropy is only observed for both unbiased samples, the X=2 nm continuous sample, and both X=5 nm samples that have undergone field cooling treatment from 500 to 5 K under −2 T. Usually, HC(AAO)>HC(CML), ∣HEXC(AAO)∣>∣HEXC(CML)∣, and ∣HA(AAO)∣<∣HA(CML)∣ (HA designating the anisotropy field). However, for certain conditions, as, for instance, for FC-procedures starting from high temperatures and/or strong magnetic field, other situations might be observed. A discussion pertaining to the amplitudes of HC, ∣HEXC∣ and the anisotropy field (∣HA∣) of continuous and discontinuous samples is given for our experimental results as well as for published data in the literature, in the light of structural characteristics (wedge-to-wedge distance, porosity, or coverage ratio). Such biased perpendicular antidots might be particularly used in specific nanomaterials devoted to spintronics.

Active control of mid-wavelength infrared non-linearity in silicon photonic crystal slab.

Natural materials' inherently weak nonlinear response demands the design of artificial substitutes to avoid optically large samples and complex phase-matching techniques. Silicon photonic crystals are promising artificial materials for this quest. Their nonlinear properties can be modulated optically, paving the way for applications ranging from ultrafast information processing to quantum technologies. A two-dimensional 15-μm-thick silicon photonic structure, comprising a hexagonal array of air holes traversing the slab's thickness, has been designed to support a guided resonance for the light with a wavelength of 4-μm. At the resonance conditions, a transverse mode of the light is strongly confined between the holes in the "veins" of the silicon component. Owing to the confinement, the structure exhibits a ratio of nonlinear to linear absorption coefficients threefold higher than the uniform silicon slab of the same thickness. A customised time-resolved Z-scan method with provisions to accommodate ultrafast pump-probe measurements was used to investigate and quantify the non-linear response. We show that optically pumping free charge carriers into the structure decouples the incoming light from the resonance and reduces the non-linear response. The time-resolved measurements suggest that the decoupling is a relatively long-lived effect on the scale comparable to the non-radiative recombination in the bulk material. Moreover, we demonstrate that the excited free carriers are not the source of the nonlinearity, as this property is determined by the structure design.

Stability of electronical properties of Antimony antidot nanostructure in the presence and in the absence of substrate

Using the multi-orbital (sp3) tight-binding (TB) model, we have obtained the band structure of the Antimony monolayer with a hexagonal array of holes. These structures are named Antimony antidot lattice (AAL). In our tight-binding model, the interaction of the nearest and next-nearest neighbors of atoms has been considered. Our numerical results present that band structures of the system depend on the size of embedded holes in the Antimony monolayer as points of the antidot lattice. Also, we have shown that AALs with a large radius of holes are almost conductors with tiny energy gaps. Besides, we have represented that spin–orbit coupling magnificently affects the band gap for Antimony antidot lattice in the different ranges of hole radius. Moreover, we have investigated the effect of distance between holes of AALs on the band structure and energy gap. Also, the variations of the band gap of Antimony antidot by changing the inserted strain on it have been investigated. Furthermore taking into account the effect of strain, we have compared the band structure and energy gap of antidot lattice (Highlightslayer with hole) and ideal sheet (monolayer without hole).Finally, the effect of substrate on the electronical properties of Antimony monolayer with holes (Antidot) and without holes (ideal sheet) is analyzed.

Fabrication of three-dimensional opal nanolattices using template-directed colloidal assembly

Three-dimensional (3D) nanostructures play a crucial role in nanophotonics, lasers, and optical systems. This article reports on the fabrication of 3D nanostructures consisting of opal structures that are spatially aligned to an array of holes defined in the photoresist. The proposed method uses colloidal lithography to pattern a hexagonal array of holes, which are then used to direct the subsequent 3D assembly of colloidal particles. This approach allows the 3D opal structures to be aligned with the 2D array of holes, which can enhance spatial-phase coherence and reduce defects. The polymer patterns can be used as a sacrificial template for atomic layer deposition and create free-standing nanolattices. The final structure consists of a combination of nanolattice, upon which controlled deposition of opal structures is achieved. These structures result in nanostructured materials with high porosity, which is essential to create low-index materials for nanophotonics. A thick layer of titanium oxide with high refractive index is deposited over nanolattices to demonstrate the mechanical stability of underlying structures. These nanolattice structures with precisely controlled height can serve as a low-index layer and can find applications in Bragg reflectors, nanophotonics, and optical multilayers.

Hydrogen passivated [formula omitted]-borophene nanoribbon: A propitious one-dimensional metallic anode for sodium-ion rechargeable batteries

Synthesized borophene nanoribbons are the single-atom-thick self-assembly of light weight boron atoms separated by hexagonal hole arrays. In this current study, van der Waals corrected density functional theory have been employed to inspect the interaction of Na with both edge hydrogen passivated β12-borophene nanoribbon, based upon first-principles calculations. It is found that the six member ring site is favorable location for Na insertion. Gradual decrease in adsorption energies is observed with increasing Na concentration owing to the fact that the number of vacant sites on anchoring nanoribbon decreases and metal–metal electrostatic repulsion increases. The chemical stoichiometry of full sodiated nanoribbon corresponds to B11H2Na7 with a maximum gravimetric capacity of 1551.65 mAhg−1 without the sacrifice of Na mobility (diffusion barrier of 0.38 eV). Bader charge investigation reveals that 0.83|e| is transferred from Na atom, implying high electronic conductivity of B33H6 nanoribbon during the adsorption process. The average insertion potential is obtained as 0.66 V versus Na/Na+, lying into 0.1–1 V desirable anode potential range. Our study confirm that β12-borophene nanoribbon holds a great potential to act as a propitious negative electrode material for sodium-ion rechargeable batteries based on its structural, electronic and electrochemical properties.

Characterization of damage morphology of structural SiO2 film induced by nanosecond pulsed laser

Abstract We investigated the damage morphology of porous silicon oxide film with a periodic hexagonal hole array irradiated by nanosecond pulsed laser, both experimentally and numerically. To understand the damage morphology, the temperature field distribution and the thermal stress distribution during the laser radiation process were investigated by finite element method. The simulation results show that the thermal stress regulated by periodic structural surface is the reason for the circumferential and discrete distribution of the damage points. The results provide ideas for improving the laser damage resistance of the structural surfaces.

Enhanced Faraday effects of magneto-plasmonic crystals with plasmonic hexagonal hole arrays.

Magneto-optical (MO) properties of the bilayed Au/BIG and trilayered Au/BIG/Au magneto-plasmonic crystals (MPCs) were analyzed by the finite-difference time-domain method. In contrast to the low deflection angle and transmission of the smooth thin film, all the heterostructures with perforated holes in the top Au film displayed a similar trend with two strong resonant bands in Faraday rotation and transmittance in the near infrared wavelength range. The bands and electric distribution relative to the component and hole structure were revealed. The MPC with plasmonic hexagonal holes was found to own superior Faraday effects with distinctive anisotropy. The evolution of the resonant bands with the size and period of hexagonal holes, the thickness of different layers, and the incident light polarization was illustrated. The Faraday rotation of the optimized bilayed and trilayered hexagonal MPCs was improved 15.3 and 17.5 times, and the transmittance was enhanced 12.1 and 11.1 folds respectively at the resonant wavelength in comparison to the continuous Au/BIG film, indicating that the systems might find potential application in MO devices.

Microstructural and optical characterization of polymer nanotemplates with different morphologies

The block copolymer (BCP) templates with nanoscale structures and diverse morphologies have attracted much attention for nanofabrication in photonic and plasmonic research fields. Herein, PS-b-PMMA BCP nanotemplates with different morphologies, including the patterns of hexagonal hole arrays, fingerprint stripes, and their mixture, are successfully prepared for in-depth microstructural and optical characterizations. Apart from direct microstructure observations and component characterization, ellipsometric measurements are mainly applied to characterize structural and optical evolution of these BCP nanotemplates undergoing different preparation stages. Interestingly, among different BCP nanotemplates, two typical reflectance peaks in the ultraviolet region (200–250 nm) are distinctly different from each other, due to their PS- and thickness-dependent optical behaviors. With the preparation proceeding, there are synchronous diminution in the intensity of the two peaks, accompanying with an obvious blue-shift for the shorter wavelength peak. In addition, the variation of refractive index and ellipsometric parameters greatly demonstrates morphology-dependent optical behaviors among different BCP nanotemplates. Finally, Au-covered BCP nanotemplates present the distinct template-morphology-dependent visible-light responses, demonstrating different light-matter interactions occurred in their plasmonic Au nanostructures.

High Sensitivity Temperature Sensor Based on Two-Dimensional Photonic Crystal

In this work, a two-dimensional photonic crystal ring resonator based temperature sensor is investigated. The device consists of a hexagonal array of air holes surrounded by the base material of germanium (Ge). The sensing mechanism is based on the shifting of transmission peak with refractive index changes in Ge induced by variation of temperature. Simulation results are obtained using finite difference time domain method (FDTD). The photonic band gap is studied by plane wave expansion method (PWE). The sensor has high sensitivity of 270 pm/K and high quality factor of 2028.86 with` wide range of temperature detection between 300 K to 800 K. The size of the structure is 112.91 (µm)2 and appropriate for sensing applications in nanotechnology.

Wide-band self-collimation in a low-refractive-index hexagonal lattice.

Wide-angle, broadband self-collimation (SC) is demonstrated in a hexagonal photonic crystal (PhC) fabricated in a low-refractive-index photopolymer by multiphoton lithography. The PhC can be described as a hexagonal array of cylindrical air holes in a block of dielectric material having a low-refractive index. Optical characterization shows that the device strongly self-collimates light at near-infrared wavelengths that span 1360 to 1610 nm. SC forces light to flow along the extrusion direction of the lattice without diffractive spreading, even when light couples into the device at high oblique angles. Numerical simulations corroborate the experimental findings.

Low loss flat dispersive hexagonal porous core THz fiber

A low loss porous core hexagonal lattice photonic crystal fiber (PCF) is proposed. Design procedure and properties are investigated in terahertz (THz) band. We explore different important properties of the PCF to evaluate the merit of proposed design. Circular air hole at the center surrounded by rectangular air hole array in the fiber core is designed in such a way that maximizes the core porosity of the fiber. The structure of cladding, on the other hand, is designed with a large hexagonal air hole array with tiny isolation width, resulting in a minimum total loss. The bulk material loss is diminished by 85.8% using core porosity of 80%. Numerical result shows relatively low confinement loss at 1 THz with flattened dispersion. Withal, single-mode propagation is exhibited in the proposed PCF design. The novel approach of this proposed PCF is unique to ensure good confinement in the core with a reasonable amount of power fraction.

Fabrication of a diamond concave microlens array for laser beam homogenization

An ultra-smooth diamond concave microlens array was realized using a chemical reflow process assisted by dry etching technique, which has a great performance for homogenization of light beam. Firstly, photolithography was used to produce a regular hexagonal hole array in the photoresist layer on a diamond substrate. Then, the hole array was reflowed in ethanol solvent vapor atmosphere at 20 ℃ to form the concave spherical structure array. Finally, diamond concave microlens array was formed by transferring photoresist concave structures onto diamond substrate through dry etching technique. The fabricated diamond concave microlens array was characterized, demonstrating a well-uniformity of the surface morphology, high quality imaging and focusing performances. Furthermore, laser beam homogenization was realized by the diamond concave microlens array.

High fracture toughness micro-architectured materials

We investigate the possibility of achieving high fracture toughness and high strength by the design of lightweight (density below water) metallic micro-architectured materials. The micro-architectured materials were manufactured by drilling a hexagonal array of holes in plates of an aluminum alloy, and the fracture toughness was evaluated via three-point bend tests of single-edge notch specimens. The results show that the fracture toughness of micro-architectured materials increases with increasing relative density and remarkably, a micro-architectured material can be 50% lighter than the parent material but maintain the same fracture toughness. Additional tests on geometrically similar specimens revealed that the fracture toughness increases linearly with the square-root of the cell size. The experiments are complemented by finite element calculations of ductile fracture. In the calculations, the fracture toughness of single-edge notch specimens subjected to three-point bending are evaluated using both, a procedure similar to the experiments and direct computation of the J-contour integral. The fracture toughness as calculated by both methods are consistent with the experimental results. In addition, the calculations are also carried out for single-edge notch specimens subjected to tensile loading, confirming the validity of the measured fracture toughness as a useful material property independent of specimen geometry.

Plasmonic Janus Microspheres Created from Pickering Emulsion Drops.

Metal nanostructures have been created in a film format to develop unique plasmonic properties. Here, well-defined metal nanostructures are designed on the surface of microspheres to provide plasmonic microgranules. As conventional techniques are inadequate for nanofabrication on spherical surfaces, photocurable emulsion drops with a regular array of silica particles are employed at the interface to create periodic nanostructures. The silica particles, originating from the dispersed phase, fully cover the interface by forming a non-close-packed hexagonal array after drop generation, and slowly protrude to the continuous phase during aging while their interparticle separation decreases. Therefore, hexagonal arrays of spherical dimples with controlled geometry and separation are created on the surface of microspheres by photocuring the drops and removing the particles. Directional deposition of either aluminum or gold results in a continuous film with a hexagonal array of holes on the outermost surface and isolated curved disks in dimples, which renders the hemisphere of microspheres plasmonically colored. The resonant wavelength is controlled by adjusting the aging time, metal thickness, and size of silica particles, providing various plasmonic colors. This granular format of the plasmonic Janus microspheres will open a new avenue of optical applications including active color pixels, optical barcodes, and microsensors.

Soft mold of microlens arrays fabricated by surface self-assembly

As a micro-optical element, microlens arrays (MLAs) play an important role in imaging, optical communication, lithography, and sensing. This paper reports a new method of fabricating a soft mold of MLAs by surface self-assembly. First, a hexagonal hole array mask is placed on a polydimethylsiloxane (PDMS) film in a radially pre-stretched state, and the masked PDMS film is irradiated using a scanning UV lamp in air, to form a hexagonal array of a silica-like hard film. Because of the surface self-assembly effect, this array bends inward, yielding a soft mold of concave MLAs after the pre-strain is spontaneously released. The focal length of the MLAs could be controlled in the range of 140–330 μm with a 37-μm-diameter hexagonal hole array by varying the exposure time of the ultraviolet ozone. Moreover, the diameter of the hexagonal hole array and the wrinkle wavelength, which are key influencing factors, were systematically studied. The soft mold of MLAs was applied to fabricate replica positive MLAs on planar and curved surfaces, which may have potential applications in image sensing and artificial compound eyes.

Experimental characterisation of the bound acoustic surface modes supported by honeycomb and hexagonal hole arrays

The Dirac point and associated linear dispersion exhibited in the band structure of bound (non-radiative) acoustic surface modes supported on a honeycomb array of holes is explored. An aluminium plate with a honeycomb lattice of periodic sub-wavelength perforations is characterised by local pressure field measurements above the sample surface to obtain the full band-structure of bound modes. The local pressure fields of the bound modes at the K and M symmetry points are imaged, and the losses at frequencies near the Dirac frequency are shown to increase monotonically as the mode travels through the K point at the Dirac frequency on the honeycomb lattice. Results are contrasted with those from a simple hexagonal array of similar holes, and both experimentally obtained dispersion relations are shown to agree well with the predictions of a numerical model.

Honeycomb‐Inspired Surface‐Patterned Cu@CuO Composite Current Collector for Lithium‐Ion Batteries

The interface design between the current collector and active materials has a non‐negligible effect on the performance of lithium‐ion batteries (LIBs). Inspired by the honeycomb with a large specific surface area, this work validates the use of a surface‐patterned cellular Cu@CuO composite current collector with a hexagonal blind hole array and a nanostructured film of CuO nanoflowers for LIBs. This unique structure is synthesized by lithography and solution immersion. Mesocarbon microbead graphite powders are used as the active materials. Results show that the battery with the cellular Cu@CuO current collector maintains a reversible charge capacity of 354.1 mAh g−1 at 100 mA g−1 after 200 cycles. Compared with the Cu‐based current collector with a hexagonal blind hole array and a bare Cu current collector, the new pattern gains a performance improvement in the reversible capacity by 27.9% and 153.2%, respectively. The excellent electrochemical property of the cellular Cu@CuO current collector can be ascribed to strengthened adhesion with the active materials, reduced contact resistance, and improved Li‐ion diffusion capability. Due to the enhanced electrochemical performance, facile preparation processes, and cost‐effective raw materials, the new current collector shows great potential to replace the conventional current collector of energy storage systems.

Programmable Aerodynamic Drag on Active Dimpled Cylinders

This study demonstrates that the aerodynamic drag profile of a patterned deformable cylinder at high Reynolds numbers can be programmed, on demand, by modifying its topography through pneumatic actuation. The samples used in the experiments comprise a rigid tube containing a hexagonal array of holes, over which an elastomeric cylindrical membrane is stretched. Decreasing the internal pressure of the structure causes an inward deflection of the outer membrane over the holes, thereby producing a regular pattern of dimples. The depth of these dimples can be controlled by tuning the pressure differential. The relationship between the mechanical deformation of the membrane and the pneumatic loading is characterized using a combination of finite element simulations and precision mechanical experiments. Wind tunnel experiments are performed to study how both the depth and the diameter of the dimples dictate the aerodynamic performance of the samples in the critical Reynolds number regime. Finally, the tunable nature of the specimens is exploited to automatically control the dependence of the drag coefficient on the Reynolds number, toward targeting predefined drag profiles.

Enhanced quantum dots spontaneous emission with metamaterial perfect absorbers

Metamaterial perfect absorbers (PAs) made of a hexagonal array of holes on Ag-SiO2-Ag thin films have been realized and utilized to enhance the spontaneous emission rate and photoluminescence intensity of CdSe/ZnS quantum dots (QDs) spin-coated on the absorber top surface. Perfect absorption of incoming light occurs at the wavelength where the impedance is matched to that of the free space. When QDs strongly excite both the electric and magnetic resonances at this perfect absorption wavelength, a significant Purcell effect on the spontaneous emission process and enhanced radiative outcoupling of photoluminescence intensity are expected. For perfect absorbers with near-unity absorption at the QD emission wavelength of 620 nm, 5-fold Purcell enhancement of the spontaneous emission rate and 3.6-fold enhancement of photoluminescence intensity are demonstrated in the time-resolved photoluminescence experiments, which are in good agreement with three-dimensional finite-difference time-domain simulation. These results will advance the understanding and applications of metamaterial PA-based light harvesting and emitting devices.

Differential plasma chemokines profile in dual depression - the role of tumor necrosis factor alpha (TNF-alpha) and stromal cell-factor one (SDF-1)

We investigate the possibility of achieving high fracture toughness and high strength by the design of lightweight (density below water) metallic micro-architectured materials. The micro-architectured materials were manufactured by drilling a hexagonal array of holes in plates of an aluminum alloy, and the fracture toughness was evaluated via three-point bend tests of single-edge notch specimens. The results show that the fracture toughness of micro-architectured materials increases with increasing relative density and remarkably, a micro-architectured material can be 50% lighter than the parent material but maintain the same fracture toughness. Additional tests on geometrically similar specimens revealed that the fracture toughness increases linearly with the square-root of the cell size. The experiments are complemented by finite element calculations of ductile fracture. In the calculations, the fracture toughness of single-edge notch specimens subjected to three-point bending are evaluated using both, a procedure similar to the experiments and direct computation of the J-contour integral. The fracture toughness as calculated by both methods are consistent with the experimental results. In addition, the calculations are also carried out for single-edge notch specimens subjected to tensile loading, confirming the validity of the measured fracture toughness as a useful material property independent of specimen geometry.