Slit Arrays Research Articles

Sub-Wavelength Slit-Assisted Binary Metallic Lens Design for Effective Multifocusing via Phase Superposition Method

This paper proposes a multifocal lens design method named phase superposition method (PSM) and a methodology to increase the efficiency of metallic binary zone plates through a sub-wavelength slit array. The phase superposition method calculates the intensity at each focus, assuming a perfectly phase-matched modulation based on input light, and the size of the lens. Subsequently, based on the calculated focus intensity, the phases linearly related to the target intensity at each focus are combined. The sub-wavelength slit-assisted metallic binary plate assumes the sub-wavelength slit array as a dielectric in the light polarized perpendicular to the slit. It then controls the condition of the slit to invert the phase difference between the opened region and the slit region. By applying these methods, the trifocal lens was designed to verify our idea. The light was focused on three target positions. After applying the sub-wavelength slit structure, the amplitude was increased up to 90.11%, and the efficiency increase was 303.99% compared to that of a binary plate without the sub-wavelength slit.

Anomalous birefringence through metasurface-based cavities with linear-to-circular polarization conversion

We propose the metasurface-based cavities for splitting a linearly polarized (LP) electromagnetic wave into two symmetric circularly polarized (CP) beams with opposite helicities. More specifically, periodic strip slits and rectangular C-slits etched on substrate integrated waveguide cavities are shown to be capable of perfectly coupling the LP wave into the cavities from the strip slit array, and then releasing them with specific polarization directions according to the orientations of the C-slits. Such metasurface-based cavities with gradient oriented C-slit array would further create the opposite equivalent phase gradients for the right-hand and left-hand CP transmitting waves that are decomposed from the released LP wave with anomalous birefractions and polarization conversion simultaneously.

Tailoring radiative properties with magnetic polaritons in deep gratings and slit arrays based on structural transformation

Magnetic polaritons which refer to the couplings between incident electromagnetic waves with magnetic excitation inside the structures play an important role in tailoring thermal radiative properties. Many researchers devote themselves to studying the geometric effects of the micro/nanostructures based on magnetic polaritons, and many wavelength-selective devices are designed. However, the links between different structures on thermal radiative properties based on structural transformation are not investigated yet. In this work, the idea that deep slit arrays and deep gratings can be equivalent conversion approximately when studying the radiative properties is proposed. Magnetic polariton resonant conditions of deep metal gratings and slit arrays are investigated. Considering the symmetry of the structures, when gratings and slit arrays have same geometry parameters except that the slit heights are twice that of the grating depths, the phenomena that they have almost identical magnetic polariton resonant frequencies are observed. The reason for almost identical magnetic polariton resonant conditions is attributed to that the symmetry of the structures leads to the similar electromagnetic field distributions. Moreover, equivalent inductor-capacitor circuit model is also used to analytically illuminate and confirm almost same MP resonant frequencies. The idea provided in this paper may benefit design of the wavelength-selective devices based on magnetic polaritons and further studies on tailoring radiative properties from perspective of structural transformation.

Periodic Metallic Stepped Slits for Entire Transmission of Optical Wave and Efficient Transmission of Terahertz Wave

The presence of metallic structures with array of slits, when slits have dimensions much less than the wavelength of the incident optical wave, typically causes extraordinary power transmission. This excellent power transmission indeed comes from the surface waves on the periodic structure excited by the transverse magnetic wave. Here we show that if slits are stepped, then the incident wave power is transmitted into the substrate at desired optical frequency entirely and simultaneously in terahertz frequency band by 70%, for substrate. Power transmitted through the proposed structure is derived both in a closed form by an analytical model as well as numerically by finite element method. It is found that with the increase of field intensity in the substrate, the structure with array of stepped slits (as opposed to uniform slits) favorably has no reduction of frequency range at maximum transmitted power.

Marked effects of lateral displacement on the optical transmission properties of stacked artificial dielectric systems composed of metallic sub-wavelength slit arrays

Metallic sub-wavelength slit-array slabs act just like dielectrics when the incident wavelength is longer than the slit interval. However, their stacked structures do not always exhibit optical properties equivalent to conventional dielectric multilayers, which should be due to the lateral periodicity. This work investigated in detail the lateral displacement dependence on the characteristics of the Fabry–Perot-like waveguide resonance modes of two-tier systems. Simulation studies clarified the behavior of the resonance modes and the physical mechanism underpinning the transmission disappearance by the lateral displacement. Furthermore, we determined that the critical frequency, above which the even-order modes exhibit blue-shifts with increases in the air gap spacing, can be predicted very accurately using a simple equation including the lateral displacement. Additionally, the transmission characteristics assessed experimentally were compared with the simulation results. The geometrically-tunable unique optical characteristics are expected to promote the future applications of artificial dielectrics operating in low-frequency optical regions.

Heat Transfer Enhancement for Wake Zone Using Slit Pillar in Microchannel Heat Sinks

Abstract Pillar microchannel heat sinks have been widely used for chip cooling, while their overall heat transfer performance is restricted by the stagnation flow in pillar wake zone. In this work, a simple but effective method using slit microstructure modified on pillar was proposed to enhance wake zone heat transfer. It enables a special flow path for the incoming fluid that intensively disturbs the wake fluid. To validate the proposed method, a three-dimensional simulation was employed to study the laminar flow and heat transfer characteristics in the slit pillar microchannel. The pillar without slit design was also investigated for comparative analysis. Effects of slit angle (θ), height over diameter ratio (H/D), and blocking ratio (D/W) of a single pillar were systematically studied at the Reynolds numbers of 26–260. Results showed the case with θ = 0 deg always demonstrated lower surface temperature, higher Nusselt number and higher thermal performance index (TPI) compared to other cases with different slit angles at the same conditions. Furthermore, it was interesting to find that the slit configuration was not suitable for long pillar microchannel, but preferred for high blocking ratio pillar microchannel at present ranges (H/D ≤ 1, D/W ≤ 0.5). The slit pillar array microchannel was also explored and observed with improved overall heat transfer performance. The proposed slit microstructure well prevents the heat transfer deterioration in pillar wake zone, which is promisingly to be used for cooling performance improvement of electronic device.

Embedded dual-layered dielectric slit arrays under second Bragg incidence for 1×3 splitting field propagation

Embedded double-layer transmission grating beam splitter under the second Bragg incidence is presented in this paper. With a given duty cycle of 0.5 and a period of 940 nm, we use rigorous coupled-wave analysis method to investigate this beam splitter. The grating parameters are optimized for high-efficiency output. Each efficiency of the three orders under polarizations of TE and TM is close to 33.3 % respectively. Compared with the single-layer fused-silica grating, the three-port double-layer grating is flexible with complex structure and stable in performance. The field distribution modal method are given, which can well explain the propagation process and coupling mechanism.

Fabrication of vertical van der Waals gap array using single-and multi-layer graphene

Arrays of van der Waals gaps were manufactured by synthesizing the vertically aligned graphene layer stacked between two copper (Cu) catalytic films. The Cu–graphene–Cu laminated structure was obtained by directly synthesizing graphene on a patterned Cu film followed by depositing a second copper layer for optical measurements. The synthesis of graphene on the Cu surface was optimized by adjusting the synthesis temperatures and pre-annealing time using plasma enhanced chemical vapor deposition (PECVD). Resonant Raman spectroscopy measurements reveal that graphene can be synthesized on both bulk Cu foil and relatively thin Cu film under the same growth mechanism using PECVD. Structural and optical characterizations of the array of graphene van der Waals gaps were implemented by the transmission electron microscope and terahertz-time domain spectroscopy (THz–TDS). In THz–TDS, the measured THz amplitude transmitted through the graphene van der Waals gap slit array was constant regardless of the gap width determined by the number of graphene layers between the Cu thin films in a single slit. These results imply that the optical dielectric constant of graphene at THz frequencies in the out-of-plane direction is linearly proportional to the gap width. Our results of the manufacturing method can be adopted to investigate mechanical, electrical, and optical properties of other 2D materials such as h-BN, MoS2, and others. Furthermore, metal-graphene-metal structures with vertical orientations can be used in many electronic, optic, and optoelectronic applications.

Light Transmission of a Thin Metal Screen with an Infinite Array of Periodic Nanoslits

A rational method for calculating the transmission of light by a thin metal screen with an infinite periodic array of subwavelength slits is proposed. The calculation results agree well with available experimental and theoretical data.

Mechanism of polaritons coupling from perspective of equivalent MLC circuits model in slit arrays.

Magnetic polariton is a significant mode in tailoring thermal radiative properties with micro/nanostructures metamaterials and can be explained by equivalent inductor-capacitor circuit model. However, the equivalent inductor-capacitor circuit model is out of operation when the magnetic polariton resonance frequency is close to the surface plasmon polariton excitation frequency and cases of oblique incidence. In this work, we present a mutual inductor-inductor-capacitor circuit model to describe magnetic polariton resonance conditions. The mechanism of coupling between the surface plasmon polariton and magnetic polariton is explained from the perspective of equivalent circuits. The interaction between the surface plasmon polariton and magnetic polariton is studied and considered as a mutual inductance in the MLC circuit model. This model is still applicable in the case of oblique incidence. Slit arrays with different geometric parameters and incident angles are calculated to verify the rationality of the mutual inductor-inductor-capacitor circuit model. This study may allow us to predict features and parameters and achieve tailoring of the thermal radiative properties of the micro/nano-structures metamaterials.

PDMS membrane filter with nano-slit array fabricated using three-dimensional silicon mold for the concentration of particles with bacterial size range

Herein we report a microfluidic device integrated with a PDMS membrane nano-filter for the size-based trapping of particles. We propose a novel cost-effective fabrication process to form submicron-sized nano-pore and slit arrays in the PDMS membrane by using a reusable three-dimensional silicon mold with the shape of microneedles and microblades. The PDMS is coated onto the silicon mold, etched, and detached from the mold to form nano-pores and slits in the PDMS membrane filter. The sizes of the nano-pores and slits could be adjusted in the range of several hundreds of nanometers by varying the etching time of the PDMS coated on the sharp silicon mold. The diameter of the pores and the width of the slits were controlled to be 354 ± 79 nm and 389 ± 84 nm, respectively, with PDMS etching times of 60 s. Moreover, it was confirmed that the deposition of a parylene-C thin film on the PDMS membrane filter can be effectively used to control the nano-slit size and improve the trapping efficiency. We have demonstrated size-based filtration of microbeads with diameters of 0.4, 0.8, 1.0, and 2.2 μm using the proposed microfluidic device. In addition, the microfluidic device integrated with the proposed PDMS membrane filter was successfully applied to the trapping of submicron-sized E. coli bacteria with a filter recovery efficiency of 89.1%.

Terahertz Radiation from Combined Metallic Slit Arrays

We report an approach to efficiently generate terahertz radiation from a combined periodic structure. The proposed configuration is composed of two metallic slit arrays deliberately designed with different periodic length, slit width and depth. We found that the combination of the two slit arrays could provide special electromagnetic modes, which exhibit nonradiative property above the surface of one slit array and radiative property inside the other one. An electron beam holding proper energy could resonate with those modes to generate strong and directional electromagnetic radiations in the terahertz regime, indicating that the approach has the potential in developing high-performance terahertz radiation sources.

Micro/nano-scale Characterization and Fatigue Fracture Resistance of Mechanoreceptor with Crack-shaped Slit Arrays in Scorpion

The nocturnal scorpion Heterometrus petersii uses Basitarsal Compound Slit Sensilla (BCSS) as mechanoreceptor to detect mechanical signal (e.g. substrate vibration, cyclic loads caused by walking) without fatigue failure such as initiation of fatigue crack and further propagation of crack-shaped slit. The outstanding perceptive function has been discovered for over half a century. However, it is not yet clear about the microstructure, material composition and micromechanical property which are all important factors that determine the fatigue fracture resistance of the BCSS. Here, the microscopic characteristics of the BCSS were thoroughly studied. The results indicate that anti-fatigue resilin and stiff chitinous cuticle form multilayered composite as the main body of the BCSS. Meanwhile, the pre-existing slit as mechanosensory structure is covered by cuticular membrane which has different mechanical property with the epicuticle. Theoretical analysis shows that the structure-composition-property synergistic relations of composites confer on the BCSS with extreme fatigue fracture tolerance.

Workpiece reciprocating movement aided wire electrochemical machining using a tube electrode with an array of holes

Parts with a ruled surface manufactured to high accuracy and good surface integrity are widely used in industry. Wire electrochemical machining (WECM) has great advantages and potential in the processing of these parts because there is no residual machining stress, the surface is free of burrs and has no recast layer, and there is no tool wear. In this paper, WECM using a tube electrode with an array of holes was introduced to further improve the machining efficiency and capability for thick workpiece. Additionally, the reciprocating movement of the workpiece was used to change the spraying position of the electrolyte on the machining surface. The simulation results indicate that this method can improve the velocity and uniformity of the electrolyte flow field in the machining gap. Experiments verified that when the amplitude of the reciprocating movement is greater than the spacing of holes, machining efficiency and accuracy are greatly improved. Finally, using optimized machining parameters, an array of slits with a width of 550 μm (standard deviation of 5.99 μm) was fabricated on 10-mm-thick stainless steel 304 at a cutting rate of 3 mm²/min.

Coupling between subwavelength nano-slit lattice modes and metal-insulator-graphene cavity modes: a semi-analytical model

We present a semi-analytical model of the resonance phenomena occurring in a hybrid system made of a 1D array of periodic subwavelength slits deposited on an insulator/graphene layer. We show that the spectral response of this hybrid system can be fully explained by a simple semi-analytical model based on weak and strong couplings between two elementary sub-systems. The first elementary sub-system consists of a 1D array of periodic subwavelength slits viewed as a homogeneous medium. In this medium lives a metal-insulator-metal lattice mode interacting with surface and cavity plasmon modes. A weak coupling with surface plasmon modes on both faces of the perforated metal film leads to a broadband spectrum while a strong coupling between this first sub-system and a second one made of a graphene-insulator-metal gap leads to a narrow band spectrum. We provide a semi-analytical model based on these two interactions thus allowing efficient access of the full spectrum of the hybrid system.

Complete optical transmission through nano-scale periodic metallic structure with three-stepped slits

Metallic structures with periodic array of subwavelength slits are well-known to lead to extraordinary optical transmission (EOT). Excellent power transmission originates from surface waves excited by incident transverse magnetic wave. Here we show that the metallic structure with array of three-stepped slits can transmit almost the entire power of incident wave into the substrate at desired optical frequency for low-temperature-grown gallium arsenide (LT-GaAs) substrate. Transmitted power of the proposed structure is studied both by an analytical model as well as numerical method by finite element (FEM). It is found that existence of three corners in the proposed structure can favorably confine generated carriers in the vicinity of the electrodes as this structure is used in photoconductive antenna (PCA); this feature contributes to increase terahertz (THz) current and subsequently THz radiation power of PCAs.

Exceptional points and spectral singularities in active epsilon-near-zero plasmonic waveguides

We present a nanoscale active plasmonic waveguide system consisting of an array of periodic slits that can exhibit exceptional points and spectral singularities leading to several novel functionalities. The proposed symmetric active system operates near its cut-off wavelength and behaves as an effective epsilon-near-zero (ENZ) medium. We demonstrate the formation of an exceptional point (EP) that is accessed with very low gain coefficient values, a unique feature of the proposed nanoscale symmetric plasmonic configuration. Reflectionless ENZ transmission and perfect loss-compensation are realized at the EP which coincides with the effective ENZ resonance wavelength of the proposed array of active plasmonic waveguides. When we further increase the gain coefficient of the dielectric material loaded in the slits, a spectral singularity occurs at the ENZ resonance leading to super scattering (lasing) response at both forward and backward directions. These interesting effects are achieved by materials characterized by very small gain coefficients with practical values and at subwavelength scales due to the strong and homogeneous field enhancement inside the active slits at the ENZ resonance leading to enhanced light-matter interaction. We theoretically analyze the obtained EP, as well as the divergent spectral singularity, using a transmission-line model and investigate the addition of a second incident wave and nonlinearities in the response of the proposed active ENZ plasmonic system. Our findings provide a novel route towards interesting nanophotonic applications, such as reflectionless active ENZ media, unidirectional coherent perfect absorbers, nanolasers, and strong optical bistability and all-optical switching nanodevices.

A design to tune the frequency in a terahertz filter based on dual-layered metallic slit arrays

A tunable terahertz filter based on reconfigurable dual-layered subwavelength metallic slit arrays is designed and theoretically analyzed. Reconfigurability is attained via mechanical tuning of relative shift between the slits on adjacent metallic films. Due to the desired application, the device parameters can be determined such that it operates in any frequency range. In this research, a filter with tuning range between 530 and 860 GHz is proposed, which is highly demanded in terahertz imaging systems. This polarization-sensitive filter has low insertion loss throughout its operating frequency range.

Deep Electrical Modulation of Terahertz Wave Based on Hybrid Metamaterial-Dielectric-Graphene Structure

A terahertz modulation structure based on hybrid metamaterial and graphene is proposed and demonstrated in this work. The metamaterial with a square slit ring array excites terahertz resonance in the slits and enhances the interaction between the terahertz wave and graphene. The graphene layer acting as the active material is tuned by the applied electrical field. With the separation by a dielectric layer between the graphene and the metallic structure, the resonant frequency and transmitted energy are both modulated by the graphene. Experimental result indicates that the modulation depth of the terahertz transmitted amplitude is 65.1% when the applied modulation voltage is tuned 5 V.

A Compact Single-Layer Q-Band Tapered Slot Antenna Array With Phase-Shifting Inductive Windows for Endfire Patterns

A wideband millimeter-wave (mmW) tapered slot antenna (TSA) array operating in the Q-band is reported, aiming at the upcoming fifth-generation wireless communications. The antenna element is comprised by a TSA with two arrays of comb-shaped slits symmetrically cut on the edges of the two fins, respectively, which enables a degree of miniaturization for the radiator. Importantly, the geometrical structures of adjacent elements are flipped such that the fins of two neighboring TSAs are electrically connected. A miniaturized T-shaped eight-way power divider based on substrate integrated waveguide (SIW) technology is used to feed the linear array, where adjacent output channels are out-of-phase. In order to compensate the phase errors caused by the conventional compact T-shaped eight-way power divider, a well-tuned matching circuit composed of two consecutive inductive windows embedded inside the SIW is employed behind each TSA element, resulting in an enhanced gain and a low sidelobe level over a wide bandwidth. A prototype of the proposed antenna is fabricated using standard printed circuit board process and characterized, showing a good agreement between measured results and simulation predictions. The fabricated antenna experimentally achieves an $S_{11} dB bandwidth of 13.0% (38.97–44.45 GHz), within which the gain is higher than 13.4 dBi and the sidelobe level is below −14.6 dB. Its directive endfire radiation patterns and wideband operation make it a suitable candidate for various mmW wireless communication systems.