Transverse Magnetic Polarization Research Articles

Ultra‐Broadband and Wide‐Angle Metamaterial Absorber with Carbon Black/Carbonyl Iron Composites Fabricated by Direct‐Ink‐Write 3D Printing

This study proposes the fabrication of an asymmetric woodpile metamaterial absorber using direct‐ink‐writing 3D printing technology. The prepared inks are compounded using two loss fillers: carbon black and carbonyl iron powder. The synergistic effect enhances the electromagnetic loss performance and the synergistic mechanism is analyzed. The designed asymmetric woodpile absorber has a wider bandwidth than a simple tetragonal woodpile, and the advantage presented by the asymmetric woodpile arrangement overcomes the local impedance mismatch. A simulation is then performed, which demonstrates that the designed asymmetric woodpile metamaterial with a thickness of 8.6 mm can achieve a −10 dB absorbing bandwidth in the frequency range of 3.9–18 GHz, and the maximum reflection loss reaches up to −39 dB. Additionally, the absorber exhibits excellent angular performance and the absorption bandwidth of the transverse electric polarization or transverse magnetic polarization waves can reach more than 10 GHz with incident angles from 0° to 50°. Furthermore, the manufacturing process of the absorber is simple, efficient, and inexpensive, which presents considerable potential for its widespread implementation in practical engineering applications.

Multiplication of photonic band gaps in one-dimensional photonic crystals by using hyperbolic metamaterial in IR range

The light-slowing effect near band endpoints is frequently exploited in photonic crystals to enhance the optical transmittance. In a one-dimensional binary photonic crystal (1DPC) made of hyperbolic metamaterials (HMMs), we theoretically examined the angle-dependent omnidirectional photonic bandgap (PBG) for TM polarization. Using the transfer matrix approach, the optical characteristics of the 1DPC structure having dielectric and HMM layers were examined at the infrared range (IR). As such, we observed the existing of numerous PBGs in this operating wavelength range (IR). Meanwhile, the HMM layer is engineered by the subwavelength dielectric- nanocomposite multilayers. The filling fraction of nanoparticles have been explored to show how they affect the effective permittivity of the HMM layer. Furthermore, the transmittance properties of the suggested structure are investigated at various incident angles for transverse magnetic (TM) and transverse electric polarizations. Other parameters such as, the permittivity of the host material, the filling fraction of nanoparticles, and the thickness of the second layer (HMM) are also taken into account. Finally, we investigated the effect of these parameters on the number and the width of the (PBGs). With the optimum values of the optical parameters of the nanocomposite (NC) layer, this research could open the way for better multi-channel filter photonic crystals.

Ultra-sensitive terahertz sensing based on Rayleigh anomaly in hyperbolic metamaterial gratings

Terahertz sensors are promising for biomedical, environmental and security applications. Challenges in terahertz sensing have been the limited sensitivity and small figure of merit (FOM). Here we propose an ultra-sensitive terahertz sensing platform based on the Rayleigh anomaly (RA) in hyperbolic metamaterial gratings, which are composed of periodic slits patterned in paired metal-dielectric multilayers. Simulation results show that, under normal incidence with transverse-magnetic polarization and designed with proper parameters, the proposed structure has a super-high quality factor of 516, an ultra-high sensitivity of 1.56 THz RIU−1 (or nm RIU−1), and extremely large FOM of 355 RIU−1 at frequency of 2.272 THz, corresponding to a large normalized sensitivity of 0.69 RIU−1. By comparing with other multilayered gratings, we attribute this exceptionally high sensing performance to the strong local field enhancement over an extremely large area in the sensing region at the RA frequency, which is found to be unique for hyperbolic metamaterial gratings. We further show that the resonance frequency decreases linearly with the grating period, following the RA equation, whereas the large normalized sensitivity keeps constant. We expect this work provides a novel strategy to design extremely sensitive sensors in terahertz as well as other frequency regimes.

Bifunctional terahertz sensor based on tunable graphene metamaterial absorber

In this paper, we propose a tunable graphene metamaterial absorber (TGMA) as a bifunctional terahertz sensor. The TGMA can excite one peak with a maximum absorption of 99.9%, and the physical mechanism of the absorption is explained by introducing the coupled mode theory. The simulation results show that the absorptivity can be dynamically adjusted by changing the chemical potential of graphene. The TGMA exhibits excellent absorption performance under a wide range of incident angles for transverse-electric (TE) and transverse-magnetic (TM) polarizations, respectively. More importantly, the TGMA can operate as a temperature sensor with a sensitivity of 6.2 GHz/K and a refractive index sensor with a sensitivity of 1.43 THz/RIU in both TE and TM polarizations under normal incidence. In addition, the effect of polarization angle on the sensing sensitivity is investigated. The simulation results indicate that the sensitivity is completely independent of the polarization angle for temperature and refractive index sensing, respectively. In a word, the demonstration of this bifunctional terahertz sensor offers an alternative solution for developing multifunctional metamaterial devices in the THz range.

Design and Characterization of the Fully Metallic Gap Waveguide-Based Frequency Selective Radome for Millimeter Wave Fixed Beam Array Antenna

This article presents a bandpass frequency selective surface (FSS) radome based on fully metallic gap waveguide (GW) technology. The element of the proposed FSS radome consists of a conventional cross-dipole slot etched on metallic plates and positioned over a groove GW cavity. A design with a single GW-cavity layer was initially produced which was later optimized for performance, to comprise a dual GW-cavity layer, while considering both functionality and manufacturability. It is shown that the proposed FSS element offers a stable and wide bandpass (from 26 to 30 GHz) performance in the broadside direction for both transverse electric (TE) and transverse magnetic (TM) polarizations. For oblique angle of incidence, the suggested FSS element works up to 30° with a reduction in usable bandpass bandwidth performance to 26–28 GHz for both TE and TM polarizations. A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20 \times 20$ </tex-math></inline-formula> -element GW-FSS array prototype has been fabricated and measured, which was integrated with a fixed-beam array antenna to further validate its functionality as a filtering radome. The findings show an excellent agreement between simulations and measurements. Hence, the proposed GW-FSS represents a great opportunity to develop an all-metallic FSS with low insertion loss, sharp-roll-off filtering, wideband performance, and inexpensive fabrication cost.

A redshifted photonic bandgap and wide-angle polarization selection in an all-hyperbolic-metamaterial one-dimensional photonic crystal.

According to the Bragg scattering theory, photonic bandgaps (PBGs) in all-dielectric one-dimensional (1-D) photonic crystals (PhCs) are polarization-insensitive. This polarization-insensitive property of PBGs poses a challenge in wide-angle high-performance polarization selection. Herein, we theoretically investigate the angle dependence of the PBGs in a novel kind of 1-D PhC called all-hyperbolic-metamaterial (all-HMM) 1-D PhC entirely composed of HMMs. As the incident angle increases, the PBGs in all-HMM 1-D PhCs exhibit the redshift property under transverse magnetic polarization while exhibiting the blueshift property under transverse electric polarization. Empowered by the polarization-sensitive property of the PBGs, wide-angle high-performance polarization selection can be theoretically achieved. Such a wide-angle polarizer would be useful in liquid crystal displays, quantum interferometers, and Q-switched lasers.

Tunable ultra-broadband terahertz metamaterial absorber based on vanadium dioxide strips

A simple design of an ultra-broadband metamaterial absorber (MMA) of terahertz (THz) radiation based on vanadium dioxide (VO2) configurations is proposed. The system is composed of a top pattern representing orderly distributed VO2 strips, a dielectric spacer and an Au reflector. Theoretical analysis based on the electric dipole approximation is performed to characterize the absorption and scattering properties of an individual VO2 strip. The results then are used to design an MMA composed of such configurations. It is shown that the efficient absorption characteristics of the Au-insulator-VO2 metamaterial structure can be ensured in a broad spectrum of 0.66-1.84 THz with an absorption band relative to the center frequency reaching as high as 94.4%. The spectrum of the efficient absorption can be easily tuned via the corresponding choice of strip dimensions. Wide polarization and incidence angle tolerance for both transverse electric (TE) and transverse magnetic (TM) polarizations are ensured by adding an identical parallel layer rotated by 90 degrees in respect to the first one. Interference theory is applied to elucidate the absorption mechanism of the structure. The possibility of modulation of the electromagnetic response of MMA relying on the tunable THz optical properties of VO2 is demonstrated.

A New Design of Optical Logic Gates with Transverse Electric and Magnetic

This paper presents a new design of optical NOR and XNOR logic gates using a two-dimensional-hexagonal photonic crystal (2D-HPhC) that allows for both Transverse Electric (TE) and Transverse Magnetic (TM) polarization modes. The structure is very small in size and has a low delay time. The design includes three inputs (A, B, C) and one output (Q) waveguide, with the NOR gate having a delay period of 0.75 ps and the XNOR gate having a delay period of 0.9 ps. The contrast ratio between the input and output for both gates is 7-8 dB. The XNOR gate has an optimum transmission signal rate of T = 96%. The purpose of the structure is to use a reference input to create the fundamental logic gates NOR and XNOR by adjusting the signal phase angle.

A Multi-layered GaGeTe Electro-Optic Device Integrated in Silicon Photonics

Electrically tunable devices contribute significantly to key functions of photonics integrated circuits. Here, we demonstrate the tuning of the optical index of refraction based on hybrid integration of multi-layered anisotropic GaGeTe on a silicon micro-ring resonator (Si-MRR). Under static applied (DC) bias and transverse-electric (TE) polarization, the device exhibits a linear resonance shift without any amplitude modulation. However, for the transverse-magnetic (TM) polarization, both amplitude and phase modulation is observed. The corresponding wavelength shift and half-wave voltage length product <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$(V_{\pi} . l)$</tex-math></inline-formula> for the TE polarization are -1.78 pm/V and 0.9 V.cm, respectively. These values are enhanced for the TM polarizations and correspond to - 6.65 pm/V and 0.28 V.cm, respectively. The dynamic radio frequency (RF) response of the devices was also tested at different bias conditions. Remarkably, the device exhibits a 1.6 MHz and 2.1 MHz response at 0 V and 7 V bias, respectively. Based on these findings, the integration of 2D GaGeTe on the silicon photonics platform has great potential for the next generation of integrated photonic applications such as switches and phase shifters.

Switchable optical trapping based on vortex-pair beams generated by a polarization-multiplexed dielectric metasurface.

Optical trapping is a state-of-the-art methodology that plays an integral role in manipulating and investigating microscopic objects but faces formidable challenges in multiparticle trapping, flexible manipulation, and high-integration applications. In this study, we propose and demonstrate a switchable optical scheme for trapping microparticles incorporating disparate vortex-pair beams generated by a polarization-multiplexed metasurface. The miniaturized all-dielectric metasurface, which comprises an array of titanium dioxide nanoposts, was manufactured and characterized to provide polarization-tuned two-fold vortex-pair beams. The profiles of the created vortices can be flexibly tailored by adjusting the combination of topological charges and the separation among phase singularities. Under transverse electric polarized light conditions, a vortex-pair beam with opposite topological charge combinations traps a single microparticle within one beam spot, while under transverse magnetic polarization conditions, two microparticles are captured simultaneously by a vortex-pair beam with the same topological charge signs. The proposed switchable trapping scheme (incorporating a vortex-pair light beam) is expected to feature enhanced integration and flexible manipulation of multiple particles with potential applications in biophysics, nanotechnology, and photonics.

An Artificial Immune System Model Tightly Coupled to the Inverse Design of Photonic Crystals

We propose a model based on an Artificial Immune System (AIS) to be used in conjunction with any electromagnetic solver in order to solve inverse design problems of integrated photonics. The proposed AIS was inspired by the Opt-aiNET algorithm coupled to a homemade electromagnetic solver based on the frequency domain Finite Element Method (FEM), which was used for calculating the fitness function. To validate the proposed model, we evaluate the photonic bandgap (PBG) of photonic crystals by the inverse design approach by assuming triangular lattices comprised of Si and air, and compared the results with other metaheuristics. The proposed AIS model is tightly coupled to such type of problem when the photonic structure is discretized in the binary-valued domain. Interestingly, different from many bio-immune-inspired metaheuristics, this proposed model has one of its metaheuristics parameters highly related to the characteristics of the application, which allows the algorithm itself to get a better “learning” of the problem and, additionally, reveals the designer some features intrinsic to the application. We have obtained midgap ratios of the first PBGs in triangular lattices of 47.55% and 49.11% for transversal magnetic (TM) and transversal electric (TE) polarizations, respectively. Additionally, the proposed model was applied to achieve large PBGs assuming the TM polarization in PhCs based on silicon carbide, and a new structure presented a PBG equals to 33.94%.

Impact of random alloy fluctuations on the electronic and optical properties of (Al,Ga)N quantum wells: Insights from tight-binding calculations.

Light emitters based on the semiconductor alloy aluminum gallium nitride [(Al,Ga)N] have gained significant attention in recent years due to their potential for a wide range of applications in the ultraviolet (UV) spectral window. However, current state-of-the-art (Al,Ga)N light emitters exhibit very low internal quantum efficiencies (IQEs). Therefore, understanding the fundamental electronic and optical properties of (Al,Ga)N-based quantum wells is key to improving the IQE. Here, we target the electronic and optical properties of c-plane AlxGa1-xN/AlN quantum wells by means of an empirical atomistic tight-binding model. Special attention is paid to the impact of random alloy fluctuations on the results as well as the Al content x in the well. We find that across the studied Al content range (from 10% to 75% Al), strong hole wave function localization effects are observed. Additionally, with increasing Al content, electron wave functions may also start to exhibit carrier localization features. Overall, our investigations on the electronic structure of c-plane AlxGa1-xN/AlN quantum wells reveal that already random alloy fluctuations are sufficient to lead to (strong) carrier localization effects. Furthermore, our results indicate that random alloy fluctuations impact the degree of optical polarization in c-plane AlxGa1-xN quantum wells. We find that the switching from transverse electric to transverse magnetic light polarization occurs at higher Al contents in the atomistic calculation, which accounts for random alloy fluctuations, compared to the widely used virtual crystal approximation approach. This observation is important for light extraction efficiencies in (Al,Ga)N-based light emitting diodes operating in the deep UV.

Argon flow rate effects on the optical waveguide properties of DC sputtered TiO2 thin films

TiO2 thin films were successfully prepared by DC sputtering using pure titanium target in mixed gases (Argon, Oxygen) plasma on glass substrates. The films were deposited at a constant substrate temperature (350 °C) with different Argon flow rate values (15, 30, 45, and 60 sccm). Raman spectroscopy analysis shows that all films crystallized in the Anatase phase. The crystallinity was found to improve with increasing Argon flow rate up to 45 sccm and then deteriorate sharply at 60 sccm. The crystallite size varied between 9.1 and 9.7 nm. Atomic force microscopy (AFM) revealed that the roughness fluctuated between 1.30 and 6.01 nm with an overall increase. The grain shape went from sharp needles like shape to dome like shape with an enlargement in their size by up to 60 nm. The UV–Vis spectrophotometer displayed that the films were highly transparent. The optical band gap ranged from 3.65 to 3.49 eV. Prism coupler analysis exhibited single guided modes in both transverse electric and transverse magnetic polarizations for the sample prepared at 15 sccm Argon flow rate and bi-guided modes in both polarizations for the rest of the samples. Both ordinary and extraordinary refractive indices decreased over Argon flow by 9.912% and 6.441% respectively. The thickness, porosity as well as the birefringence where found to increase by 155 nm, 16.16% and 0.0832 respectively as Argon flow rate went from 15 to 60 sccm.

Design and Fabrication of Tapered Dielectric for Broadband and Wide Incident Angle Transmission

We propose a novel design procedure for a 2-D array of tapered dielectric structures for broadband and wide-incident-angle transmission. We exploit a stereolithography apparatus (SLA)-type 3-D printer to fabricate tapered dielectric samples. This technology eliminates the costly and complicated processes (e.g., perforation and etching) that are required in the fabrication of tapered dielectric structures. Specifically, we adopt the rigorous coupled wave analysis (RCWA) technique to extract the effective permittivity (EP) of a tapered dielectric unit structure. An optimization algorithm is employed to determine the ultimate optimal design parameters for the maximum transmittance. The free-space measurement is performed to evaluate the electromagnetic (EM) performance of designed samples. We demonstrate that a 2-D array of optimized tapered dielectric unit structures can exhibit enhanced transmission property in comparison to a half-wavelength dielectric layer and tapered dielectric structure without optimization, viz. above 60% transmittance over the entire Ka-band up to 60° of the incidence angle for both transverse electric (TE) and magnetic polarizations.

Ultrawideband metamaterial absorber for oblique incidence using characteristic mode analysis

An ultrawideband, polarization-insensitive, metamaterial absorber for oblique angle of incidence is presented using characteristic mode analysis. The absorber consists of conductive meander square loops and symmetric bent metallic strips, which are embedded with lumped resistors. With the aid of modal currents and modal weighting coefficients, the positions of the lumped resistors are determined. After that, the equivalent circuit (EC) model and admittance formula are proposed and analyzed to further understand the working principle and ultrawide bandwidth. The proposed absorber measures an absorption bandwidth of 4.3–26.5 GHz (144.1% in fractional bandwidth) for 90% absorptivity under normal incidence. At the oblique angle of incidence of 45°, the bandwidth of 90% absorptivity is still 5.1–21.3 GHz (122.72%) for transverse electric (TE) polarization, and 6.8–29.5 GHz (125.07%) for transverse magnetic (TM) polarization. The good agreement among simulation, measurement, and EC calculation demonstrates the validity of the proposed method and indicates that the method can be applied to other microwave and optical frequency bands. The proposed metamaterial absorber can be widely applied in electromagnetic compatibility, electromagnetic interference, radar stealth, and biomedical detection.

Broadband Bilayer Antireflective Coating with Metasurfaces and Chebyshev Transformer

Antireflection coatings with a broad relative bandwidth (RBW) and reduced electrical length are desirable for a variety of applications in the microwave frequencies. Here, we propose the design principle of bilayer metasurfaces with 100% RBW and subwavelength thickness based on the traditional Chebyshev transformer, with excellent agreement with our experimental validation for both transverse electric and magnetic polarizations. We utilize the multiple-beam interference theory to reveal the underlying mechanism of the preferred choices for nonresonant or weakly resonant metasurface unit cells. Being able to manipulate the transmissions and reflections within the bilayer structure, we further demonstrate the universality and sensitivity of the designs capable of providing near-unity microwave transmission from air into the full range of natural media, with prescribed RBW and reduced electrical length.

Ultra-High Performance All-Silicon TM Polarizer Covering O-U Optical Communication Bands

Polarizers are the essential components to purify the polarization in the system demanding for the single polarization propagation. Polarizers with broad working bandwidth, low excess loss (EL) and large polarization extinction ratio (PER) are usually preferred. TE polarizers with ultrahigh performance have been demonstrated. However, the bandwidth of the previously reported transverse magnetic (TM) integrated polarizers is still limited to be < ∼160 nm when the EL < 1 dB and PER > 20 dB are simultaneously satisfied. In this paper, an all-silicon integrated TM polarizer with > 415 nm working bandwidth from 1260 nm to 1675 nm is experimentally demonstrated. The device consists of a double slot structure with 180° Euler bending on silicon waveguides. The effective refractive index (ERI) for TE polarization in the double slot structure is lower than that of the silica buffer/cladding while ERI for TM polarization is much larger than that of the silica buffer/cladding. Thus, TE polarization will leak into the substrate while TM polarization can propagate through the device with low loss. The measured transmission spectra for the fabricated polarizer show low EL < 0.9 dB and large PER > 25 dB over an ultrabroad band beyond the O-, E-, S-, C-, L-and U- bands.

Electrically controlled continuous laser beam steering in a liquid crystal based electro-optic waveguide

Non-mechanical and continuous beam steering devices are highly desirable in recent laser scanning applications and telecommunication advancements. We report a liquid crystal (LC) core waveguide-based electrically controlled, non-mechanical, and continuous laser beam steering device. The waveguide uses nematic 5CB (4-cyano-4′-pentylbiphenyl) LC as a propagation medium surrounded by PVA (polyvinyl alcohol) polymer cladding, which is realized on ITO coated glass substrates. A pattern of five triangular electrodes is transferred on the substrate using photolithography to create prisms of tunable refractive index in the LC core of the waveguide. The beam steering for transverse electric (TE) and transverse magnetic (TM) polarization is theoretically studied and experimentally characterized using a He-Ne laser of λ = 633 nm. A steering angle of −5.4° and +5.7° at 7 Vpp is obtained for TE and TM polarization, respectively, in a waveguide with the prism’s apex angle of 13°. Similarly, a higher steering angle up to 7.3° for TM polarization is demonstrated in the waveguide with an apex angle of 22°. The device is developed using ease of fabrication process and cost-effective materials that operate at relatively low voltages.

Shell-shaped active layers for omnidirectional organic photovoltaic cells

For the employment of organic photovoltaic cells in wearable electronic systems, improvements in energy conversion efficiency and omnidirectionality (angular coverage) are highly appreciated. This study aims at those improvements by introducing shell-shaped active layers. The proposed device structures enhance light absorption and angular range through light coupling to guided modes in the active layer. Two shapes, i.e., a triangle and a semicircle, are examined for the shell cross-sections. Numerical simulation using finite-element analysis and finite-difference time-domain methods demonstrates that the devices with the triangular-shell-shaped active layers exhibit an average absorption enhancement of up to 63% for transverse electric (TE)-polarization and up to 32% for transverse magnetic (TM)-polarization when compared with the flat active layers of the same thicknesses. The average enhancements of the semicircular-shell-shaped active layers are found to be slightly lower than those values, with 60% for TE and 28% for TM. The examined structures also show good omnidirectionality with decent absorption up to an 81 deg incidence angle for the triangular-shell-shaped device and up to a 76 deg angle for the semicircular one when TM polarization is considered. These absorption enhancements and improved angular coverages make the proposed structures highly attractive for wearable electronic system applications.

Near-infrared broadband polarization beam splitter with an Au nanocube array

A near-infrared broadband polarization beam splitter (PBS) is proposed and numerically simulated, which is realized by a directional coupler assisted with an Au nanocube array. The fundamental transverse electric (TE) mode can be coupled to the output of the upper waveguide by exciting the surface plasmon polaritons (SPPs), while the fundamental transverse magnetic (TM) mode is output directly from the lower waveguide. The length of the coupling region of the PBS is only 1 μm. It has a working bandwidth of 210 nm in the range of 1420 ∼ 1630 nm, covering three bands: S-, C- and L-bands. The simulation shows that the extinction ratio of TE and TM polarization is 20.31 dB and 14.15 dB, respectively.