Anisotropic Metamaterials Research Articles

Tungsten disulfide based anisotropic metalens for refractive index sensing applications in visible region

Studies on metalenses are rapidly developing, aiming to introduce compact optical devices with superior properties. In this study, which can contribute to the existing literature under the heading of anisotropic metamaterials, the focusing performance of the metalens using the aperture-based Pancharatnam-Berry phase method is numerically analyzed. The proposed structure is formed by coating tungsten disulfide layer on silicon nitride membrane and then etching the whole structure according to the aperture Pancharatnam-Berry phase method. The designed metalens is capable of focusing with efficiencies of 9.7 %, 6.7 %, and 8.4 % at red, green, and blue wavelengths, respectively. At all three wavelengths, the resonative behavior is provided by the anisotropic behavior. Additionally, high numerical aperture values as 0.81, 0.81, and 0.76 are obtained for all three wavelengths. Moreover, refractive index sensing performance is analyzed by immersing the structure into different types of alcohol media, and in this respect, another unique aspect of the study is revealed.

Compact Design for Bi-Polarization Quantum Routers on SOI Platform

An ultra-compact optical quantum router (QR) consisting of a Mach–Zehnder interferometer (MZI) and waveguide tapers is proposed and numerically simulated, using silicon-on-insulator (SOI). The interferometer is designed to work at the center wavelength of 1550 nm with visibilities of 99.65% and 98.80% for TE and TM polarizations, respectively. Using the principle of phase compensation and self-image, the length of the waveguide tapers is shortened by an order of magnitude with the transmission above 95% for both TE and TM polarizations. Furthermore, polarization beam splitters (PBS) with an ultra-compact footprint of 1.4 × 10.4 μm2 with transmissions of 98% for bi-polarizations are achieved by introducing anisotropic metamaterials. The simulated results indicate that the interferometer facilitates low loss, a broad operating spectral range, and a large tolerance to size variation in fabrications. The optical switch possesses the routing function while maintaining the polarization states, which promises to pave the point-to-point BB84 protocol into applications of network-based quantum communication.

On-chip long-wavelength infrared polarimeter for full-Stokes polarization detection

Polarization detection is a critical technique in various research and application fields, especially in the long-wavelength infrared regime for target discrimination from complex backgrounds. With the trend of miniaturization and integration, on-chip full-Stokes polarimeters are eagerly pursued. However, how to achieve efficient linear and circular polarimetry simultaneously on the same detection material chip with sufficient polarization extinction ratios and responsivities still remains a challenge. In this work, we propose an on-chip long-wavelength infrared full-Stokes polarimeter based on the integration of anisotropic and twisted metamaterials on the same quantum well material chip. The device consists of six subpixels with different principle detection polarization states. Based on the dual polarization selection by the structure and the detection material, the linear polarization extinction ratio exceeds 106 and the circular polarization extinction ratio reaches 150. In addition, the absorptance of the detection material is enhanced by more than 16 times. Through simple subtracting and normalization operations on the photocurrents of the subpixels, the Stokes parameters are obtained with small root mean square errors. The device performance remains at a high level over the incident angle range of ±5°. This work points out a promising way for on-chip long-wavelength infrared full-Stokes detection. The device architecture is compatible with focal plane arrays.

Broadband asymmetric transmission in quasiperiodic-metamaterials based on high entropy alloys ribbon

The new paradigm was introduced for achieving excellent electromagnetic (EM) waves absorption through multi-layer superimposed quasiperiodic-metamaterials, which were generated in rotary high entropy alloys ribbon. With the help of asymmetry, the designed anisotropic metamaterials with excellent absorption (ReflectionLoss≤−10dB) covered from 3.8 GHz to 18 GHz on the scale of sub-wavelength. High entropy alloys ribbon increased soft magnetic properties that design cascaded configuration metamaterials displayed distinct phase differences with increased rotation angle, which significantly focus on microwave attenuation. Meanwhile, the direction of the wave vector accompanied by rotation can realize polarized conversion, from linear polarizer to circular polarizer which is demonstrated in simulation, highly desirable in controlling EM waves and shrinking the size of traditional absorbing coating. Our study provides a fresh perspective to developing high-performance absorbers by combining soft magnetic and irregular phase patterns as possible as need for complex EM waves manipulations, arbitrary phase encoding, and vortex field.

Maximum gain enhancement in wireless power transfer using anisotropic metamaterials

We present an analysis for metamaterial (MM) enhanced wireless power transfer (WPT) that includes new results revealing the impact of magnetostatic surface waves and their degradation of WPT efficiency. Our analysis shows that the commonly used fixed loss model used by previous works leads to the incorrect conclusion regarding the highest efficeincy MM configuration. Specifically, we show that the “perfect lens” configuration provides lower WPT efficiency enhancement in comparison to many other MM configurations and operating conditions. To understand why, we introduce a model for quantifying loss in MM-enhanced WPT and introduce a new figure of merit on efficiency enhancement, G_{rho }. Using both simulation and experimental prototypes, we show that while the “perfect-lens” MM achieves a field enhancement of four times the other configurations considered, its internal loss due to magnetostatic waves significantly reduces its efficiency-enhancement. Surprisingly, all the MM configurations analyzed other than the “perfect-lens” achieved higher efficiency enhancement in simulation and in experiment than the perfect lens.

Acoustic emission enhancement and self-centering effect via an extreme anisotropic metamaterial

An extreme anisotropic metamaterial consisting of one central cavity, eight zigzag and straight channels is proposed, aiming to achieve acoustic emission enhancement and self-centering effect. By placing a monopole source in the center of the metamaterial, acoustic emission enhancement can be achieved through the resonance in the zigzag channels and the monopole resonances. Theory and simulation confirm the self-centering effect of the proposed metamaterial, that is, when monopole sources are placed away from the center of the metamaterial, the external sound field can still be regarded as a uniform sound field generated by a monopole source placed in the center.

Multiscale optimal design method of acoustic metamaterials using topology optimization

AbstractAcoustic anisotropic metamaterials are known for its potential of applications to various acoustic devices, such as hyperlens and waveguides. Anisotropic metamaterial typically has a periodic structure constituted of unit cells, and exhibits highly anisotropic wave propagation based on local resonance induced by its periodic medium. In previous works, design methods for anisotropic metamaterials which shows unusual wave propagation properties have been proposed, though since metamaterials comprised of only a single type of unit cell were in interest, the macroscopic wave propagation properties that could be obtained were limited. Therefore, in the present work, we propose a design method for acoustic structures which are composed of several unit cells, in order to realize waveguides that navigate acoustic waves and achieve complex wave propagation overall. First, we introduce the high‐frequency homogenization method, which is capable of evaluating the macroscopic property of periodic structures based on local resonance. We construct a multiscale optimal design method by formulating the macro‐scale optimization problem for obtaining the properties required for different unit cells in order to realize the waveguide, along with the micro‐scale optimization problem for designing each unit cell based on topology optimization. Numerical examples are introduced to demonstrate the validity of our proposed method.

All-optical image edge detection based on the two-dimensional photonic spin Hall effect in anisotropic metamaterial.

Optical image processing based on the photonic spin Hall effect (SHE) has been gaining significant attention as a convenient and an accurate way for image edge detection. However, the recent edge imaging techniques depending on optical differentiation is mainly achieved by modulation of one-dimensional photonic SHE. Here, we theoretically predict the two-dimensional photonic SHE in the anisotropic metamaterial, and find that its longitudinal and transverse displacements exhibit spin-dependent property at filling factors with increasing incidence angle. As the transverse and in-plane displacements induced by the photonic SHE can be controlled by the filling factor of the crystal structure, the optical axis angle, and the incident angle, this intrinsical effect can be used to realize a tunable edge imaging. Interestingly, by changing the optical axis of the anisotropic metamaterial, the in-plane displacements are equal to the transverse displacements for a certain filling factor and the corresponding incident angle. Therefore, we propose a two-dimensional image edge detection method based on the photonic SHE in anisotropic metamaterial. Further numerical results validate the theoretical proposal.

A Wideband Circularly Polarized Antenna Based on Anisotropic Metamaterial

This article presents a novel circularly polarized (CP) antenna based on an anisotropic metamaterial. The antenna is capable of performing linear-to-circular polarization conversion over a wideband of frequencies in the X-band (8–12 GHz). The proposed antenna was constructed from a metamaterial-based polarizer mounted above the aperture of a rectangular waveguide. The polarizer was oriented at 45° to E-plane of the waveguide. The proposed polarizer is composed of multiple metamaterial layers. The unit cell of the proposed polarizer consists of a single dielectric slab incorporating a series of rectangular split ring resonators that are printed on both sides of the slab. Impedance matching layers (IML) are introduced to enhance the axial-ratio (AR) bandwidth. The polarizer has a low profile in terms of electrical length (thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.41 \lambda _{0}$ </tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> is the free-space wavelength at 10.5 GHz). A full-wave electromagnetic simulator was used to verify the anisotropic characteristics of the unit cell. This was achieved by showing that the metamaterial exhibits two different refractive indices along the orthogonal components of the incident electric field. The prototype of the proposed design is fabricated and measured to validate the performance. The measured results agree with the simulated ones and demonstrated a wide impedance bandwidth of 62.4% ranging from 7.08 to 13.5 GHz with a 3 dB AR bandwidth of 29.9% (9.25–12.5 GHz).

Enhancement of Optical Spin-Orbit Coupling in Anisotropic ENZ Metamaterials

Vector vortex beams which possess complex intensity and polarisation patterns, and phase or polarisation singularities are important in a variety of applications ranging from high resolution imaging and data transmission to nonlinear optics and quantum technologies. Here, we theoretically analyse the effect of vector vortex beams propagation through strongly anisotropic metamaterials exhibiting epsilon-near-zero behaviour. The interaction of the metamaterial with longitudinal field of the vector beams results in a strong modification of the polarisation distribution, with the metamaterial acting as an azimuthal polariser. The dependence on the local ellipticity of the field is also investigated revealing conversion of spin angular momentum to orbital angular momentum and polarisation structure vorticity, mediated by the longitudinal field. The results show the promise of an anisotropic metamaterial platform for manipulation of polarisation properties of and spin-orbit coupling in vector vortex beams.

Flexibly Enhanced Photonic Spin Hall Effect via Selective Brewster Angle

AbstractThe manipulating of photonic spin Hall effect (SHE) plays a crucial role for development of spin‐dependent nanodevices and systems. Since the photonic SHE is generally enhanced near the Brewster angle, the choice of incident angle usually has low flexibility with natural materials due to their dielectric constants. Herein, an efficient method to flexibly enhance the photonic SHE by utilizing selective Brewster angle in an anisotropic metamaterial is proposed. Through adjusting the thickness ratio of two media in metamaterial, the Brewster angle can be flexibly adjusted in a broad range (nearly 0–90°). With the selective Brewster angle, the spin‐dependent transverse shift can be enhanced at nearly arbitrary incident angles. Furthermore, based on this structure, a binary encoding system is demonstrated, realizing information conversion around incident angles. This research work provides more possibilities for applications in manipulating photonic SHE.

Hierarchical peroxiredoxin assembly through orthogonal pH-response and electrostatic interactions.

Morpheeins are proteins that adapt their morphology and function to the environment. Therefore, their use in nanotechnology opens up the bottom-up preparation of anisotropic metamaterials, based on the sequential use of different stimuli. A prominent member of this family of proteins is peroxiredoxins (Prx), with dual peroxidase and chaperone function, depending on the pH of the media. At high pH, they show a toroidal morphology that turns into tubular stacks upon acidification. While the toroidal conformers have been explored as building blocks to yield 1D and 2D structures, the obtention of higher ordered materials remain unexplored. In this research, the morpheein behaviour of Prx is exploited to yield columnar aggregates, that are subsequently self-assembled into 3D anisotropic bundles. This is achieved by electrostatic recognition between the negatively charged protein rim and a positively charged porphyrin acting as molecular glue. The subsequent and orthogonal input lead to the alignment of the monodimensional stacks side-by-side, leading to the precise assembly of this anisotropic materials.

A New Class of Single-Material, Non-Reciprocal Microactuators.

A crucial component in designing soft actuating structures with controllable shape changes is programming internal, mismatching stresses. In this work, a new paradigm for achieving anisotropic dynamics between isotropic end-states-yielding a non-reciprocal shrinking/swelling response over a full actuation cycle-in a microscale actuator made of a single material, purely through microscale design is demonstrated. Anisotropic dynamics is achieved by incorporating micro-sized pores into certain segments of the structures; by arranging porous and non-porous segments (specifically, struts) into a 2D hexagonally-shaped microscopic poly(N-isopropyl acrylamide) hydrogel particle, the rate of isotropic shrinking/swelling in the structure is locally modulated, generating global anisotropic, non-reciprocal, dynamics. A simple mathematical model is introduced that reveals the physics that underlies these dynamics. This design has the potential to be used as a foundational tool for inducing non-reciprocal actuation cycles with a single material structure, and enables new possibilities in producing customized soft actuators and modular anisotropic metamaterials for a range of real-world applications, such as artificialcilia.

Two-Dimensional Distributed Transmission-Line Models for Broadband Full-Tensor Anisotropic Acoustic Metamaterials Based on Transformation Acoustics

We propose the design method for broadband acoustic metamaterials based on the concept of transformation acoustics. Two-dimensional distributed transmission-line (TL) models for full-tensor anisotropic electromagnetic metamaterials are applied to full-tensor anisotropic acoustic metamaterials and the design formulas are shown to uniquely determine the structural parameters of the unit cells. Two-dimensional acoustic waveguide unit cell structures for realizing the TL models are proposed and an acoustic carpet cloak and an acoustic illusion medium are designed according to the introduced theory. The complex sound pressure distributions in the acoustic waveguides of the unit cells are calculated by full-wave simulations to verify the validity of the proposed method, and the broadband operations of the designed carpet cloak and illusion medium are confirmed from the results.

Integrated Subwavelength Grating Waveguide Bragg Gratings for Microwave Photonics

Bragg gratings (BGs) have numerous applications in microwave photonic (MWP) signal processing and MWP systems due to the ability to obtain custom-designed amplitude and phase responses. Moreover, waveguide BGs (WBGs) are compact and can be readily integrated with other passive and active components to realize MWP building blocks and processing systems. Subwavelength grating (SWG) waveguides and passive devices in silicon-on-insulator (SOI) have emerged as an enabler for developing break-through devices based on anisotropic metamaterials. In this paper, we review recent advances on developing SWG WBGs in SOI, including chirped, phase-shifted, sampled, and polarization (in)dependent structures. Applications of these SWG WBGs in MWP are described.

Gain-Assisted Giant Third-Order Nonlinearity of Epsilon-Near-Zero Multilayered Metamaterials.

We investigate the third-order nonlinear optical properties of epsilon-near-zero (ENZ) Au/dye-doped fused silica multilayered metamaterials in the visible spectral range for TM incident by using nonlocal effective medium theory at different incidence angles. The nonlocal response affects the permittivity of anisotropic metamaterials when the thickness of the layer cannot be much smaller than the incident wavelength. By doping pump dye gain material within the dielectric layer to compensate for the metal loss, the imaginary part of the effective permittivity is reduced to 10−4, and the optical nonlinear refractive index and nonlinear absorption coefficient are enhanced. The real and imaginary parts of the permittivity are simultaneously minimized when the central emission wavelength of the gain material is close to the ENZ wavelength, and the nonlinear refraction coefficient reaches the order of 10−5 cm2/W, which is five orders of magnitude larger than that of the nonlinear response of the metamaterial without the gain medium. Our results demonstrate that a smaller imaginary part of the permittivity can be obtained by doping gain materials within the dielectric layer; it offers the promise of designing metamaterials with large nonlinearity at arbitrary wavelengths.

Tunable anisotropic parameters realized by metal-dielectric hybrid meta-atoms.

Rational design of the structure enables metamaterials to go beyond the ingredients and achieve unprecedented material properties. However, the realization of complicated and anisotropic electromagnetic parameters relies on the elaborate design of building blocks, and the mutual coupling between the anisotropic responses makes precise control of material parameters even more difficult. Here, we propose a metal-dielectric hybrid metamaterial, not only realizing the decoupling between anisotropic electromagnetic responses, but also establishing a one-to-one correspondence between independent geometric dimensions and anisotropic parameter components. Moreover, a tuning theoretical paradigm applied to an anisotropic and resonant system is further suggested, which proves that the operating frequency of this hybrid metamaterial can be easily adjusted by changing external fields. As prototypes, two typical and tunable microwave meta-devices, a transformation-optics cloak and a frequency splitter, are constructed with Ba-Sm-La-Ti ferroelectric ceramic and flexible printed circuit board, which successfully demonstrate our proposed design theory. This work provides a simple strategy for the design and fabrication of tunable anisotropic metamaterials, and boost the development of meta-devices toward practical application.

An Optically Anisotropic Crystal with Large Birefringence Arising from Cooperative π Orbitals.

Birefringent materials are highly demanded for high-performance polarized optics. As compared with artificial anisotropic metamaterials, anisotropic crystals have advantages of low optical losses and easy processing, but their birefringence is still limited. Herein, based on first-principles studies, we identified a new type of functional anion units, (Hx C6 N9 )(3-x)- (x=0, 1, 2), and then successfully synthesized a new anisotropic crystal, namely, CsH2 C6 N9 ⋅H2 O (I), whose crystal structure consists of (H2 C6 N9 )- anions. Remarkably, I is ultraviolet transparent and exhibits very large birefringence of about 0.55@550 nm, which is much larger than those of commercial birefringent crystals. These results make I a candidate for highly efficient manipulation of optics and light in optical modulation devices. Theoretical calculations reveal that large birefringence mainly arises from the cooperative π orbitals in (H2 C6 N9 )- anions. This work provides a new insight on the underlying structure-property relationships of anisotropic crystals.

An Optically Anisotropic Crystal with Large Birefringence Arising from Cooperative π Orbitals

AbstractBirefringent materials are highly demanded for high‐performance polarized optics. As compared with artificial anisotropic metamaterials, anisotropic crystals have advantages of low optical losses and easy processing, but their birefringence is still limited. Herein, based on first‐principles studies, we identified a new type of functional anion units, (HxC6N9)(3−x)− (x=0, 1, 2), and then successfully synthesized a new anisotropic crystal, namely, CsH2C6N9⋅H2O (I), whose crystal structure consists of (H2C6N9)− anions. Remarkably, I is ultraviolet transparent and exhibits very large birefringence of about 0.55@550 nm, which is much larger than those of commercial birefringent crystals. These results make I a candidate for highly efficient manipulation of optics and light in optical modulation devices. Theoretical calculations reveal that large birefringence mainly arises from the cooperative π orbitals in (H2C6N9)− anions. This work provides a new insight on the underlying structure–property relationships of anisotropic crystals.

Topology optimization of thermal cloak using the adjoint lattice Boltzmann method and the level-set method

This paper presents a topology optimization (TO) method for thermal cloak design based on the adjoint lattice Boltzmann method and the level-set method. The gradient-type TO method presented is of high computational efficiency and of strong parallelism scalability since it completely circumvents large-scale matrix operations. After validated, the TO method is applied to design the thermal cloak composed of common isotropic bulk materials instead of anisotropic meta-materials. A clearly defined material distribution is obtained without non-physical grayscales. Excellent camouflage performance is achieved in the final design with the objective, namely the dimensionless squared norm of the temperature deviation, reduced from 1 to 7.57 × 10−5. Effects of the dam structures and bypass structures in the designed cloak on the heat transfer are discussed in detail. All results in this work are verified by the finite volume method with the relative error lower than 2 %. A comprehensive parametric study is conducted to reveal the effects of the initial structure, mesh number, thermal conductivity, the subdomain size, as well as the location, shape and topology of the cloaking object. The results have demonstrated the effectiveness, robustness and flexibility of the presented TO method on the practical design of thermal cloak.