Transmission-type Metasurface Research Articles

Bidirectional triple-band truly incident angle insensitive polarization converter using graphene-based transmissive metasurface for terahertz frequency

Abstract The design and response of a triple-band linear-to-circular polarization converter (LTCPC) in the terahertz frequency regime using a graphene-based transmission-type metasurface have been studied numerically and analytically. The unit cell of the converter is constructed using lossy silicon dioxide (SiO2) with a relative permittivity of 3.9 as the substrate. And two similar layers of graphene-based sub-wavelength structures are used at the top and bottom of it. Multiband linear to circular polarization conversion is achieved from 0.74 THz to 0.98 THz, 1.70 THz to 2.33 THz, and 4.06 THz to 5.19 THz, i.e., about 28 %, 31 %, and 24 % fractional bandwidths, respectively. As the same metasurface configuration is used at the top and bottom layers of the transmission-type unit cell, it can be used as a bidirectional polarization converter with identical responses from both sides. The geometrical optimization of the unit cell is done, and an equivalent lumped parameter circuit model is also proposed for the same, considering analogous responses. Tunability over the operating frequency is achieved by varying the chemical potential and relaxation time of graphene. Moreover, due to the ultrathin width and special types of identical configuration of the metasurfaces, the response of the system remains excellent over a wide range of incident angle variations up to 80° angle of incidence.

Acoustic metasurface development for transmitted wavefront manipulation using a level set-based topology optimization approach

A customized transmission-type metasurface design is developed for high-efficiency wavefront modulation by employing a level set-based topology optimization approach. The metasurface consists of periodically arranged supercells made of discrete unit cells aiming to form a precisely linear phase gradient of the transmitted wave ranging from 0 to 2π with high transmittances. The configuration of each unit is optimized to generate a desired phase and amplitude response of the transmitted wave at the interface. The transmitted wavefront steering realized by the phase-gradient metasurface is numerically and experimentally verified according to the diffraction theory, demonstrating that the level set-based topology optimization approach is a viable and effective technique for developing a transmitted phase-gradient metasurface. Afterwards, the designed optimal unit cells are elaborately arranged to form desired phase shift distributions for realizing fascinating acoustic functionalities, including acoustic energy focusing and acoustic Airy-like beam generation, further demonstrating the ability of the optimal units to achieve unprecedented transmitted wavefront control. This study presents a promising approach for customized transmission-type metasurface design based on the level set-based topology optimization, which is expected to be significantly valuable for acoustic device development.

A Design Method of Transmission-Type Metasurfaces Using Circuit Synthesis Theory of Microwave Bandpass Filters

In this paper, we applied the circuit synthesis theory of filters to the design of transmission-type metasurface cells and arbitrarily designed the amplitude and phase of the transmission and reflection by adjusting the resonant frequency and coupling coefficient. In addition, we successfully designed the phase of the unit cell by using the frequency conversion of filter theory. Moreover, we designed a refractive transmission-type metasurface plate with a novel cell structure that reacts to both polarizations. The prototype operated at the desired refraction angle, confirming the design theory.

Amplitude and phase independent modulation based on transmission-type metasurfaces

We propose an ultra-thin I-type metasurface that independently controls the amplitude and phase of transmitted waves simultaneously by changing the geometry and direction of rotation of I-shaped metal particles. It is found that the direction of rotation and the size of the geometrical opening of the I-shaped copper particle independently control the amplitude and phase of the incident wave. In order to better demonstrate the independent regulation of the amplitude and phase of the metasurface, we arranged the element structure and constructed a grating structure. We can adjust the diffraction order of grating freely by arranging different element structures. Using classical lithography techniques, we have experimentally prepared the array structure samples and compared the theoretical results with the experimental measurements. Theory and experiment agree well. We further propose a two-layer structure to simultaneously regulate the amplitude and phase of terahertz waves. The amplitude and phase of transmitted terahertz waves can be independently regulated by the rotation angle and opening angle of the double-layer cell structure. We further prepared the double-layer structure sample, and the theoretical calculation results are in good agreement with the experimental measurement results.

Transmission-type coding metasurface with ultra-broadband polarization conversion

The advent of coding metasurfaces (MSs) has established a bond between electromagnetic scattering and digital control. Given that most of the current coding MSs are based on reflection-type, we investigate a novel transmission-type MS that can achieve ultra-wideband and efficient polarization conversion. A new double diagonal split ring resonator structure is designed to mainly constitute the unit. The 1-bit and 2-bit coding arrays are constructed using the proposed polarization rotation mechanism of the MS unit combined with the dimensional transformation to achieve beam focusing, beam deflection, and beam splitting. Finally, the broadband focusing characteristic of the coding MS is also demonstrated. Numerical simulation results demonstrate that the transmission coefficient of the proposed MS is above 90% in the frequency range of 0.4 to 2 THz, with a relative bandwidth of 133.3%. Asymmetric transmission phenomenon is observed in the range of 0.4 to 2 THz, with coefficients remaining above 80% in all ranges.The proposed coding MS can be used in terahertz polarization converters and unidirectional transmission devices due to its superior polarization conversion performance.

Side lobe level reduction of metasurface transmit array

This paper presents a study on the side-lobe level (SLL) reduction with transmission type metasurface (MTS) using the concept of non-uniform array technique and transmit array antenna. According to the non-uniform antenna array technique, the SLL can be controlled by controlling the amplitude and phase distribution simultaneously. Initially, transmit array concept is used to design the MTS. The radiation characteristics of the MTS antenna are analyzed by exciting with a rectangular patch antenna at a fixed feed position. The simulation results show a gain of 17.6 dB with SLL of −18.3 and −19 dB in ϕ = 0° and 90°-planes, respectively. Later, non-uniform array techniques are used to reduce the SLL by introducing various amplitude distribution functions on the MTS for a fixed focal distance. The MTS incorporates the amplitude tapering functions such as Kaiser, Hamming, Hanning, and Blackman distribution functions. The resulting SLLs are analyzed, and the achieved SLLs are far from the theoretical values. In order to further reduce and control the SLL, the Gaussian distribution function is used, which minimizes the SLL by optimizing the standard deviation(σ). A MTS with 16 × 16 unitcells is fabricated and measured with an optimized σ = 0.31. The measured SLL are −25.1 and −26.3 dB in ϕ = 0° and 90°-planes, respectively.

Absorption-transmission-type multifunctional coding metasurface

Reflection- and transmission-type metasurfaces have great potentials in wireless communications. In this letter, we propose an absorption-transmission-type multifunctional coding metasurface composed of an absorptive layer and a transmissive layer. For incident waves at different frequencies with different polarizations, the proposed coding metasurface can perform three different functions independently: wave absorption at low frequencies, vortex-wave generation for the x-polarized wave and beam deflection for the y-polarized wave at high frequencies. Both numerical and experimental results verify the excellent performance of low-frequency absorptions and high-frequency dual-polarization transmissions. This work provides a method for developing multifunctional and reconfigurable components for the wireless communications in the future.

Highly efficient wavefront control based on extremely anisotropic materials

In this paper, a new strategy to design phase gradient metasurfaces (PGMs) are proposed. Using this way, we can achieve highly efficient control of the electromagnetic wavefronts. Specifically, we design reflection-type PGMs formed by an extremely anisotropic medium and a perfect electric conductor boundary. Instead of using refractive index distributions of materials to control the phases, we use a uniform anisotropic medium to control the reflection phases by changing the shape of the boundary. Using this strategy, three devices of the reflection focusing lens, the generator of the quasi airy beam and the PGMs have been realized, which show excellent performance in wavefront control. Our method is also suitable for the design of transmission-type metasurfaces and other wavefront transformations, which provides a novel approach for wave manipulation.

Research on Vortex Electromagnetic Wave Based on Coding Metasurface

The characteristics of vortex electromagnetic wave generated by coding metasurface were studied, and the transmission-type metasurface of vortex wave was designed based on coding unit. It was found that the coding metasurface could replace the quasi-continuous phase to reduce the difficulty of the unit design. But when the orbital angular momentum increased gradually the spacing of two main lobes increased, the side lobes increased, and the vortex waves of other orbital angular momentum reduced. Based on this limitation, this paper optimizes the arrangement of units by genetic algorithm in order to maximize the performance of vortex electromagnetic wave in the limited metasurface size, so that the vortex electromagnetic wave energy is concentrated in the main lobes, thereby enhancing the purity of the orbital angular momentum mode and reducing the electromagnetic wave carrying the orbital angular momentum with other modes, and reduce the main lobes spacing is conducive to propagation. Finally, the feasibility of the method is verified by simulation measurement.

Optical spin-symmetry breaking for high-efficiency directional helicity-multiplexed metaholograms

Helicity-multiplexed metasurfaces based on symmetric spin–orbit interactions (SOIs) have practical limits because they cannot provide central-symmetric holographic imaging. Asymmetric SOIs can effectively address such limitations, with several exciting applications in various fields ranging from asymmetric data inscription in communications to dual side displays in smart mobile devices. Low-loss dielectric materials provide an excellent platform for realizing such exotic phenomena efficiently. In this paper, we demonstrate an asymmetric SOI-dependent transmission-type metasurface in the visible domain using hydrogenated amorphous silicon (a-Si:H) nanoresonators. The proposed design approach is equipped with an additional degree of freedom in designing bi-directional helicity-multiplexed metasurfaces by breaking the conventional limit imposed by the symmetric SOI in half employment of metasurfaces for one circular handedness. Two on-axis, distinct wavefronts are produced with high transmission efficiencies, demonstrating the concept of asymmetric wavefront generation in two antiparallel directions. Additionally, the CMOS compatibility of a-Si:H makes it a cost-effective alternative to gallium nitride (GaN) and titanium dioxide (TiO2) for visible light. The cost-effective fabrication and simplicity of the proposed design technique provide an excellent candidate for high-efficiency, multifunctional, and chip-integrated demonstration of various phenomena.

Bianisotropic Acoustic Metasurface for Surface-Wave-Enhanced Wavefront Transformation

Acoustic metasurfaces offer remarkable control of wave transmission and reflection, but with limited power efficiency in traditional systems. Perfect wavefront transformation with perfect efficiency traditionally requires either nonreciprocal or nonlocal responses, which are challenging to implement with passive designs. This study uses the automatically excited surface waves to design impedance-based acoustic metasurfaces requiring only reciprocal and local responses; thus passive structures suffice. Counterintuitively, transmission-type metasurfaces require nonzero reflected fields for maximum efficiency, and reflection-type metasurfaces need nonzero transmitted fields.

High-Efficiency All-Dielectric Metasurfaces for the Generation and Detection of Focused Optical Vortex for the Ultraviolet Domain

The optical vortex (OV) has drawn considerable attention owing to its tremendous advanced applications, such as optical communication, quantum entanglement, and on-chip detectors. However, traditional OV generators suffer from a bulky configuration and limited performance, especially in the ultraviolet range. In this paper, we utilize a large bandgap dielectric material, niobium pentoxide (Nb2O5), to construct ultra-thin and compact transmission-type metasurfaces to generate and detect the OV at a wavelength of 355 nm. The meta-atom, which operates as a miniature half-wave plate and demonstrates a large tolerance to fabrication error, manipulates the phase of an incident right-handed circular polarized wave with high cross-polarized conversion efficiency (around 86.9%). The phase delay of π between the orthogonal electric field component is attributed to the anti-parallel magnetic dipoles induced in the nanobar. Besides, focused vortex generation (topological charge l from 1 to 3) and multichannel detection (l from −2 to 2) are demonstrated with high efficiency, up to 79.2%. We envision that our devices of high flexibility may have potential applications in high-performance micron-scale integrated ultraviolet nanophotonics and meta-optics.

Polarization insensitive all-dielectric metasurfaces for the ultraviolet domain

In recent years, metasurfaces have provided a tempting path to replace conventional optical components where an abrupt phase change is imposed on an incident wave using a periodic array of unit cells. Till date, highly efficient dielectric metasurfaces have been demonstrated in infrared and visible domains. However, due to the lower bandgap of typical dielectric materials, such metasurfaces present strong absorption in the ultraviolet (UV) domain, and thus, hamper their realization at shorter wavelengths. In this paper, we utilize a large bandgap dielectric material, niobium pentoxide (Nb2O5), to construct an ultra-thin and compact transmission-type metasurface that manipulates the phase of an incident wave using an array of Nb2O5 nano-cylinder. By the virtue of numerical optimization, complete 2π phase coverage along with the high transmission efficiency (around 88.5%) is achieved at 355nm. Such efficient control over the phase of the incident wave enabled us to realize the polarisation insensitive self-accelerating parabolic, reciprocal, and logarithmic Airy beams (ABs) generating metasurfaces with the efficiency of 70%, 72% and 77%, respectively. In addition to this, we also demonstrate auto focusing Airy optical vortex (AFAOV) generators where the metasurfaces are designed to combine the phase profiles of an abruptly focusing Airy (AFA) beam and that of spiral phase plate (SPP). The AFAOV is generated with efficiency of 70% (for l = 3) and 72% (for l = 5).

Phase engineering with all-dielectric metasurfaces for focused-optical-vortex (FOV) beams with high cross-polarization efficiency

Metasurfaces, the two-dimensional (2D) metamaterials, facilitate the implementation of abrupt phase discontinuities using an array of ultrathin and subwavelength features. These metasurfaces are considered as one of the propitious candidates for realization and development of miniaturized, surface-confined, and flat optical devices. This is because of their unprecedented capabilities to engineer the wavefronts of electromagnetic waves in reflection or transmission mode. The transmission-type metasurfaces are indispensable as the majority of optical devices operate in transmission mode. Along with other innovative applications, previous research has shown that Optical-Vortex (OV) generators based on transmission-type plasmonic metasurfaces overcome the limitations imposed by conventional OV generators. However, significant ohmic losses and the strong dispersion hampered the performance and their integration with state-of-the-art technologies. Therefore, a high contrast all-dielectric metasurface provides a compact and versatile platform to realize the OV generation. The design of this type of metasurfaces relies on the concept of Pancharatnam-Berry (PB) phase aiming to achieve a complete 2π phase control of a spin-inverted transmitted wave. Here, in this paper, we present an ultrathin, highly efficient, all-dielectric metasurface comprising nano-structured silicon on a quartz substrate. With the help of a parameter-sweep optimization, a nanoscale spatial resolution is achieved with a cross-polarized transmission efficiency as high as 95.6% at an operational wavelength of 1.55 µm. Significantly high cross-polarized transmission efficiency has been achieved due to the excitation of electric quadrupole resonances with a very high magnitude. The highly efficient control over the phase has enabled a riveting optical phenomenon. Specifically, the phase profiles of two distinct optical devices, a lens and Spiral-Phase-Plate (SPP), can be merged together, thus producing a highly Focused-Optical-Vortex (FOV) with a maximum focusing efficiency of 75.3%.

Ultra-broadband microwave metasurfaces for polarizer and beam splitting

Realization of ultra-broadband transmission-type functional metasurfaces is of high priority for the development of modern electromagnetic systems. In this paper, we propose a new strategy to design highly efficient ultra-broadband microwave metasurfaces with ultra-thin profiles that enable linear polarization and asymmetric transmission by merging two differently sized meta-atoms operating in neighbour frequency bands into a meta-molecule featuring an extended bandwidth. Capitalizing on the meta-molecule approach, we realize a transmission-type beam splitter based on the coding concept, which can, at higher frequencies, divide a normally incident wave into two beams propagating along symmetrical oblique directions and, at lower frequencies, couple the incident wave into two surface waves propagating away from the splitter in opposite directions. Our findings demonstrate thereby a flexible and robust approach for the realization of efficient ultra-broadband transmission-type metasurfaces that can readily be integrated into complex systems in order to implement multiple functionalities in microwave communication networks for diverse applications.

Broadband high-efficiency cross-polarization conversion and multi-functional wavefront manipulation based on chiral structure metasurface for terahertz wave

In this paper, a broadband and high-efficient tri-layered chiral structure metasurface (CSM) for linear polarization conversion and multi-functional wavefront manipulation was proposed and investigated numerically in terahertz region. The unit-cell of the proposed CSM consist of two orthogonal metal wires sandwiched with square split-ring resonator (SRR) structure, which can manipulate the polarization and wavefront of the transmitted wave simultaneously. Based on the Fabry–Perot-like cavity-enhanced effect of proposed CSM, broadband and high-efficiency cross-polarization conversion can be achieved. Numerical simulation results indicate that the cross-polarization transmission coefficient is higher than 0.9 from 0.4 THz to 1.0 THz, with a fractional bandwidth of 85.7%. The proposed CSM can achieve complete 2π phase coverage by adjusting the geometric parameters of the unit-cell structure. Anomalous refraction with a wide angle, two kinds of focusing metalenses and vortex beam generation are investigated to verify the multi-functional wavefront manipulation performance of the proposed CSM. Our work provides an effective method of enhancing the performance of transmission-type metasurface and the proposed devices show endless potential in wavefront control and communication applications in terahertz region.

High-Transmission Ultrathin Huygens’ Metasurface with 360° Phase Control by Using Double-Layer Transmitarray Elements

Metasurfaces (two-dimensional metamaterials) have attracted much attention in recent years. However, due to the symmetric design of its cascaded elements, a traditional transmission-type metasurface must be at least three layers thick to achieve high transmission and 360\ifmmode^\circ\else\textdegree\fi{} phase control simultaneously. This study presents an ultrathin Huygens' metasurface, composed of double-layer antisymmetric transmitarray elements, that can realize high transmission and 360\ifmmode^\circ\else\textdegree\fi{} phase control at once. Based on this metasurface, a high-gain ultrathin (3.3% of the wavelength) transmitarray antenna is created, with an aperture efficiency of 61.04% at the working frequency.

Broadband terahertz vortex beam generation based on metasurface of double-split resonant rings

Terahertz vortex beam generators have potential applications in optical micro-manipulation, terahertz communications and many other fields. A broadband vortex beam generator in a terahertz frequency range is proposed based on the metasurface of double-split resonant rings’ array. The designed structure consists of two layers, i.e., the top layer, which is a metasurface of double-split resonant rings, and the bottom layer, which is the dielectric layer of polymide. The numerical simulation of the cell structure array is performed by using the CST microwave studio. In order to obtain the best performance, the structure parameters of metasurface are continuously optimized and a set of optimal geometric parameters is finally determined. The simulation results show that the circularly polarized incident light can be converted into corresponding cross-polarized transmitted light. By rotating the metal resonant ring on the top layer, the cross-polarized transmitted light can be controlled to have the same amplitude and correspondingly different phases. The relationship between the phase change and the angle of rotation conforms to the P-B phase principle. These cell structures are arranged according to a specific order and can form the vortex phase plates for generating the vortex beams with different topological charges. Taking the topological charge numbers 1 and 2 for example, two kinds of vortex phase plates are designed. The characteristics of the circularly cross-polarized vortex beams generated by a circularly polarized wave perpendicularly incident on the vortex phase plates are numerically analyzed. The results show that the ideal vortex beams with different topological charge numbers are generated. The characteristics of vortex beams appear to be consistent with those theoretical results. Moreover, the vortex beams can be generated in a frequency range from 1.39 THz to 1.91 THz. The operating bandwidth is much wider than the previously obtained result of the transmission terahertz vortex phase plates. The transmission is higher than 20%, and the maximum value of transmission can reach 24%, which is close to the theoretical limit value of the single-layered transmission-type metasurface. This work provides a reference for generating the terahertz vortex beams based on metasurface. It is expected to possess a practical application in generating the device of terahertz vortex beam.

Planar Efficient Metasurface for Vortex Beam Generating and Converging in Microwave Region

In this paper, single-layer transmission-type metasurfaces are proposed to generate a converged vortex beam and vortex beams with different topological charges. A new metasurface design is used to generate the vortex beam, in which the required accumulated phase difference along the propagation path is realized by designing and arranging artificial single-layer unit cells. Furthermore, the converged vortex beam is generated by metasurface that superimposes the phase distribution for the vortex beam and converged beam. The transmitted efficiency of the metalens can reach 20% as designed, which approaches to the efficiency limit of this type of metasurface.

Ultrathin Metasurface for Controlling Electromagnetic Wave With Broad Bandwidth

In this paper, ultrathin metasurface based on phase discontinuities is proposed to manipulate electromagnetic waves in the microwave region. The gradual phase changes accumulated along the transmission path in the conventional lens are substituted with abrupt phase changes at the interface of the metasurface, which are realized by the Pancharatnam-Berry phase element. An ultrathin metasurface with a linear phase gradient is designed, fabricated, and measured. The phenomenon of anomalous refraction can be observed in a broad bandwidth, in which the cross-polarization conversion efficiency keeps approaching the theoretical limit. The measured result shows that the anomalous refraction angle can be well predicted by the generalized Snell's laws, and the efficiency keeps >20% in a broad bandwidth of 24%. Our design makes a remarkable progress in the cross-polarization conversion efficiency of the transmission-type metasurface, which is beneficial to put metasurface into practical application.