Efficient Metasurface Research Articles

Prediction of multi-layer metasurface design using conditional deep convolutional generative adversarial networks

Designing metasurfaces is a challenging task. Traditional methodologies, which primarily depend on iterative procedures, are both time-intensive and require specialized expertise. The proposed algorithm uses conditional deep convolutional generative adversarial networks (cDCGAN) to design metasurfaces. This method instantly create a 2D image of a multi-layer metasurface using the scattering parameter S11 as the input vector. The algorithm significantly reduces the size of the training dataset by applying pre-training and post-generating steps. The pre-training step involves aliasing and modifying images using a limited color palette. The post-generating step consists of separating the color channels, converting the pixels to vector based images, and fine-tuning the borders. The algorithm is evaluated for three metasurfaces that have unique features compared to the training dataset samples: a single-band metasurface unitcell, a dual-band metasurface unitcell, and a partially trained sample improved by magnetic field analysis. The results show that the proposed algorithm can accurately predict the images of these metasurface unitcells, demonstrating its potential for fast and efficient metasurface design.

Broadband and efficient terahertz helicity inversion by a reconfigurable reflective metasurface based on vanadium dioxide

We experimentally demonstrate the broadband and highly efficient inversion of the rotation direction of the terahertz circular polarization achieved by a dynamic metasurface with vanadium dioxide (VO2). This metasurface converts linear polarization into circular polarization. The rotational direction of the output circular polarization can be reversed based on the VO2 phase transition. The VO2 state can be dynamically switched by injecting direct current into an on-chip circuit. To improve the terahertz-wave utilization efficiency, the metasurface is operated at an incident angle of 45°, thereby eliminating the beam splitter typically used to separate the outgoing wave from the incident wave at normal incidence. To achieve broadband operation, we employ a dispersion-cancelation strategy that cancels the dispersive phase responses between orthogonal linear polarizations. This strategy was previously developed for static metasurfaces; we have applied it to the design of dynamic metasurfaces. At a center frequency of 0.99 THz, the experimental evaluation demonstrated helicity switching with conversion efficiencies of 92% and 87% for the insulating and metallic states of VO2, respectively. Dispersion cancelation is simultaneously achieved for the insulating and metallic states of VO2, resulting in a relative bandwidth of 0.26, which is 3.2 times broader than that of a previously developed efficient dynamic metasurface.

Dispersive terahertz metasurface fed by a horn antenna for highly oriented 2D beam steering

Highly oriented beam steering will enhance power density and field of view (FOV) in terahertz wireless links. Metasurface can be constructed by deliberate arrangement of subwavelength meta-cells to manipulate the wavefront. This paper explores a dispersive metasurface with a specific phase gradient patterned in a 2-inch aperture, allowing for collimated beamforming and two-dimensional (2D) beam steering by a combination of frequency tuning and metasurface rotation. The metasurface is directly fed by a horn antenna, ensuring a compact integration. Simulation and experiment in the 80-110 GHz band revealed that the gain band crucial for FOV and efficiency is mainly constrained by the nonlinear phase dispersion of the meta-cells. Efforts to optimize the phase linearity resulted in a more efficient metasurface with a gain of 35.7 dBi and an efficiency of 76.6% at 400 GHz. A FOV of 22.5° in the elevation was guaranteed with gain in the 325-500 GHz band (a bandwidth of 42.4%). Imaging of two scattering balls was demonstrated at a distance of 4.1 meters by using the metasurface for 2D beam steering.

Bessel beams generation with biphase transition of vanadium dioxide metasurface

Bessel beams are highly attractive due to their non-diffraction properties, parallel processing capabilities, and large capacity. However, conventional methods for generating Bessel beams, such as using spatial light modulators, axicons, and diffraction optical elements, face limitations in terms of system complexity, bulkiness, low uniformity, and limited numerical aperture (NA). In this work, we exploited the phase change material vanadium dioxide (VO2) to generate both transmitted and reflected Bessel beams. Moreover, the self-healing property of Bessel beams was verified. Our results reveal that VO2 in the insulating state achieves a transmittance of 85% in the transmitting mode, while VO2 in the metallic state exhibits a reflection efficiency of 77% in the reflecting mode. This performance indicates the potential applications in efficient switchable metasurfaces.

Highly efficient multifunctional metasurface integrating lens, prism, and wave plate

The miniaturization of optical systems is crucial for various applications, including compact augmented reality/virtual reality devices, microelectromechanical system sensors, ranging technologies, and microfabricated atomic clocks. However, reliance on bulky discrete optical elements has been a significant obstacle to achieving this miniaturization. This work introduces a highly efficient multifunctional metasurface (MFMS) that seamlessly integrates a lens, prism, and quarter-wave plate (QWP). This innovation allows simultaneous collimation, beam deflection, and polarization conversion within a singular thin element. Specifically, for the prism-QWP bifunctional integration, we achieved a high diffraction efficiency of 72.8% and a degree of circular polarization of −0.955 under exposure to linearly polarized light at a wavelength of 795 nm, proving its potential for ultracompact atomic clock applications. Moreover, the lens-prism-QWP trifunctional integration successfully showed diffraction-limited focusing performance with a numerical aperture of 0.4, which was sufficient to collimate a beam with a divergence angle of 20∘, corresponding to the light emitted from a standard vertical-cavity surface-emitting laser.

Highly efficient nonlocal metasurface for transmission-type acoustic focusing

Highly efficient nonlocal metasurface for transmission-type acoustic focusing

All-dielectric achromatic hole-array metasurfaces

Metalens, an ultra-thin optical element composed of subwavelength structure, offers a new path for the development of miniaturized device imaging. Through the design of subwavelength units, arbitrary modulation of the phase, amplitude, and polarization of electromagnetic waves can be achieved. However, current metalenses generally suffer from the disadvantages of narrowband and Ohmic losses, which limits the development of high-quality imaging. In this work, using all-dielectric hole-array structure within the framework of effective medium theory, we demonstrate the achromatic focusing effect based on the basic Huygens principle. Moreover, by changing the position of the point source, this metalens can also achieve directional refraction with flexible angles. Our results provide a new solution for the practical application of miniaturized integrated high-quality imaging, and may open up avenues for constructing efficient artificial metasurface with metamaterials for energy harvesting and other wave manipulation.

Ultrafast modulable 2DEG Huygens metasurface

Huygens metasurfaces have demonstrated remarkable potential in perfect transmission and precise wavefront modulation through the synergistic integration of electric resonance and magnetic resonance. However, prevailing active or reconfigurable Huygens metasurfaces, based on all-optical systems, encounter formidable challenges associated with the intricate control of bulk dielectric using laser equipment and the presence of residual thermal effects, leading to limitations in continuous modulation speeds. Here, we present an ultrafast electrically driven terahertz Huygens metasurface that comprises an artificial microstructure layer featuring a two-dimensional electron gas (2DEG) provided by an AlGaN/GaN heterojunction, as well as a passive microstructure layer. Through precise manipulation of the carrier concentration within the 2DEG layer, we effectively govern the current distribution on the metasurfaces, inducing variations in electromagnetic resonance modes to modulate terahertz waves. This modulation mechanism achieves high efficiency and contrast for terahertz wave manipulation. Experimental investigations demonstrate continuous modulation capabilities of up to 6 GHz, a modulation efficiency of 90%, a transmission of 91%, and a remarkable relative operating bandwidth of 55.5%. These significant advancements substantially enhance the performance of terahertz metasurface modulators. Importantly, our work not only enables efficient amplitude modulation but also introduces an approach for the development of high-speed and efficient intelligent transmissive metasurfaces.

High-efficiency terahertz wavefront manipulation based on optimized catenary metasurface

Catenary optics, as a new development object in the field of subwavelength optics, has been paid close attention and studied by scientists since it was proposed. As a two-dimensional planar structure, metasurface can realize the free control of electromagnetic wave in space. Traditional metasurface structures are often designed with discrete unit structures, which process is complicated and prone to error. In this paper, we introduce the catenary structure and use the principle of continuous geometric phase and the all-dielectric material silicon to design and optimize the catenary structure at 2 THz frequency, thereby realizing efficient transmission functional metasurfaces such as abnormal deflectors and focusing lenses that can generate Bessel beams. By optimizing the unit of catenary structure, the generation of parasitic transmission phase can be effectively avoided, and the working efficiency of the metasurfaces can be effectively improved. It is believed that the design in our work may pave the way for wavefront manipulation of high-performance terahertz waves and has potential for broadband terahertz devices.

Polarization conversion filter based on electromagnetically induced transparency-like effect

Owing to the large losses in the conversion process of traditional polarization converters, there is an increasing demand for metasurfaces with excellent transmission performance. In this work, an efficient polarization conversion metasurface is proposed based on electromagnetically induced transparency-like (EIT-like) effect in the terahertz band. The multi-level bright mode paths are excited by an asymmetric structure to obtain orthogonal circular polarization conversion windows. The transmission window is generated by the mutual interference of two sets of bright modes with similar resonant frequencies. Then an asymmetric structure is constructed to achieve transmission window shift under TE polarization and TM polarization, thereby realizing dual-frequency polarization conversion. The metamaterial unit structure consists of four open metal resonant rings and four metal resonant strips. The working mechanism is explored by analyzing the surface current distribution, frequency response, and incident angle characteristics. The results show that electromagnetically induced transparency can be achieved under different polarizations. Furthermore, based on the EIT resonance between the two incident polarizations, the conversion from linear polarization to right-hand circular polarization is achieved at 0.692 THz, and the conversion from linear polarization to left-hand circular polarization is realized at 0.782 THz, transmission coefficients are 0.7 and 0.68 respectively. According to the Stokes parameters, the corresponding ellipticity <i>η</i> values are 96% and 98%, respectively. This EIT-based polarization conversion metasurface with low loss and ultra-thin characteristics has great potential applications in compact antennas, derived radar phased arrays, and military detectors.

Efficient dual-wavelength metasurface for second-order differential edge detection in the ultraviolet.

Metasurfaces for edge detection through spatial analog calculations have attracted much attention due to advantages such as a flexible design and small footprint. Up until now, most studies have focused on single-wavelength operation in the near-infrared or visible regions, while little work has been done in the ultraviolet band. It is of significance to explore metasurfaces for edge detection in the ultraviolet band for their great potential in high-resolution imaging and lithography. Here, we propose a dual-wavelength H f O 2 metasurface for edge detection working at 273nm and 293nm, with 25% and 72% efficiency, respectively, controlled by the linear polarization of the incident light. The efficient dual-wavelength second-order differential calculation in the ultraviolet band of the metasurface has been confirmed by 1D signal and 2D image processing. It may find applications in the fields of computer vision and bioimaging.

Near-field enhancement by waveguide-plasmon polaritons in a nonlocal metasurface

Localized surface plasmons in metal nanoparticles are widely used in nano-optics to confine and enhance optical fields. It has been previously shown that, if the nanoparticles are distributed periodically, an additional enhancement can be achieved by coupling the localized surface plasmons to the diffraction orders of the lattice, forming surface lattice resonances. In this work, we study an even further improvement of the near-field enhancement by placing a metal-dielectric slab waveguide beneath the lattice of the particles to excite coupled waveguide-plasmon polaritons. These excitations can extend over many periods of the lattice, making the metasurface highly nonlocal. We numerically demonstrate that the approach can provide a significant extra increase in the near-field intensity—by a factor of 80 over that produced by a single-particle plasmon resonance and by 7 over the lattice-resonance enhancement. The described enhancement mechanism can be used to design extraordinarily efficient nonlocal optical metasurfaces for many applications, including surface-enhanced Raman spectroscopy, fluorescence spectroscopy, nonlinear optics, and solar energy harvesting.

Highly efficient broadband spin-multiplexed metadevices for futuristic imaging applications

Achieving broadband and multifunctional response through a single optical device is a highly sought-after capability for various applications like microscopy and optical imaging, which provide detailed morphological information of a targeted object. Traditional optical systems require a chain of elements to achieve such functionality, resulting in bulky systems hindering integration with modern photonics devices. Metasurfaces are exquisitely designed and exhibit unprecedented light-manipulation capability at the nanoscale, thus creating easy-to-integrate miniaturized photonic devices. Recently, broadband multifunctional metasurfaces with polarization’s degree of freedom have attracted considerable interest due to their exceptional ability of wavefront shaping, which may find their applications in imaging and data communication. However, realizing single-cell driven, highly efficient multifunctional metasurfaces operating over broadband wavelengths remains challenging. This study presents a unique helicity-dependent broadband multifunctional metasurface platform to manipulate visible light for futuristic imaging applications. The proposed platform can integrate multiple optical phenomena into a single-layered metadevice to generate several uncorrelated, spin-dependent responses across the targeted visible spectrum (470–650 nm). For proof of concept, we demonstrated several all-dielectric transmissive metadevices carrying various spin-multiplexed phase profiles, efficiently generating diffraction-limited focusing and structured light beams with different characteristics. Each designed metasurface was studied under selected visible wavelengths, and the diffracted light exhibited excellent and stable broadband performance. With the advantages of broadband multifunctional platform and miniaturized metadevices, the presented strategy may open new vistas for various applications, including biomedical imaging, optical interconnects, and microscopy.

Highly efficient flexible structured metasurface by roll-to-roll printing for diurnal radiative cooling

An ideal radiative cooler requires accurate spectral control capability to achieve efficient thermal emission in the atmospheric transparency window (8–13 μm), low solar absorption, good stability, scalability, and a simple structure for effective diurnal radiative cooling. Flexible cooling films made from polymer relying on polymer intrinsic absorbance represent a cost-effective solution but lack accuracy in spectral control. Here, we propose and demonstrate a metasurface concept enabled by periodically arranged three-dimensional (3D) trench-like structures in a thin layer of polymer for high-performance radiative cooling. The structured polymer metasurface radiative cooler is manufactured by a roll-to-roll printing method. It exhibits superior spectral breadth and selectivity, which offers outstanding omnidirectional absorption/emission (96.1%) in the atmospheric transparency window, low solar absorption (4.8%), and high stability. Impressive cooling power of 129.8 W m−2 and temperature deduction of 7 °C on a clear sky midday have been achieved, promising broad practical applications in energy saving and passive heat dispersion fields.

Si-Based Polarizer and 1-Bit Phase-Controlled Non-Polarizing Beam Splitter-Based Integrated Metasurface for Extended Shortwave Infrared.

Metasurfaces, composed of micro-nano-structured planar materials, offer highly tunable control over incident light and find significant applications in imaging, navigation, and sensing. However, highly efficient polarization devices are scarce for the extended shortwave infrared (ESWIR) range (1.7~2.5 μm). This paper proposes and demonstrates a highly efficient all-dielectric diatomic metasurface composed of single-crystalline Si nanocylinders and nanocubes on SiO2. This metasurface can serve as a nanoscale linear polarizer for generating polarization-angle-controllable linearly polarized light. At the wavelength of 2172 nm, the maximum transmission efficiency, extinction ratio, and linear polarization degree can reach 93.43%, 45.06 dB, and 0.9973, respectively. Moreover, a nonpolarizing beam splitter (NPBS) was designed and deduced theoretically based on this polarizer, which can achieve a splitting angle of ±13.18° and a phase difference of π. This beam splitter can be equivalently represented as an integration of a linear polarizer with controllable polarization angles and an NPBS with one-bit phase modulation. It is envisaged that through further design optimization, the phase tuning range of the metasurface can be expanded, allowing for the extension of the operational wavelength into the mid-wave infrared range, and the splitting angle is adjustable. Moreover, it can be utilized for integrated polarization detectors and be a potential application for optical digital encoding metasurfaces.

Efficiently spatial field localization enabled second-harmonic and sum-frequency generation in an etchless LiNbO3 layer by guided resonant quasi-bound states in the continuum

High-Q resonances on metasurfaces have yielded a feasible approach for nonlinear frequency conversion in recent years. In this work, we propose a highly efficient nonlinear metasurface based on an etchless lithium niobate (LiNbO3) layer by combining periodic silicon nanodisks. These nanodisks introduce two kinds of capabilities for achieving ultra-sharp spectral lineshapes owing to the high-Q resonances of bound states in the continuum and simultaneously attain a significantly enhanced spatial field localization within the LiNbO3 cavity owing to guided resonant behaviors. This distinctive mode engineering yielded a total efficiency of conversion of 2.8% in the second-harmonic generation and 1% for sum-frequency generation when the peak intensity of the fundamental pump was as low as 1 MW/cm2. Our method eliminates challenges to the manufacture of etched LiNbO3 thin films and allows for the efficient conversion of frequency. It is suitable for various applications, such as integrated, nonlinear nanophotonic chips, and high-speed communication.

Highly efficient second harmonic generation assisted by the quasi-bound states in the continuum from AlGaAs meta-gratings

Due to the design freedom of metasurfaces in tailoring wavefronts, they have become a general platform for miniaturized functional nonlinear optics. Recently, with the help of the quasi-bound states in the continuum (Q-BIC), the all-dielectric resonance metasurfaces with high quality factor (Q) boost the interaction between pump light and dielectric structure, which have become an effective way to achieve high nonlinear efficiency. Here, an asymmetric grating structure composed of AlGaAs is utilized to enhance the second harmonic generation (SHG) near Q-BIC. By breaking the symmetry of the grating structure, a simple normal incidence linearly polarized plane wave can be treated to excite stronger localized fields within the metasurfaces, allowing strong light–matter​ interactions in nonlinear metasurfaces. Numerical investigations depict that the Q-factor and resonant field enhancement noticed on the metasurface are intimately related to the asymmetric structure of the system. In addition, the high Q-factor resonance facilitates the SHG conversion process in AlGaAs metasurfaces. Especially, for the AlGaAs metasurface with Q-factor of 4.0314 × 104, the conversion efficiency of SHG can reach 10.5% when the pumping intensity is 5 MW/cm 2. This study is of great importance for analyzing the efficient nonlinear metasurfaces with high Q-factors.

High-performance polarization-controlled perfect optical vortex generation via all-dielectric metasurfaces in the visible region

Metasurfaces, with superiority in integration and flexibility compared to conventional bulky optical elements, open a perspective in generating perfect optical vortex (POV). However, the generation of POV beams with simultaneous high-efficiency, multi-functionality, and broadband is still challenging. Here, we design POV generators based on highly efficient all-dielectric metasurfaces and produce POV beams in simulation in the visible region with maximum generation efficiency of 74%. Such high efficiency originates from the stimulation of the resonance modes of polarization-dependent parity in the well-designed unit cell. By superposing two sets of phase profiles, polarization-controlled POV generators are obtained. Furthermore, the POV generator exhibits broadband property due to the geometric phase independent of the incident wavelengths. Our work provides an effective approach for generating controllable POV beams, promoting the construction of integrated and multi-functional optical systems.

Efficient dynamic tunable metasurface based onGe2Sb2Te5 in the near infrared band.

For effective wavefront management in the optical infrared range, dynamic all-dielectric metasurfaces, always based on phase transition materials, particularly G e 2 S b 2 T e 5 (GST), can be used. In this paper, we propose a GST-based tunable metasurface by structuring the phase-change material GST. We confirm that the nanopillar we designed has high transmittance in the wavelength band around 1550nm and can fully cover the 0∼2π phase. Based on these characteristics, we can achieve beam steering and a focusing effect in amorphous phase by elaborately arranging GST nanopillars, while the aforementioned optical phenomena disappear in crystalline phase. Additionally, by arranging the array of vortex phases, we also realize switching the perfect composite vortex beam (PCVB) when changing the crystal state of GST, and simulate the generation of PCVB with different topological charges and sizes in amorphous phase. We believe that our research results can serve as a reference for multifunctional optical surfaces, dynamic optical control, optical communication, and information processing.

Dispersion study of a broadband terahertz focusing reflecting metasurface for 6G wireless communication

In 6G wireless communications, functional terahertz reflecting metasurfaces are expected to play increasingly important roles such as beamforming and beamsteering. However, their performance optimization as a function of multiple competing design parameters remains a significant challenge. This paper demonstrates a multi-dimensionally optimized design of a functional and efficient focusing metasurface reflector in the burgeoning D-band (0.11-0.17 THz). In addition to achieving 20% fractional bandwidth (0.135-0.165 THz), this design is polarization-maintaining, diffraction limited, simple in design, and exhibits 64.1% broadband power efficiency (1.9 dB insertion loss). We perform a unique study of the magnitude of temporal (group-delay) dispersion introduced by this design to the broadband signal at the focus. Despite being formed by an array of highly dispersive resonators, the metasurface exhibits very low dispersion, which avoids pulse reshaping and its consequent limitations on achievable data rate. The low dispersion characteristic of the metasurface is a consequence of its optimized design. The design and performance of the focusing reflector are presented followed by a dispersion analysis revealing that only 2.83% temporal broadening of the pulse is observed at the focus.