Bilayer Metasurface Research Articles

Bilayer Metasurface With High-Q Terahertz Chiroptical Response Empowered by Intrinsic Quasi-Multi-Bound States in the Continuum

Bilayer Metasurface With High-Q Terahertz Chiroptical Response Empowered by Intrinsic Quasi-Multi-Bound States in the Continuum

MEMS-tunable topological bilayer metasurfaces for reconfigurable dual-state phase control

Tunable optical metasurfaces (MSs) have demonstrated exceptional capabilities in actively manipulating light fields. However, most existing tunable MSs are limited to controlling only one functionality. Here, by combining a MEMS mirror with a plasmonic bilayer MS (BMS), we develop an electrically driven MEMS-BMS platform enabling complete reflection phase transformation and switching between two encoded functionalities by actuating the MEMS mirror. This capability stems from different optical responses of each MS layer at distinct MEMS-BMS separations, due to evolving topological singularities in a defined parameter space. With this tunable topological MEMS-BMS platform, we demonstrate polarization-independent MEMS-BMS for reconfigurable diffraction gratings, achieving ∼25% efficiency, ∼0.75 contrast at 850-nm wavelength, and fast response (∼5µs). The MEMS-BMS arrangement for generating vortex beams with switchable topological charges of ±1 is also demonstrated, evidenced by distinct near- and far-field interferograms. Our work expands the scope of tunable MSs by exploiting dynamic topological phases in the MEMS-BMS arrangement, paving the way for multifunctional tunable meta-optics.

Bidirectional Vectorial Holography Using Bi-Layer Metasurfaces and Its Application to Optical Encryption.

The field of optical systems with asymmetric responses has grown significantly due to their various potential applications. Janus metasurfaces are noteworthy for their ability to control light asymmetrically at the pixel level within thin films. However, previous demonstrations are restricted to the partial control of asymmetric transmission for a limited set of input polarizations, focusing primarily on scalar functionalities. Here, optical bi-layer metasurfaces that achieve a fully generalized form of asymmetric transmission for any input polarization are presented. The designs owe much to the theoretical model of asymmetric transmission in reciprocal systems, which elucidates the relationship between front- and back-side Jones matrices in general cases. This model reveals a fundamental correlation between the polarization-direction channels of opposing sides. To circumvent this constraint, partitioning the transmission space is utilizedto realize four distinct vector functionalities within the target volume. As a proof of concept, polarization-direction-multiplexed Janus vectorial holograms generating four vectorial holographic images are experimentally demonstrated. When integrated with computational vector polarizer arrays, this approach enables optical encryption with a high level of obscurity. The proposedmathematical framework and novel material systems for generalized asymmetric transmission may pave the way for applications such as optical computation, sensing, and imaging.

High dispersion and bistability of the light transmission through a bilayer metasurface with resonant plasmonic elements

The optical response of the bilayer metasurface consisting of thin gold nanobars featuring localized plasmon resonance was considered. Detailed theoretical and numerical analysis demonstrates that a sharp narrow dip in the transmittance spectrum of the metasurface can be observed when the layer-to-layer coupling is taken into account. Here gold nanobars, supporting localized plasmon modes, act as open resonators giving rise to the interaction of the unit-cell elements in adjacent layers. Through a careful choice of the geometrical parameters of the metasurface, one can achieve resonance in the near-infrared spectrum with a linewidth as narrow as 4 nm, and resonant values of the transmittance and reflectance of 0.06 and 0.46, respectively. The resonance accompanied by a noticeable increase in the wave field intensity in the interlayer area creates favorable conditions to boost nonlinear processes thus enabling all-optical light control and optical bistability.

Full-space metasurfaces for independent manipulation of transmission and reflection.

In recent years, beam manipulation using metasurfaces has evolved from being limited to either a transmission or reflection space to encompassing a full space. However, existing methods still inevitably require complex systems and are unable to achieve continuous and arbitrary phase manipulation. Here, one type of a bilayer metasurface is proposed to simultaneously manipulate reflection and transmission phases continuously and independently, which also makes the optical system more compact without requiring any analyzers and enhances the degree of freedom for full-space beam manipulation. As a proof-of-concept demonstration, one device is designed to show different holograms in transmission and reflection spaces. Additionally, the Dammann grating designed in the reflection hologram increases the information capacity. The proposed method may pave the way toward achieving a variety of applications such as multi-channel beam manipulation and multifunctional optical devices.

Light Field Transformation of Metasurface Based on Arbitrary Jones Matrix

AbstractMetasurfaces are ultra‐thin micro‐nano elements, have abundant freedoms, and are widely used in light field transformation. The Jones matrix of single‐layer birefringent metasurfaces have at most 6 degrees of freedom, which limits the richness and diversity of light field transformation. Bilayer metasurfaces have more design channels and functionality than single‐layer metasurfaces. By mathematical derivation, the decomposition method of the bilayer metasurface unitary Jones matrix is provided. Further, inspired by double‐phase holography, arbitrary complex Jones matrix is decomposed and realized by bilayer metasurface and achieves analytical decoupling of all 4 complex components of the Jones matrix. As the theoretical verification, spin‐orbit angular momentum mapping and four‐channel complex holography are realized by the bilayer metasurface. The decomposition method is completely analytical, universal, and easy to implement. It is of great significance for the transformation of light field, information transmission, and optical encryption based on metasurfaces.

Terahertz Sensing Based on Floating Bilayer Metasurface with Toroidal Dipole Resonance Toward Ultra‐High Sensitivity

AbstractMetasurface structures have proven to be effective in enhancing terahertz sensing signals and can thus be used as sensors to improve terahertz detection sensitivity. However, the sensitivity is limited by the poor spatial overlap between the analytes and the local electric field of the metasurface. In this work, a novel design of a floating bilayer metasurface structure for terahertz sensing is proposed and investigated. This structure supports a sharp toroidal dipole resonance and can concentrate near‐field energy on the analyte and metal atoms rather than on the substrate surface by floating the metal atoms. Consequently, the sensitivity is significantly improved to as high as 362 GHz RIU−1; theoretically, this is approximately 2.6 times higher than that of the common metasurface. The ability of the floating bilayer metasurface to quantitatively detect chlorothalonil is experimentally demonstrated. The resonance peak shows a significant frequency shift of 7 GHz for a change of 0.0001 mg dL−1 in chlorothalonil concentration, reaching up to 86 GHz when the change in chlorothalonil concentration is 100 mg dL−1; this is approximately 6.6 times higher than that of the common metasurface. This work provides opportunities for metasurface to realize ultrasensitive sensing in the terahertz regime.

Highly dispersive transmission conditions for a conductive rods-based ultrathin bilayer metastructure

We experimentally demonstrate that the transmission of microwave electromagnetic fields through a bilayer metasurface (BMS) composed of thin conductive rods can abruptly change in a narrow frequency range. A theoretical analysis based on the coupled oscillator model is performed to reveal the physical mechanism behind the frequency-dependent properties of such a structure. Two conditions primarily facilitate the observed high dispersion in the BMS. The first one is the resonant interaction between the incident microwaves and rods, leading to the formation of surface standing waves. These waves with radial electric fields enable the coupling of the near-field of rods in structural layers. The second condition is the complex value of the coupling coefficient between rods of different owing to the delayed interaction process between them. The electromagnetic response here can be effectively controlled by varying the distance between layers and the dielectric properties of the interlayer medium.

WITHDRAWN: Cross polarization converter with high transmittance and near unity polarization conversion efficiency for visible light based on bilayer all-dielectric metasurface

It is highly desired to achieve high efficiency polarization converters in visible wavelength region for ultra-compact optical systems which have shown potential applications in optical sensing, communication and imaging. Due to the intrinsic property of low loss, all dielectric metasurfaces are more competitive candidates to be utilized for fabrication of polarization converters than the counterparts based on metals. In this paper, we design a bilayer all dielectric metasurface polarization converter composed of a Si double-rod resonator placed on the top of a glass block with a polymer substrate for linearly polarized visible light. The proposed bilayer metasurface structure can convert linearly polarized light to the cross-polarized light efficiently within the visible region from 542 nm to 563 nm in the transmission mode with high transmittance and near unity polarization conversion efficiency. The transmittance is larger than 0.8 in the mentioned wavelength range with a maximum transmission of 0.86 at 556 nm and polarization conversion ratio is larger than 90% in the same wavelength range with a maximum polarization conversion ratio of 99.01% at 556 nm. The underline physical mechanisms of high conversion efficiency and high transmittance are analyzed in detail with simulated distributions of surface current, electrical field distributions and the calculated multipole decompositions of the scattering power.

Rotational photonic spin Hall effect on twisted bilayer metasurfaces

Optical differential operations possess the remarkable advantages of high efficiency and real-time imaging, which can improve the ability of image processing to enhance image edge detection. The photonic spin Hall effect has been demonstrated to realize optical differential operations. However, the direction of optical differential operation generated by the conventional photonic spin Hall effect is single and fixed. Here, we propose a scheme for the rotational photonic spin Hall effect on twisted bilayer metasurfaces to obtain the optical differential operations in any desirable directions and phase reconstruction. The rotation angle of the rotational photonic spin Hall effect is defined as the tangent of the in-plane spin-dependent shift to the transverse spin-dependent shift. The twist angle of the bilayer metasurfaces influence the Fresnel reflection coefficients which affect the transverse spin-dependent shift and the in-plane spin-dependent shift. Therefore, the direction of rotational photonic spin Hall effect can be controlled by the twist angle of the bilayer metasurfaces. Moreover, we show applications of the rotational photonic spin Hall effect including image edge detection and phase reconstruction. These findings open up new horizons for future applications of the rotational photonic spin Hall effect in optics.

Do dielectric bilayer metasurfaces behave as a stack of decoupled single-layer metasurfaces?

Flat optics or metasurfaces have opened new frontiers in wavefront shaping and its applications. Polarization optics is one prominent area which has greatly benefited from the shape-birefringence of metasurfaces. However, flat optics comprising a single layer of meta-atoms can only perform a subset of polarization transformations, constrained by a symmetric Jones matrix. This limitation can be tackled using metasurfaces composed of bilayer meta-atoms but exhausting all possible combinations of geometries to build a bilayer metasurface library is a very daunting task. Consequently, bilayer metasurfaces have been widely treated as a cascade (product) of two decoupled single-layer metasurfaces. Here, we test the validity of this assumption for dielectric metasurfaces by considering a metasurface made of titanium dioxide on fused silica substrate at a design wavelength of 532 nm. We explore regions in the design space where the coupling between the top and bottom layers can be neglected, i.e., producing a far-field response which approximates that of two decoupled single-layer metasurfaces. We complement this picture with the near-field analysis to explore the underlying physics in regions where both layers are strongly coupled. We also show the generality of our analysis by applying it to silicon metasurfaces at telecom wavelengths. Our unified approach allows the designer to efficiently build a multi-layer dielectric metasurface, either in transmission or reflection, by only running one full-wave simulation for a single-layer metasurface.

Acoustic Bilayer Gradient Metasurfaces for Perfect and Asymmetric Beam Splitting

We experimentally and theoretically present a paradigm for the accurate bilayer design of gradient metasurfaces for wave beam manipulation, producing an extremely asymmetric splitting effect by simply tailoring the interlayer size. This concept arises from anomalous diffraction in phase gradient metasurfaces and the precise combination of the phase gradient in bilayer metasurfaces. Ensured by different diffraction routes in momentum space for incident beams from opposite directions, extremely asymmetric acoustic beam splitting can be generated in a robust way, as demonstrated in experiments through a designed bilayer system. Our work provides a novel approach and feasible platform for designing tunable devices to control wave propagation.

Phase Change Materials‐Based Bilayer Metasurfaces for Near‐Infrared Photonic Routing

AbstractPhotonic routing holds immense significance in the fields of photonic integrated circuits (PICs) for free‐space optical linking. The emergence of programmable metasurfaces with powerful phase regulation capability provides a promising strategy for achieving designated wavefront modulation and thus photonic routing in the optical communication waveband. Here, a bilayer metasurfaces with four‐level codable unit pixels, which realize controllable photonic routing in near‐infrared (NIR) region is reported. The unit pixel of the metasurfaces consists of the underneath amorphous silicon (α‐Si) nanofin and the upper Ge2Sb2Te5 (GST) nano‐antenna, which contribute to the cross‐polarization generation and amplitude modulation of cross‐polarization light, respectively. Each unit pixel can be designated with desired states, thus achieving on‐demand non‐volatile photonic routing for coherent NIR light. In particular, polarization conversion modulation of the tunable metasurfaces comprising identical unit cell with large cross‐polarization switching (≈55%) at ≈1550 nm is demonstrated. Moreover, the metasurfaces with coded unit pixels to realize designated photonic routing are prototyped. The metasurfaces with four‐level programmability portend a new paradigm in NIR photonic routing for PICs.

Full-space and multi-polarization holograms realized by a non-Hermitian bilayer metasurface.

Multilayer metasurfaces break the mirror symmetry along the path of light propagation, thereby increasing the potential for light manipulation. Herein, a paradigm is proposed that building a non-Hermitian bilayer metasurface, which is composed of two identical, orthogonally oriented, chiral J-shaped Au structures in each layer, allows exceptional points (EPs) to exist in full-space. Specifically, in the reflected half-space that adheres to mirror symmetry, the circularly polarized eigenstates coalesce at the EP, while in the transmission half-space, where mirror symmetry is broken, the linearly polarized eigenstates converge at the EP. By considering the intrinsic property of topologically protected 2π-phase accumulation encircling both EPs, we investigated full-space holography through using circularly polarized light (in reflection half-space) and linearly polarized light (in transmission half-space).

Full-space wavefront control enabled by a bilayer metasurface sandwiching 1D photonic crystal.

Metasurfaces, composed of sub-wavelength structures, have a powerful capability to manipulate light propagations. However, metasurfaces usually work either in pure reflection mode or pure transmission mode. Achieving full-space manipulation of light at will in the optical region is still challenging. Here we propose a design method of full-space meta-device containing a bilayer metasurface sandwiching 1D photonic crystal to manipulate the transmitted and reflected wave independently. To provide a proof-of-concept demonstration, a device is proposed to show the light focusing in transmission and a vortex beam in reflection. Meanwhile, a device focusing the reflected light with oblique 45° incidence and the transmitted light with normal incidence is designed to indicate its application potential in augmented reality (AR) application. Our design provides a promising way to enrich the multifunctional meta-devices for potential applications.

Broadband large-angle beam scanning with dynamic spin energy distribution based on liquid crystal cascaded bilayer metasurface.

Dynamic manipulation of terahertz (THz) beams plays an important role in THz application systems. The PB metasurface provides an effective scheme for space separation and deflection of the spin beam. However, mirror symmetry locking of the conjugated spin states severely limits the versatility of the device. In this work, we demonstrate a liquid crystal (LC) cascaded bilayer metasurface that includes an LC layer, anisotropic metasurface, and PB metasurface. By controlling anisotropy and polarization conversion effects, dynamic spin asymmetric transmission is realized. Meanwhile, two different dynamic energy distribution processes are realized between the L and R state with the corresponding deflection side. The results show that the device achieves a large angular spatial dispersion within the frequency-angle scanning range of ±35° to ±75° corresponding to the broadband range of 0.6-1.1 THz. Moreover, it achieves a spin beam spatial separation with a maximum proportion of energy distribution greater than 26 dB, and the active modulation rate in the energy distribution process reaches 98 %. This work provides a dynamic THz spin conversion and efficient large-angle beam scanning, with important potentials in wavelength/polarization division multiplexing and frequency-scanning antenna for large-capacity THz wireless communication, radar, and imaging systems.

Scattering exceptional point in the visible

Exceptional point (EP) is a special degeneracy of non-Hermitian systems. One-dimensional transmission systems operating at EPs are widely studied and applied to chiral conversion and sensing. Lately, two-dimensional systems at EPs have been exploited for their exotic scattering features, yet so far been limited to only the non-visible waveband. Here, we report a universal paradigm for achieving a high-efficiency EP in the visible by leveraging interlayer loss to accurately control the interplay between the lossy structure and scattering lightwaves. A bilayer framework is demonstrated to reflect back the incident light from the left side ( | r−1 | >0.999) and absorb the incident light from the right side ( | r+1 | < 10–4). As a proof of concept, a bilayer metasurface is demonstrated to reflect and absorb the incident light with experimental efficiencies of 88% and 85%, respectively, at 532 nm. Our results open the way for a new class of nanoscale devices and power up new opportunities for EP physics.

Dynamically Metasurface-Modulated Electrochemiluminescence Polarization Coupling Angle Strategy for miR-142-3p Detection.

The combination of the electrochemiluminescence (ECL) technique with nanophotonics research can spark new analytical and sensing applications. Herein, we developed a novel modulation strategy of the ECL polarization angle based on the dynamically tunable few-layer metasurface. The bilayer metasurface consisted of a fixed Au-Ag core-shell nanocube array (Au@Ag NCA) layer with strong plasmonic hot spots and different amounts of the Au nanoparticles@MoS2 heterostructure nanosheet (0D-2D HNS) layer with strong metal-support interaction. Due to the interference and near-field coupling between layers, the bilayer metasurface can strongly redistribute the local electromagnetic field and energy in the ECL system, which not only significantly amplified the ECL signal but also modulated the polarization coupling angle. Therefore, the novel ECL polarization angle-resolved sensing strategy has been developed, which was beneficial to improve the sensitivity and resolution of ECL sensing. A dynamically tunable metasurface-based ECL biosensor was successfully used to detect the asthma-related miRNA-142-3p (miR-142-3p). Moreover, the simulation calculations of the electromagnetic field revealed the unique optical activity of the metasurface. This study brought the insightful understanding of the metasurface-modulated optical signal and provided a new idea to construct novel sensing platforms.

A Mid-Infrared Multifunctional Optical Device Based on Fiber Integrated Metasurfaces.

A metasurface is a two-dimensional structure with a subwavelength thickness that can be used to control electromagnetic waves. The integration of optical fibers and metasurfaces has received much attention in recent years. This integrated device has high flexibility and versatility. We propose an optical device based on fiber-integrated metasurfaces in the mid-infrared, which uses a hollow core anti-resonant fiber (HC-ARF) to confine light transmission in an air core. The integrated bilayer metasurfaces at the fiber end face can achieve transmissive modulation of the optical field emitted from the HC-ARF, and the Fano resonance excited by the metasurface can also be used to achieve refractive index (RI) sensing with high sensitivity and high figure of merit (FOM) in the mid-infrared band. In addition, we introduce a polydimethylsiloxane (PDMS) layer between the two metasurfaces; thus, we can achieve tunable function through temperature. This provides an integrated fiber multifunctional optical device in the mid-infrared band, which is expected to play an important role in the fields of high-power mid-infrared lasers, mid-infrared laser biomedicine, and gas trace detection.

A bi-Layered chiral metasurface as a multi-functional polarization convertor with high incidence angle stability

This paper introduces a polarization converter with multiple functions constructed using a bi-layered chiral metasurface. The chiral metasurface has double-modified metal layers on a dielectric substrate. The results authenticate the superior efficiency of the structure in achieving both circular birefringence and circular dichroism characteristics. The chiral metasurface can convert y-(x-) polarized electromagnetic lights to circularly polarized waves at 7.18 GHz and 8.85 GHz. When the x-(y-) polarized wave interacts with chiral metamaterial in –z(+z) orientations, it results in a transmitted wave of y-(x-) polarized light (15–17 GHz). The extracted significant polarization conversion ratio above 0.9 (15–17 GHz) and the considerable polarization extinction ratio (PER) over 40 dB (7.18 GHz) and -22dB (8.85 GHz) confirm the considerable potential of the offered chiral metasurface in acquiring the transmitted waves with approximately pure circular and linear polarization. The proposed structure has reliable asymmetric transmission properties and can function as a stable polarizer in the microwave spectrum.