Dielectric Metasurfaces Research Articles

Optimizing Metasurface-Component Performance by Improving Transmittance and Phase Match of the Nanopillars.

In the propagation phase of a dielectric metasurface, there are two important problems. Firstly, the range of transmittance of the nanopillars for a building metasurface is usually between 60% and 100%, which reduces the metasurface's overall transmittance and affects the uniformity of the transmitted light. Secondly, the realistic phase provided by the nanopillar cannot be matched very well with the theoretical phase at each lattice location.The phase difference (between a realistic phase and theoretical phase) may reach tens of degrees. Here, we propose an interesting method to solve these problems. With this new method, a metalens is designed in this paper. The nanopillars for building the metalens have transmittance over 0.95, which increases the metalens transmittance and improves the light uniformity. In addition, with the new method, the phase differences of all elements in the metalens can also be reduced to be below 0.05°, decreasing the metalens spherical aberration dramatically. This method not only helps us to optimize the metalens but also provides a useful way for designing high-quality metasurfaces.

Enhancing light emission near low-refractive-index contrast nanostructures via mirror-symmetry breaking

A platform of recent interest and large application potential is one in which the light emitters are directly integrated with an optical metasurface. Plasmonic metals and high-refractive-index, dielectric Mie metasurfaces have been explored in this connection but have their challenges. We propose low-refractive-index, contrast nanostructured thin films for studying metasurface-emitter systems, specifically focusing on two configurations of practical importance: LED white light generation using color converting polymers and fluorescence-based sensing in aqueous media. To achieve light emission enhancement in a low-index contrast, low optical absorption setting, we exploit the excitation of quasi-bound states in the continuum modes in a mirror-symmetry broken grating. Our numerical study predicts widely tuneable sharp-linewidth emission enhancements, near-zero quenching, and large and controllable active volume in the grating vicinity, which are significant improvements in comparison with both plasmonic and high-index contrast, all-dielectric platforms. When compared with simple gratings, mirror-symmetry broken gratings give four times larger radiative enhancement. Our results are of interest in furthering experimental activity and realizing applications such as light converter for efficient white LEDs and smart detection electronics-integrated substrates for sensing.

Engineering Electromagnetic Field Distribution and Resonance Quality Factor Using Slotted Quasi-BIC Metasurfaces.

Dielectric metasurfaces governed by bound states in the continuum (BIC) are actively investigated for achieving high-quality factors and strong electromagnetic field enhancements. Traditional approaches reported for tuning the performance of quasi-BIC metasurfaces include tuning the resonator size, period, and structure symmetry. Here we propose and experimentally demonstrate an alternative approach through engineering slots within a zigzag array of elliptical silicon resonators. Through analytical theory, three-dimensional electromagnetic modeling, and infrared spectroscopy, we systematically investigate the spectral responses and field distributions of the slotted metasurface in the mid-IR. Our results show that by introducing slots, the electric field intensity enhancement near the apex and the quality factor of the quasi-BIC resonance are increased by a factor of 2.1 and 3.3, respectively, in comparison to the metasurface without slots. Furthermore, the slotted metasurface also provides extra regions of electromagnetic enhancement and confinement, which holds enormous potential in particle trapping, sensing, and emission enhancement.

Far-field sub-wavelength imaging using high-order dielectric continuous metasurfaces.

Due to the wave nature of light, the resolution achieved in conventional imaging systems is limited to around half of the wavelength. The reason behind this limitation, called diffraction limit, is that part of the information of the object carried by the evanescent waves scattered from an abject. Although retrieving information from propagating waves is not difficult in the far-field region, it is very challenging in the case of evanescent waves, which decay exponentially as travel and lose their power in the far-field region. In this paper, we design a high-order continuous dielectric metasurface to convert evanescent waves into propagating modes and subsequently to reconstruct super-resolution images in the far field. The designed metasurface is characterized and its performance for sub-wavelength imaging is verified using full wave numerical simulations. Simulation results show that the designed continuous high-order metasurface can convert a large group of evanescent waves into propagating ones. The designed metasurface is then used to reconstruct the image of objects with sub-wavelength features, and an image with the resolution of λ/5.5 is achieved.

Electrically controlled molecular fingerprint retrieval with van der Waals metasurface

Polaritons in two-dimensional van der Waals (vdW) materials possess extreme light confinement, which have emerged as a potential platform for next-generation biosensing and infrared spectroscopy. Here, we propose an ultra-thin and electric tunable graphene/hexagonal boron nitride/graphene metasurface for detecting molecular fingerprints over a broad spectrum. The vdW metasurface supports hybrid plasmon–phonon polariton resonance with high-quality factor (Q > 120) and electrically controlled broadband spectra tunability from 6.5 to 7 μm. After coating a thin layer of bio-molecular (e.g., CBP) on top of the metasurface, the molecular absorption signatures can be readout at multiple spectral points and, thus, achieve broadband fingerprint retrieval of bio-molecules. Additionally, our electric tunable metasurface works as an integrated graphene-based field-effect transistor device, without the need of multiple resonance generators such as angle-resolved or pixelated dielectric metasurfaces for broadband spectra scanning, thereby paving the way for highly sensitive, miniaturized, and electrically addressed biosensing and infrared spectroscopy.

Nano-electromechanical spatial light modulator enabled by asymmetric resonant dielectric metasurfaces

Spatial light modulators (SLMs) play essential roles in various free-space optical technologies, offering spatio-temporal control of amplitude, phase, or polarization of light. Beyond conventional SLMs based on liquid crystals or microelectromechanical systems, active metasurfaces are considered as promising SLM platforms because they could simultaneously provide high-speed and small pixel size. However, the active metasurfaces reported so far have achieved either limited phase modulation or low efficiency. Here, we propose nano-electromechanically tunable asymmetric dielectric metasurfaces as a platform for reflective SLMs. Exploiting the strong asymmetric radiation of perturbed high-order Mie resonances, the metasurfaces experimentally achieve a phase-shift close to 290∘, over 50% reflectivity, and a wavelength-scale pixel size. Electrical control of diffraction patterns is also achieved by displacing the Mie resonators using nano-electro-mechanical forces. This work paves the ways for future exploration of the asymmetric metasurfaces and for their application to the next-generation SLMs.

Scalable direct printing of visible-light metasurfaces composed of an industrial ZrO2-composite imprint material

Optical metasurfaces offer a promising route to the replacement of traditional bulky optics with thinner and more functional ones, but their practical application has been challenging due to the lack of scalable and cost-effective manufacturing methods. One promising path to meeting this challenge could be single-step, direct printing of resin-based metasurfaces using well-established nanoimprint lithography (NIL). However, for its full-scale use, there are still practical issues such as fabrication accuracy as well as resin stability and mass-productivity. In this study, we address these issues by highly leveraging the recent advances in industrial nanoimprint resins. Here, a zirconium dioxide (ZrO2)-composite resin for industrial NIL is used for the direct printing of metasurfaces. We show that such a resin well meets the fundamental requirements for constructing dielectric metasurfaces, while affording practical advantages such as high formability, high resistance to long-term light exposure, and the feasibility of mass production. Our experimental demonstration validates that industrial resin is fully applicable for scalable metasurface manufacturing, which could be an important milestone toward the commercialization of metasurface optics.

Фрактальные приложения радиоэлектроники как фрактальная инженерия

The use of the fractal paradigm is presented - the main directions for introducing textures, fractals, fractional operators, dynamic chaos and methods of nonlinear dynamics for the design and creation of real technical projects in radio electronics - fractal radio systems, taking into account the hereditarity, non-Gaussianity and scaling of physical signals and fields. The substantiation of the use of fractal-scaling and texture methods for the synthesis of fundamentally new topological texture-fractal methods for detecting signals in the space-time channel of scattering waves (a new type of radar) is discussed. It is shown that the use of fractal systems, sensors and nodes is a fundamentally new solution that significantly changes the principles of constructing intelligent radio engineering systems and devices. It is shown that the use of computational dielectric metasurfaces brings to a new level all the functional characteristics of a multifunctional system of topological texture-fractal processing of signals and fields in solving classical problems of detection, measurement, recognition and classification by intelligent radio engineering systems and devices. The concept of "fractal engineering" is introduced, the methodology of its use is discussed.

Nanoscale refractive index sensor with ultrahigh figure of merit based on toroidal dielectric metasurfaces

Dielectric metasurfaces exhibiting low intrinsic loss and strong localized field are an emergent platform in light manipulation and sensing. Toroidal dipole, a new eigenmode distinct to the common electric and magnetic multipoles, expands the toolbox of nonlinear optics, filters, and multi-channel biosensors. Here, we propose a dielectric metasurface with two semicircle disks, which supports the high Q-factor toroidal dipole for the refractive index sensing. The performance of toroidal dipole is sensitive to the parameters of two semicircle disks. Via optimizing the geometrical parameters, we realize a Q-factor up to 69000. Moreover, the corresponding sensitivity and figure of merit can reach up to 70nm/RIU and 2970/RIU, respectively. The figure of merit is substantially higher than the previous works in near-infrared region. The enhancement of electric field located in the gap of two semicircles enables the strong interaction of light and the analyte. The introduction of the analyte will introduce the redshift of the resonant frequency of toroidal dipoles. Our findings expand the scope of toroidal dipoles and may serve as a new platform for high sensitivity sensing.

Structured Vector Field Manipulation of Terahertz Wave along the Propagation Direction Based on Dielectric Metasurfaces

AbstractThe generation and manipulation of vector light fields are of great significance for both fundamental research and industrial applications of polarized optics. In recent years, the spatial domain control of structured vector fields has gradually expanded from two‐ to three‐dimensional, including traditional optics and meta‐optics. Here, a new method to generate and manipulate structured vector light fields along the propagation direction is proposed, and the functionality in terahertz band using all‐silicon metasurfaces is demonstrated. The coherent superposition of orthogonal circularly polarized terahertz waves through long focal depth and multifocal metalens is completed, and varying phase differences between them in the propagation direction via path accumulation or initial phase design are introduced, thereby continuous variation or independently designed vector polarization distributions in multiple planes are obtained. It is worth mentioning that the proposed scheme is not only for the design of transverse electric field components, but also shows a strong ability for manipulation of the longitudinal component. This scheme realizes the polarization distribution designs of three‐dimensional vector fields in three‐dimensional space, and provides a new inspiration for the generation and manipulation of vector beams based on meta‐optics.

Realization of maximum optical intrinsic chirality with bilayer polyatomic metasurfaces.

Optical chirality plays a key role in optical biosensing and spin-selective optical field manipulation. However, the maximum optical intrinsic chirality, which is represented by near-unity circular dichroism (CD), is yet to be achieved in a wide bandwidth range based on nanostructures. Here, we utilize dielectric bilayer polyatomic metasurfaces to realize the maximum optical intrinsic chirality over a wide bandwidth range. The CD efficiency of the two designed metasurfaces with opposite chirality is 99.9% at 1350 nm and over 98% from 1340 nm to 1361 nm. Our work provides a straightforward and powerful method for the realization of maximum optical intrinsic chirality, which has great potential in spin-selective optical wave manipulation.

Optimization and design on multi-layers of dielectric metasurface as broadband terahertz quarter wave plate

Planar metasurface-based quarter wave plates (QWPs) have significant advantages over conventional devices in terms of compactness, flexibility, and simplicity of manufacture; however, they offer a relatively narrow operational bandwidth. A broadband terahertz achromatic QWP is realized in the 0.68–1.48 THz spectral region in this work, which consists of several separate metasurface layers of dielectric elliptic pillars stacked together with various rotation angles. Meanwhile, an improved simulated annealing method is proposed, which introduces the evolutionary strategies to optimize the distinct fundamental microstructural unit cells, and the running speed is greatly increased. Furthermore, the proposed multi-layers metasurface may pave the way for arbitrary polarization control of incident waves and be ideally suited for application by virtue of subwavelength thickness in the other frequency bands as well.

Terahertz reconfigurable dielectric metasurface hybridized with vanadium dioxide for two-dimensional multichannel multiplexing

The metasurface hybridized with vanadium dioxide (VO2) can be dynamically tuned, which has attracted enormous attention in recent years and orbital angular momentum (OAM) multiplexing based on metasurfaces has shown promising prospects in terahertz communications. However, existing research on VO2 metasurface focuses on the metallic metasurface. The dielectric VO2 metasurface used for OAM multiplexing is rarely reported to the present. This paper proposed a terahertz reconfigurable dielectric metasurface hybridized with VO2 for two-dimensional multichannel multiplexing combing with spatial and frequency domains. The metasurface works in both reflection and transmission modes and simultaneously the polarization control and operating frequency band regulation can be realized by switching the VO2 from the metallic state to the insulator state. For the reflective or transmissive metasurface, when 4×M-channel (M is a positive integer) off-axis plane waves are incident on the metasurface, the co-polarization reflected or cross-polarization transmitted waves are transformed into 4×M-channel orthogonal on-axis beams with topological or frequency orthogonality. A metasurface composed of 14 × 14 unit cells is designed for verification. The simulated result shows that two-dimensional 12-channel multiplexing combing with OAM and frequency by the designed metasurface can be realized on the reflection and transmission modes in two different frequency bands. The proposed metasurface has great potential in terahertz communications.

Exciton resonances for atomically-thin optics

Metasurfaces enable flat optical elements by leveraging optical resonances in metallic or dielectric nanoparticles to obtain accurate control over the amplitude and phase of the scattered light. While highly efficient, these resonances are static and difficult to tune actively. Exciton resonances in atomically thin 2D semiconductors provide a novel and uniquely strong resonant light–matter interaction, which presents a new opportunity for optical metasurfaces. Their resonant properties are intrinsic to the band structure of the material, do not rely on nanoscale patterns, and are highly tunable using external stimuli. In this tutorial, we present the role that exciton resonances can play for atomically thin optics. We describe the essentials of metasurface physics and provide background on exciton physics and a comprehensive overview of excitonic materials. Excitons demonstrate to provide new degrees of freedom and enhanced light–matter interactions in hybrid metasurfaces through coupling with metallic and dielectric metasurfaces. Using the high sensitivity of excitons to the medium's electron density, the first demonstrations of electrically tunable nanophotonic devices and atomically thin optical elements are also discussed. The future of excitons in metasurfaces looks promising, while the main challenge lies in large-area growth and precise integration of high-quality materials.

Kerker Condition for Enhancing Emission Rate and Directivity of Single Emitter Coupled to Dielectric Metasurfaces (Advanced Optical Materials 17/2022)

Kerker Condition for Enhancing Emission Rate and Directivity of Single Emitter Coupled to Dielectric Metasurfaces (Advanced Optical Materials 17/2022)

Near-infrared toroidal dipole response supported by silicon metasurfaces.

The dielectric metasurfaces supporting non-radiative toroidal dipole resonances play important roles in nanophotonics. In this paper, toroidal dipole resonances using a double-axe nanostructure array in the near-infrared region are theoretically investigated by the characterization of the near-field distribution and far-field scattering. An experimental quality factor of 261 is obtained at the resonant wavelength of 1498 nm.

Introducing a Symmetry‐Breaking Coupler into a Dielectric Metasurface Enables Robust High‐Q Quasi‐BICs

Dielectric metasurfaces supporting quasibound states in the continuum (quasi‐BICs) exhibit very high‐quality factor resonances and electric field confinement. However, accessing the high‐Q end of the quasi‐BIC regime usually requires marginally distorting the metasurface design from a BIC condition, pushing the needed nanoscale fabrication precision to the limit. This work introduces a novel concept for generating high‐Q quasi‐BICs, which strongly relaxes this requirement by incorporating a relatively large perturbative element close to high‐symmetry points of an undistorted BIC metasurface, acting as a coupler to the radiation continuum. This approach is validated by adding a ≈100 nm diameter cylinder between two reflection‐symmetry points separated by a 300 nm gap in an elliptical disk metasurface unit cell, using gallium phosphide as the dielectric. It is found that high‐Q resonances emerge when the cylindrical coupler is placed at any position between such symmetry points. This metasurface's second harmonic generation capability in the optical range is further explored. Displacing the coupler as much as a full diameter from a BIC condition produces record‐breaking normalized conversion efficiencies >102 W−1. The strategy of enclosing a disruptive element between multiple high‐symmetry points in a BIC metasurface can be applied to construct robust high‐Q quasi‐BICs in many geometrical designs.

Second‐Harmonic Generation in Strained Silicon Metasurfaces

Dielectric metasurfaces supporting Mie resonances can allow significantly boosted and tailored nonlinear light–matter interactions at the nanoscale. However, nonlinear dielectric metasurfaces typically only have odd‐order nonlinearities, unless resorting to material systems such as GaAs, GaP, or LiNbO3, each with unique challenges in device fabrication and integration. As the most widely adopted constituent material of metasurfaces, silicon (Si) does not possess an intrinsic second‐order nonlinear susceptibility. Herein, second‐harmonic generation (SHG) in a Si metasurface strained by a silicon nitride (SiN x ) cladding layer is demonstrated. Utilizing the stress caused by the SiN x layer to break the inversion symmetry of the bulk Si crystal, greatly enhanced SHG in the strained Si metasurface compared with that from an unpatterned Si film with the SiN x cladding layer, with an experimentally measured conversion efficiency as high as 4.2 × 10−5 W−1, is observed. Experiments are further performed and it is concluded that the enhanced SHG is most likely due to the applied stress instead of charged defects in the SiN x cladding. This work opens a new route to realizing dielectric metasurfaces with second‐order nonlinearity in a complementary metal–oxide–semiconductor (CMOS)‐compatible platform.

Electromagnetic multipolar quasi-bound states in the continuum for optical sensing

Optical sensing is of vital importance in many applications, which has received extensive attention in recent years. The pursuit of optical sensing mechanism with higher figure of merit (FOM) is very important for various sensing applications. Bound states in the continuum (BIC) with theoretical infinite Q factor is an ideal candidate to effectively improve the FOM of optical sensor. Herein, we propose and numerically demonstrate an optical refractive index sensor based on the electromagnetic multipolar quasi-BICs of all-dielectric metasurfaces. Based on the eigen-solution and excitation simulation, we find that magnetic dipole (MD) quasi-BIC is the best candidate for optical refractive index sensing among the electromagnetic multipolar quasi-BICs in the proposed all dielectric metasurface. The sensitivity (S) and the value of FOM for the optical refractive index sensor are 225.27 nm/RIU and 441762.93, respectively. It is anticipated that our results might shed new light on the optical refractive index sensing of high sensitivity and ultra-high FOM.

Multifunctional resonant wavefront-shaping meta-optics based on multilayer and multi-perturbation nonlocal metasurfaces

Photonic devices rarely provide both elaborate spatial control and sharp spectral control over an incoming wavefront. In optical metasurfaces, for example, the localized modes of individual meta-units govern the wavefront shape over a broad bandwidth, while nonlocal lattice modes extended over many unit cells support high quality-factor resonances. Here, we experimentally demonstrate nonlocal dielectric metasurfaces in the near-infrared that offer both spatial and spectral control of light, realizing metalenses focusing light exclusively over a narrowband resonance while leaving off-resonant frequencies unaffected. Our devices attain this functionality by supporting a quasi-bound state in the continuum encoded with a spatially varying geometric phase. We leverage this capability to experimentally realize a versatile platform for multispectral wavefront shaping where a stack of metasurfaces, each supporting multiple independently controlled quasi-bound states in the continuum, molds the optical wavefront distinctively at multiple wavelengths and yet stay transparent over the rest of the spectrum. Such a platform is scalable to the visible for applications in augmented reality and transparent displays.