Dielectric Metasurfaces Research Articles

Dual-wavelength terahertz two-dimensional phase gratings based on all dielectric metasurfaces

Efficient and accurate phase gratings hold immense significance in the realization of large format heterodyne array receivers at terahertz frequencies. Metallic phase gratings have made substantial advancements in terms of operating wavelength and the number of diffraction beams. Like most other diffractive optical devices, metallic phase gratings are primarily optimized to operate at one specific wavelength. Metasurfaces compositing arrays of subwavelength nanostructures have been demonstrated with various optical functions, by freely modifying the polarization, phase, and amplitude of light. In this study, we present an approach to create a multi-wavelength phase grating compositing segments that incorporate multiple nanostructures. The resulting transmission phase grating not only exhibits uniform diffraction beams (2 × 2) but also achieves the same diffraction angles at both 1.31 and 2.7 THz. The measured total power efficiency of the diffraction beam pattern is 53.2% for 1.31 THz and 42.4% for 2.7 THz. These devices can be applied in terahertz astronomical observations and fluorescence microscopy applications, where multi-wavelength operation is necessary.

Ultrafast all-optical second harmonic wavefront shaping

Optical communication can be revolutionized by encoding data into the orbital angular momentum of light beams. However, state-of-the-art approaches for dynamic control of complex optical wavefronts are mainly based on liquid crystal spatial light modulators or miniaturized mirrors, which suffer from intrinsically slow (µs-ms) response times. Here, we experimentally realize a hybrid meta-optical system that enables complex control of the wavefront of light with pulse-duration limited dynamics. Specifically, by combining ultrafast polarization switching in a WSe2 monolayer with a dielectric metasurface, we demonstrate second harmonic beam deflection and structuring of orbital angular momentum on the femtosecond timescale. Our results pave the way to robust encoding of information for free space optical links, while reaching response times compatible with real-world telecom applications.

Nonlinear asymmetric imaging with AlGaAs metasurface.

Nowadays, dielectric metasurfaces are a promising platform in many different research fields such as sensing, lasing, all-optical modulation and nonlinear optics. Among all the different kinds of such thin structures, asymmetric geometries are recently attracting increasing interest. In particular, nonlinear light-matter interaction in metasurfaces constitutes a valid approach for achieving miniaturized control over light. Here, we demonstrate nonlinear asymmetric generation of light in a dielectric metasurface via second harmonic generation. By inverting the illumination direction of the pump, the nonlinear emitted power is modulated by more than one order of magnitude. Moreover, we demonstrate how a properly designed metasurface can generate two completely different images at the second harmonic when the direction of illumination is reversed. Our results may pave the way to important opportunities for the realization of compact nanophotonic devices for imaging applications by densely integrating numerous nonlinear resonators.

Вычислительные диэлектрические метаповерхности в фотонных топологических устройствах обработки многомерных сигналов

A single-layer computational dielectric broadband metasurface (MS) with silicon nanodiscs is proposed to implement spatial differentiation and contour recognition operations. The variety of physical mechanisms of dielectric MSs for performing similar mathematical operations is considered. The necessity of such computational nanostructures for improving the numerous methods of topological texture-fractal processing of signals and fields in modern radio physics and radio electronics is revealed.

Broadband angular spectrum differentiation using dielectric metasurfaces

Signal processing is of critical importance for various science and technology fields. Analog optical processing can provide an effective solution to perform large-scale and real-time data processing, superior to its digital counterparts, which have the disadvantages of low operation speed and large energy consumption. As an important branch of modern optics, Fourier optics exhibits great potential for analog optical image processing, for instance for edge detection. While these operations have been commonly explored to manipulate the spatial content of an image, mathematical operations that act directly over the angular spectrum of an image have not been pursued. Here, we demonstrate manipulation of the angular spectrum of an image, and in particular its differentiation, using dielectric metasurfaces operating across the whole visible spectrum. We experimentally show that this technique can be used to enhance desired portions of the angular spectrum of an image. Our approach can be extended to develop more general angular spectrum analog meta-processors, and may open opportunities for optical analog data processing and biological imaging.

Optimum asymmetry for nanofabricated refractometric sensors at quasi-bound states in the continuum

A symmetry-protected bound state in the continuum (BIC) is one of the bases for high-resolution photonic refractometric sensors that rely on spectral shifts. However, a trade-off exists between the quality (Q) factors and the resonance amplitudes when the asymmetries of the unit cell are changed, making it difficult to intuitively determine the optimal nanostructural geometry. In this study, we present a theoretical and experimental approach for identifying the asymmetry parameters of dielectric metasurfaces that yield the lowest limit of detection (LOD). Silicon-based metasurfaces with asymmetric pair-rod arrays are fabricated experimentally, and the minimum LOD is obtained under a critical coupling condition with equal radiative and nonradiative Q factors. The results agree well with the theoretical model derived from the temporal coupled-mode theory. We reveal that the LOD and the optimum asymmetry are significantly influenced by nonradiative losses in the nanostructure, emphasizing the importance of loss reduction in dielectric metasurfaces at quasi-BICs for high-performance refractometric sensors.

Phase noise matching in resonant metasurfaces forintrinsic sensing stability.

Interferometry offers a precise means of interrogating resonances in dielectric and plasmonic metasurfaces, surpassing spectrometer-imposed resolution limits. However, interferometry implementations often face complexity or instability issues due to heightened sensitivity. Here, we address the necessity for noise compensation and tolerance by harnessing the inherent capabilities of photonic resonances. Our proposed solution, termed "resonant phase noise matching," employs optical referencing to align the phases of equally sensitive, orthogonal components of the same mode. This effectively mitigates drift and noise, facilitating the detection of subtle phase changes induced by a target analyte through spatially selective surface functionalization. Validation of this strategy using Fano resonances in a 2D photonic crystal slab showcases noteworthy phase stability (). With demonstrated label-free detection of low-molecular-weight proteins at clinically relevant concentrations, resonant phase noise matching presents itself as a potentially valuable strategy for advancing scalable, high-performance sensing technology beyond traditional laboratory settings.

Image Processing Using High‐Index Dielectric Metasurfaces Based on Fano Resonance

AbstractThis paper presents the design and analysis of high‐index dielectric metasurfaces based on Fano resonance for image processing applications. With the aim of optical edge detection, two different metasurface structures are proposed to create first‐ and second‐order derivative operators. The metasurfaces designed for second‐order derivative is a square lattice structure consisting of silicon cylinders surrounded by a silicon dioxide layer. Moreover, another metasurface with a square lattice structure composed of three different silicon cylinders embedded in a silicon dioxide layer is proposed to perform first‐order derivation. To achieve the best performance of the proposed metasurfaces in edge detection, the physical dimensions of the proposed structures are optimized by conducting parametric studies to reach the maximum available numerical aperture . Consequently, the second‐order derivative metasurface attains a numerical aperture with a magnitude , while the first‐order derivative metasurface achieves a numerical aperture of . Finally, the applicability of the proposed structures in optical edge detection is verified in the analog domain. The proposed metasurfaces provide high spatial resolution, high efficiency, and low dependence on the horizon angle, which pave the way for high‐quality image processing applications.

Multifunctional all-dielectric quarter-wave plate metasurfaces for generating focused vector beams of Bell-like states

Abstract The generation of vector beams using metasurfaces is crucial for the manipulation of light fields and has significant application potential, ranging from classical physics to quantum science. This paper introduces a novel dielectric metasurface composed of quarter-wave plate (QWP) meta-atoms, known as a QWP metasurface, designed to generate focused vector beams (VBs) of Bell-like states under right circularly polarized illumination. The propagation phase imparted on both the co- and cross-polarized components of the output field constructs hyperbolic and helical phase profiles with topological charge l p , whereas the Pancharatnam–Berry (PB) phase acts only on the cross-polarized component to construct another helical phase profile with topological charge l g . Thus, the co- and cross-polarized components form two orthogonal vector vortex (VV) modes with topological charges l p and l p + l g , respectively. When the parameter conditions are satisfied by matching the incident polarization chirality σ and topological charges l p and l g , the focused VBs of Bell-like states are generated by simultaneously manipulating the two VV modes, in contrast to existing QWP metasurfaces. The polarization states of the generated VBs are manipulated using the initial orientation angle θ 0 of the meta-atom. Overall, this research provides an innovative strategy for metasurface design, enhancing the functionality of metasurface devices for a broader range of application scenarios.

Refractive index sensing of spoof surface plasmon polaritons excited on graphene strips based on cylindrical dielectric metasurface coupling

In this paper, we present a novel terahertz plasmonic sensing concept that utilizes a metasurface coupler to excite spoof surface plasmon polaritons (SSPs) modes on graphene strips. The dielectric metasurface unit cell consists of two silicon cylindrical rods with different radii on a quartz substrate. Vertically incident terahertz waves are deflected by the metasurface, resulting in a transverse wave-vector within the substrate that matches the SSPs wave-vector supported by the graphene strips. This enables efficient excitation of SSPs modes on the graphene strips surface, leading to absorption resonances in the range of 0.420–0.430 THz with a high quality (Q) factor of 152. We then applied this device for refractive index sensing, and the investigation results indicate that the high Q resonances in the absorption spectrum experience shifts with changes in the refractive index of the target substance, with a sensitivity of 110 GHz/RIU. The designed structure in this study exhibits significant potential for applications in trace substance absorption spectrum detection.

Optical Slot-Assisted Metasurface for IgG Protein Detection

The sensing of antigens plays a pivotal role in the early diagnosis of tumors by potentially revealing specific biomarkers associated with them. Early detection of tumors can facilitate targeted and less invasive therapeutic strategies. In the realm of Immunoglobulin G (IgG) protein detection, a high concentration may indicate tumor lesions or the presence of symptoms commonly detectable just through invasive diagnostic methods. IgG detection is feasible in blood samples or other biological fluids like saliva or urine. These non-invasive tests offer the advantage of being repeatable over time, reducing the need for invasive or painful procedures. In the pursuit of next-generation medical technologies aimed at flexible and compact sensing platforms, we proposed a refractometric system mainly based on a dielectric metasurface, operating at about 780 nm, featuring a 2D distribution of cells composed of dimers that support the optical slot effect. This effect allows for a large Q-factor, with a small footprint, and strong spatial confinement. Specifically, a Q-factor of 3.19 × 103 with a significant amplitude (> 0.7 a.u.) enables a sensitivity of 25 nm/RIU for IgG protein detection.

A metasurface-on-fiber light-sheet generator for biological imaging

Light sheet has been widely used for bioimaging. However, it is a big challenge to build a miniature light-sheet imaging system due to geometric constraints of bulky refractive and diffractive elements. Metasurface-based devices composed of subwavelength unit cells, with compact size and the ability to effectively manipulate optical fields, have been considered as potential planar analogs of conventional components to realize various functionalities. Integrating metasurfaces onto an optical fiber facet is sparking new ideas and emerging applications. Here, we propose a metasurface-on-fiber light-sheet generator, which can convert a fiber mode to a thin light sheet. By adopting a multi-core fiber and a dielectric metasurface on its facet, a light sheet with a thickness of 2 μm and a length of 83.0 μm is obtained. The generator can greatly simplify the illumination of light sheet imaging systems and be flexibly used for advanced biological imaging applications.

Pixel-integrated Mie metasurface long-wave multispectral type II superlattice detector

Dynamic multispectral imaging finds extensive applications in acquiring multidimensional information. The integration of high-performance, dynamic, and long-wavelength multispectral detectors at the pixel level is highly desirable across various applications. However, the development of such detector faces enormous challenges due to the fundamental material and optical system limitations. In this work, we present a pixel-integrated long-wavelength multispectral type II superlattice detector based Mie dielectric metasurface (Mie-multispectral detector) realized by integrating a graphene-assisted depletion Mie metasurface structure (GAMS). The GAMS is featured with a single-layer graphene and electrically gated tuned Mie dielectric grating. This pixel-integrated multispectral detector allows for 340 nm electrical dynamic tuning and D* value of 5 × 1010 Jones. The Mie-multispectral detector offers potential solutions in space exploration, pollutant retrieval, and other relevant fields.

Resonantly Enhanced Optical Birefringence in Ultrathin High‐Index WS2 Metasurfaces

AbstractOptical birefringence plays an important role in the manipulation of polarization states of light. However, the weak birefringence in common birefringent materials restricts the device thickness to tens of microns for desired phase retardation. Although recent advances in dielectric metasurfaces have enabled remarkable birefringence using structurally anisotropic nanostructures, the refractive index contrast between the dielectric and the air cladding fundamentally limits the thickness of metasurfaces to a fraction of a micron to obtain the required phase retardation. Here, birefringent resonances in high‐index tungsten disulfide (WS2) metasurfaces are utilized to push the thickness limit down to the deep subwavelength scale. The inherent high‐index property of WS2 enables birefringent resonances in WS2 metasurfaces which consist of anisotropic nanostructures with a thickness of only 50 nm. Such birefringent resonances enhance the light‐matter interaction and produce an unprecedented birefringence (Δneff) about 4. As a result, it is experimentally realized that circular‐to‐linear polarization conversion with a normalized efficiency of ≈90% within an incident angle range up to ±10°. The results break the fundamental limit of birefringent devices and provide strategies for creating ultimate thin polarization optical devices.

Dielectric‐Metasurface‐Enhanced Kerr Self‐Modulation for Passively Q‐Switching

AbstractDielectric metasurfaces are highly sought‐after since they have found a variety of linear and nonlinear applications. Here, a novel transmission modulation is presented for pulsed laser generation by directly exploiting the enhanced Kerr effect from a periodic array of silicon nanodisks. The Mie resonance of the unit cells enhances the Kerr effect of the silicon material and is redshifted by the latter, so that the transmission of the metasurface at the working wavelength is strongly modulated (the observed modulation depth is above 3% under moderate exciting power). By inserting the fabricated metasurface as a self‐modulator into a ring cavity fiber laser, stable Q‐switched pulse laser output is achieved with a duration of 1.4 µs and a repetition rate of 60 kHz. This work opens a door for ultra‐thin low‐loss dielectric metasurfaces to efficiently shape the operation of pulsed lasing based on their enhanced nonlinear effects and flexible propagation manipulation.

Quarter-Wave Plate Metasurfaces for Generating Multi-Channel Vortex Beams.

Metasurfaces of quarter-wave plate (QWP) meta-atoms have exhibited high flexibility and versatile functionalities in the manipulation of light fields. However, the generation of multi-channel vortex beams with the QWP meta-atom metasurfaces presents a significant challenge. In this study, we propose dielectric metasurfaces composed of QWP meta-atoms to manipulate multi-channel vortex beams. QWP meta-atoms, systematically arranged in concentric circular rings, are designed to introduce the modulations via the propagation phase and geometric phase, leading to the generation of co- and cross-polarized vortex beams in distinct channels. Theoretical investigations and simulations are employed to analyze the modulation process, confirming the capability of QWP meta-atom metasurfaces for generating the multi-channel vortex beams. This study presents prospective advancements for the compact, integrated, and multifunctional nanophotonic platforms, which have potential applications in classical physics and quantum domains.

Visible Meta-Displays for Anti-Counterfeiting with Printable Dielectric Metasurfaces.

Metasurfaces, 2D arrays of nanostructures, have gained significant attention in recent years due to their ability to manipulate light at the subwavelength scale. Their diverse applications range from advanced optical devices to sensing and imaging technologies. However, the mass production of dielectric metasurfaces with tailored properties for visible light has remained a challenge. Therefore, the demand for efficient and cost-effective fabrication methods for metasurfaces has driven the continuing development of various techniques. In this research article, a high-throughput production method is presented for multifunctional dielectric metasurfaces in the visible light range using one-step high-index TiO2-polymer composite (TPC) printing, which is a variant of nanoprinting lithography (NIL) for the direct replication of patterned multifunctional dielectric metasurfaces using a TPC material as the printing ink. The batch fabrication of dielectric metasurfaces is demonstrated with controlled geometry and excellent optical response, enabling high-performance light-matter interactions for potential applications of visiblemeta-displays.

Alias Transformation of Structured Beam and Holography in Real and Fourier Space Based on Dielectric Metasurface

AbstractStructured beams are analytical solutions to wave equations that possess specific spatial profiles and rigorous transmission properties. Holography enables wavefront reconstruction by recording objective wavefront information. In this study, comprehensive wavefront control by transforming a structured beam in real space is proposed and experimentally verified, and optical holography is reconstructed in Fourier space. To obtain an arbitrary profile of the structured beam, alias transformation is performed to rebuild the coordinate system to form arbitrary user‐defined shapes. Subsequently, the complex amplitude control of the metasurface to encode the alias‐transformed structured beam accordingly is applied. This allows the conventional standard Hermite–Gaussian and Laguerre–Gaussian modes to be transformed into their corresponding shapes while maintaining their distinct features. Holography is realized based on pure phase modulation by interleaving the information with a structured beam, which fully utilizes the space‐bandwidth product of the metasurface. These distinctive optical phenomena in both spaces expand the definition of conventional structured beams and enable further developments in laser fabrication, optical manipulation, and optical displays.

Three-in-one polarization detector enabled by metasurface

In the field of biomedicine, the detection of target analytes imposes requirements on polarization information detection and sensitivity. It necessitates non-destructive analysis of proteins and DNA. The combination of dielectric geometric metasurface with terahertz (THz) vortex beams present new opportunities for biomedical sensing. In this work, we employ simulation to demonstrate a novel methodology that integrates off-axis dual-focus with vector vortex beams (VVB). Moreover, the metasurface-based method provides potential solutions for miniaturized polarization detection. The capability of polarization detection is evaluated using different polarization states at three different focal planes. Specifically, the ellipticity and handedness of incident THz waves can be determined by the electric field intensity ratio of the two off-axis focused spots. The major axis direction of linear polarization waves can be determined by extracting the azimuth of the VVB. It is worth mentioning that higher-order VVB, benefiting from its high angular resolution characteristics, exhibits enhanced sensitivity in determining the direction of major axes. Our proposed scheme offers potential applications for THz communications, medical imaging, security detection and remote sensing.

Two-Color Spatially Resolved Tuning of Polymer-Coated Metasurfaces.

For the realization of truly reconfigurable metasurface technologies, dynamic spatial tuning of the metasurface resonance is required. Here we report the use of organic photoswitches as a means for the light-induced spatial tuning of metasurface resonances. Coating of a dielectric metasurface, hosting high-quality-factor resonances, with a spiropyran (SPA)-containing polymer enabled dynamic resonance tuning up to 4 times the resonance full-width at half-maximum with arbitrary spatial precision. A major benefit of employing photoswitches is the broad toolbox of chromophores available and the unique optical properties of each. In particular, SPA and azobenzene (AZO) photoswitches can both be switched with UV light but exhibit opposite refractive index changes. When applied to the metasurface, SPA induced a red shift in the metasurface resonance with a figure of merit of 97 RIU-1, while AZO caused a blue shift in the resonance with an even greater sensitivity of 100 RIU-1. Critically, SPA and AZO can be individually recovered with red and blue light, respectively. To exploit this advantage, we coated a dielectric metasurface with spatially offset SPA- and AZO-containing polymers to demonstrate wavelength-dependent, spatially resolved control over the metasurface resonance tuning.