Diffuse reflection and reciprocity-protected transmission via a random-flip metasurface

Matte surfaces with broadband transparency enabled by highly asymmetric diffusion of white light

The long-standing paradox between matte appearance and transparency has deprived traditional matte materials of optical transparency. Here, we present a solution to this centuries-old optical conundrum by harnessing the potential of disordered optical metasurfaces. Through the construction of a random array of meta-atoms tailored in asymmetric backgrounds, we have created transparent matte surfaces that maintain clear transparency regardless of the strength of disordered light scattering or their matte appearances. This remarkable property originates in the achievement of highly asymmetric light diffusion, exhibiting substantial diffusion in reflection and negligible diffusion in transmission across the entire visible spectrum. By fabricating macroscopic samples of such metasurfaces through industrial lithography, we have experimentally demonstrated transparent windows camouflaged as traditional matte materials, as well as transparent displays with high clarity, full color, and one-way visibility. Our work introduces an unprecedented frontier of transparent matte materials in optics, offering unprecedented opportunities and applications.

Macroscopic model and statistical model to characterize electromagnetic information of a digital coding metasurface

A digital coding metasurface is a platform connecting the digital space and electromagnetic wave space, and has therefore gained much attention due to its intriguing value in reshaping wireless channels and realizing new communication architectures. Correspondingly, there is an urgent need for electromagnetic information theory that reveals the upper limit of communication capacity and supports the accurate design of metasurface-based communication systems. To this end, we propose a macroscopic model and a statistical model of the digital coding metasurface. The macroscopic model uniformly accommodates both digital and electromagnetic aspects of the meta-atoms and predicts all possible scattered fields of the digital coding metasurface based on a small number of simulations or measurements. Full-wave simulations and experimental results show that the macroscopic model is feasible and accurate. A statistical model is further proposed to correlate the mutual coupling between meta-atoms with covariance and to calculate the entropy of the equivalent currents of digital coding metasurface. These two models can help reconfigurable intelligent surfaces achieve more accurate beamforming and channel estimation, and thus improve signal power and coverage. Moreover, the models will encourage the creation of a precoding codebook in metasurface-based direct digital modulation systems, with the aim of approaching the upper limit of channel capacity. With these two models, the concepts of current space and current entropy, as well as the analysis of information loss from the coding space to wave space, is established for the first time, helping to bridge the gap between the digital world and the physical world, and advancing developments of electromagnetic information theory and new-architecture wireless systems.

Chimera metasurface for multiterrain invisibility

Invisibility, a fascinating ability of hiding objects within environments, has attracted broad interest for a long time. However, current invisibility technologies are still restricted to stationary environments and narrow band. Here, we experimentally demonstrate a Chimera metasurface for multiterrain invisibility by synthesizing the natural camouflage traits of various poikilotherms. The metasurface achieves chameleon-like broadband in situ tunable microwave reflection mimicry of realistic water surface, shoal, beach/desert, grassland, and frozen ground from 8 to 12 GHz freely via the circuit-topology-transited mode evolution, while remaining optically transparent as an invisible glass frog. Additionally, the mechanic-driven Chimera metasurface without active electrothermal effect, owning a bearded dragon-like thermal acclimation, can decrease the maximum thermal imaging difference to 3.1 °C in tested realistic terrains, which cannot be recognized by human eyes. Our work transitions camouflage technologies from the constrained scenario to ever-changing terrains and constitutes a big advance toward the new-generation reconfigurable electromagnetics with circuit-topology dynamics.

Micro-DRIFTS for small area hyper-black spectroscopy

We have developed a low-cost micro-diffuse reflectance infrared Fourier transform spectroscopic (micro-DRIFTS) setup for measuring the reflectance of small area diffuse samples. The system performance is characterized and then demonstrated on small area vertically aligned carbon nanotube (VACNT) samples. We find that our system can measure samples with a spatial resolution of approximately 140 µm with sensitivities of 10s of ppm in the 2 µm - 18 µm spectral window. Our uncertainty budget is presented along with how our measured reflectance can be equated to directional-hemispherical reflectance.

Revisiting the Design Strategies for Metasurfaces: Fundamental Physics, Optimization, and Beyond

Over the last two decades, the capabilities of metasurfaces in light modulation with subwavelength thickness have been proven, and metasurfaces are expected to miniaturize conventional optical components and add various functionalities. Herein, various metasurface design strategies are reviewed thoroughly. First, the scalar diffraction theory is revisited to provide the basic principle of light propagation. Then, widely used design methods based on the unit-cell approach are discussed. The methods include a set of simplified steps, including the phase-map retrieval and meta-atom unit-cell design. Then, recently emerging metasurfaces that may not be accurately designed using unit-cell approach are introduced. Unconventional metasurfaces are examined where the conventional design methods fail and finally potential design methods for such metasurfaces are discussed.

The COPII subunit CsSEC23 mediates fruit glossiness in cucumber

To improve our understanding of the mechanism underlying cucumber glossiness regulation, a novel cucumber mutant with a glossy peel (Csgp) was identified. MutMap, genotyping, and gene editing results demonstrated that CsSEC23, which is the core component of COPII vesicles, mediates the glossiness of cucumber fruit peel. CsSEC23 is functionally conserved and located in the Golgi and endoplasmic reticulum. CsSEC23 could interact with CsSEC31, but this interaction was absent in the Csgp mutant, which decreased the efficiency of COPII vesicle transportation. Genes related to wax and cutin transport were upregulated in the Csgp mutant, and the cuticle structure of the Csgp-mutant peel became thinner. Moreover, the wax and cutin contents were also changed due to CsSEC23 mutation. Taken together, the results obtained from this study revealed that CsSEC23 mediates cucumber glossiness, and this mediating might be affected by COPII vesicle transportation.

2-bit coding metasurface with a double layer random flip structure for wide band diffuse reflection and reciprocity protected transmission

Coding metasurfaces based on random-flip structures have attracted great attention due to their ability to achieve distortion-free transmission and diffuse reflection simultaneously. However, previous implementation based on 1-bit coding metasurface has a narrow bandwidth and insufficient bandwidth coverage in the near infrared region. Here, we propose a novel vertical 2-bit coding metasurface composed of double-layer random-flip meta-atoms (DLRFM). while the main transmission lobe is unchanged, the zero-order diffraction intensity of DLRFM's reflection direction is less than 10% of the total reflection in the range of 0°∼ 30° incidence angle, which proves its excellent diffuse reflection and distortion-free transmission effect. Such design strategy can be extended to multiple wide band coverage in near-infrared regime by tailoring the geometric parameters, which indicates good application potential in advanced display and lens designs.

Kerker-type positional disorder immune metasurfaces

Metasurfaces that can operate without a strictly periodic arrangement of meta-atoms are highly desirable for practical optical micro-nano devices. In this paper, we propose two kinds of Kerker-type metasurfaces that exhibit immunity to positional disorder. These metasurfaces consist of two distinct core-shell cylinders that satisfy the first and second Kerker conditions, respectively. Despite significant positional disorder perturbations of the meta-atoms, the metasurfaces can maintain excellent performance comparable to periodic ones, including total transmission and magnetic mirror responses. This positional disorder immunity arises from the unidirectional forward or backward scattering of a single core-shell cylinder, which results in minimal lateral scattering coupling between neighboring cylinders, thereby having little impact on multiple scattering in either the forward or backward direction. In contrast, the response of positional disorder non-Kerker-type metasurfaces decreases significantly. Our findings present a new approach for designing robust metasurfaces and expanding the applications of metasurfaces in sensing and communications within complex practical scenarios.

Recent advances in metasurface design and quantum optics applications with machine learning, physics-informed neural networks, and topology optimization methods

As a two-dimensional planar material with low depth profile, a metasurface can generate non-classical phase distributions for the transmitted and reflected electromagnetic waves at its interface. Thus, it offers more flexibility to control the wave front. A traditional metasurface design process mainly adopts the forward prediction algorithm, such as Finite Difference Time Domain, combined with manual parameter optimization. However, such methods are time-consuming, and it is difficult to keep the practical meta-atom spectrum being consistent with the ideal one. In addition, since the periodic boundary condition is used in the meta-atom design process, while the aperiodic condition is used in the array simulation, the coupling between neighboring meta-atoms leads to inevitable inaccuracy. In this review, representative intelligent methods for metasurface design are introduced and discussed, including machine learning, physics-information neural network, and topology optimization method. We elaborate on the principle of each approach, analyze their advantages and limitations, and discuss their potential applications. We also summarize recent advances in enabled metasurfaces for quantum optics applications. In short, this paper highlights a promising direction for intelligent metasurface designs and applications for future quantum optics research and serves as an up-to-date reference for researchers in the metasurface and metamaterial fields.

Edge states in coupled non-Hermitian resonators

Small perturbations may dramatically influence the physical properties of a single non-Hermitian cavity. However, how these small perturbations interplay with bulk-edge properties is still to be demonstrated by experimentation. Here, we experimentally demonstrate edge states in coupled non-Hermitian resonators, based on a chain of all-dielectric coupled resonators where each resonator consists of two target particles. The evanescent coupling between the cavity and the target particles leads to tunable asymmetric backscattering, which plays a key role in the appearance of edge states in the bulk bandgap. We also demonstrate that these observed edge states are robust against weak disorders introduced to the system. Our study may inspire further explorations of the non-Hermitian bulk-edge properties.

Broadband binary-reflection-phase metasurfaces with undistorted transmission wavefront via mirror symmetry

In this work, we report the realization of broadband binary-reflection-phase metasurfaces that simultaneously exhibit undistorted transmission wavefront. Such a unique functionality is bestowed by leveraging mirror symmetry in the metasurface design. Under the normal incidence of waves polarized along the mirror surface, a broadband binary-phase pattern with π phase difference is induced in the cross-polarized reflection, while the co-polarized transmission and reflection are unaffected by the binary-phase pattern. Consequently, the cross-polarized reflection can be flexibly manipulated by designing the binary-phase pattern, without distorting the wavefront in transmission. The phenomena of reflected-beam splitting and undistorted transmission wavefront are hereby experimentally validated in a broad bandwidth from 8 GHz to 13 GHz. Our findings reveal a unique mechanism to realize independent manipulation of reflection with undistorted transmission wavefront in a broad spectrum, which has potential implications in meta-domes and reconfigurable intelligent surfaces.

Geometrical Theory of Electromagnetic Nonreciprocity

Recent advances in electromagnetic nonreciprocity raise the question of how to engineer the nonreciprocal electromagnetic response with geometrical approaches. In this Letter, we examine this problem by introducing generalized electromagnetic continua consisting structured points, which carry extra degrees of freedom over coordinate transformation. We show that general nonreciprocal media have a unique time-varying Riemannian metric structure with local spinning components. It is demonstrated that the nonreciprocity can be alternatively identified as the torsion tensor of a Riemann-Cartan space, which could provide analytic expressions for the magneto-optical effect and the axionic magnetoelectric coupling. Our theory not only gives a deeper insight into the fundamental understanding of electromagnetic nonreciprocity but also provides a practical principle to geometrically design nonreciprocal devices through frame transformation.

Spatial and frequency-selective optical field coupling absorption in an ultra-thin random metasurface

Simplified thin-film structures with the capability of spatial and frequency-selective optical field coupling and absorption are desirable for nanophotonics. Herein, we demonstrate the configuration of a 200-nm-thick random metasurface formed by refractory metal nanoresonators, showing near-unity absorption (absorptivity > 90%) covering the visible and near-infrared range (0.380-1.167 µm). Importantly, the resonant optical field is observed to be concentrated in different spatial areas according to different frequencies, paving a feasible way to artificially manipulate spatial coupling and optical absorption via the spectral frequency. The methods and conclusions derived in this work are applicable throughout a wide energy range and hold applications for frequency-selective nanoscale optical field manipulation.

Large-scale fabrication of an ultrathin broadband absorber using quasi-random dielectric Mie resonators

Ultrathin broadband absorber maintaining a near-uniform low reflectivity over a broadband wavelength is essential for many optical applications, such as light harvesting and nanoscale imaging. Recently, there has been considerable interest in employing arrays of high-index dielectric Mie resonators on surfaces to trap light and reduce the reflectivity. For such Mie-resonant metasurfaces, however, antireflection properties featuring both a flat low reflectance curve and a wide bandwidth are hard to be satisfied simultaneously, and an efficient large-scale nanofabrication technique rarely exists. Here, we present a high-throughput laser interference induced quasi-random patterning (LIIQP) technique to fabricate quasi-random Mie resonators in large scale. Mie resonators with feature sizes down to sub-100 nm have been fabricated using a 1064 nm laser source. Each Mie resonator concentrates light at its shape-dependent resonant frequency, and all such resonators are arranged quasi-randomly to provide both rich (with broadband Fourier components) and strong (with large intensities) Fourier spectra. Specifically, a near-uniform broadband reflectivity over 400-1100 nm spectrum region has been confined below 3% by fabricating a large-scale ultrathin (around 400 nm) absorber. Our concept and high-throughput fabrication technique allows the rapid production of quasi-random dielectric Mie-resonant metasurfaces in a controllable way, which can be used in various promising applications including thin-film solar cells, display, and imaging.

Hybrid resonance metasurface for a lithium niobate electro-optical modulator

Electrically tunable metasurfaces can realize two-dimensional pixelated spatial light modulation and have a wide range of applications in optical switching, free-space communication, high-speed imaging, and so on, arousing the interest of researchers. Here, a gold nanodisk metasurface on a lithium-niobate-on-insulator (LNOI) substrate is fabricated and experimentally demonstrated as an electrically tunable optical metasurface for transmissive free-space light modulation. Using the hybrid resonance formed by the localized surface plasmon resonance (LSPR) of gold nanodisks and the Fabry-Perot (FP) resonance, the incident light is trapped in the gold nanodisk edges and a thin lithium niobate layer to realize field enhancement. In this way, an extinction ratio of 40% is achieved at the resonance wavelength. In addition, the proportion of hybrid resonance components can be adjusted by the size of the gold nanodisks. By applying a driving voltage of ± 2.8 V, a dynamic modulation of 135 MHz is achieved at resonant wavelength. The highest signal-to-noise ratio (SNR) is up to 48 dB at 75 MHz. This work paves the way for the realization of spatial light modulators based on CMOS-compatible LiNbO₃ planar optics, which can be used in lidar, tunable displays, and so on.

Realizing transmissive and reflective focusing with an on-chip metalens

A metalens made of compact planar metastructure exhibits an excellent capability of focusing. The high-quality transmissive and reflective focusing simultaneously provides Fourier transform (FT) operation for optical information processing. Here we show a transflective on-chip metalens (TOM) made of orthogonal nano-grooves (ONGs). The TOM simultaneously converges transmitted and reflected (T&R) waves to the designed focal points. By adjusting the phase gradient profiles provided by the ONGs, the focal lengths of the T&R in-plane waves can be independently tuned. Our simulations show that the TOM possesses the advantages of broadband (>400 nm bandwidth) and high-focusing-efficiency (∼60%) dual-focusing capability. Further, we utilize the TOM to build a one-to-two 4-f optical system. Two different spatial filtering operations based on FT can be simultaneously implemented in axial transmission and off-axis reflection channels for one input signal. We expect that the dual-focusing metalens approach can realize parallel optical processing in on-chip optical computing, spatial filtering, and beyond.

Reciprocity Constraints on Reflection

Reciprocity is a fundamental symmetry of Maxwell's equations. It is known that reciprocity imposes constraints on transmission, absorption, and emission. Here, we reveal reciprocity constraints on reflection. We determine the sets of all attainable reflection coefficients of n-port scattering matrices with prescribed singular values, both with and without assuming reciprocity. Their difference establishes reciprocity constraints and nonreciprocal behaviors. As an application, we examine the conditions for all-zero reflections. Our results deepen the understanding of reciprocity in optics.

Tunable bilayer dielectric metasurface via stacking magnetic mirrors

Functional tunability, environmental adaptability, and easy fabrication are highly desired properties in metasurfaces. Here we provide a tunable bilayer metasurface composed of two stacked identical dielectric magnetic mirrors. The magnetic mirrors are excited by the interaction between the interference of multipoles of each cylinder and the lattice resonance of the periodic array, which exhibits nonlocal electric field enhancement near the interface and high reflection. We achieve the reversible conversion between high reflection and high transmission by manipulating the interlayer coupling near the interface between the two magnetic mirrors. Controlling the interlayer spacing leads to the controllable interlayer coupling and scattering of meta-atom. The magnetic mirror effect boosts the interlayer coupling when the interlayer spacing is small. Furthermore, the high transmission of the bilayer metasurface has good robustness due to the meta-atom with interlayer coupling can maintain scattering suppression against positional perturbation. This work provides a straightforward method to design tunable metasurface and sheds new light on high-performance optical switches applied in communication and sensing.

Flip-component metasurfaces for camouflaged meta-domes

Allowing microwaves to transmit through without changing the wavefront is one of the essential requirements of the dome structures of antenna arrays like radars. Here, we demonstrate a microwave metasurface as an array of two types of meta-atoms, which are the flip counterparts to each other. Due to the reciprocity and space-inversion symmetry, the wavefront in the transmission is unchanged by the metasurface in a broad spectrum; while at the same time, the wavefront in reflection can be manipulated independently by changing the arrangement of the meta-atoms. Specifically, a random-flip metasurface that produces diffuse reflection is realized, enabling a camouflaged meta-dome. The broadband, wide-angle, and polarization-independent diffuse reflection and undistorted transmission are numerically and experimentally verified. Our finding enables a unique meta-dome structure that has camouflage functionality.

Emerging Long-Range Order from a Freeform Disordered Metasurface

Recently, disordered metasurfaces have attracted considerable interest due to their potential applications in imaging, holography, and wavefront shaping. However, how to emerge long-range ordered phase distribution in disordered metasurfaces remains an outstanding problem. Here, a general framework is proposed to generate a spatially homogeneous in-plane phase distribution from a disordered metasurface, by engineering disorder parameters together with topology optimization. As a proof-of-concept demonstration, an all-dielectric disordered supercell metasurface with relatively homogeneous in-plane phase fluctuation is designed by disorder parameter engineering, manifesting as polarization conversion-dependent random scattering or unidirectional transmission. Then, a topology optimization approach is utilized to overcome the lattice coupling effect and to further improve the homogeneity of complex electric field fluctuation. In comparison with the initial supercell metasurface, both the phase fluctuation range and the relative efficiency of the topology-optimized freeform metasurface are significantly improved, leading to a long-range ordered electric field distribution. Moreover, three experimental realizations are performed, all of which agree well with the theoretical results. This methodology may inspire more exotic optical phenomena and find more promising applications in disordered metasurfaces and disordered optics.