Volume 14, Issue 8 - Special Issue: Metamaterials and Plasmonics in Asia

The burgeoning demand for high-performance computing, robust data processing, and rapid growth of big data necessitates the emergence of novel optical devices to efficiently execute demanding computational processes. The field of meta-devices, such as metamaterial or metasurface, has experienced unprecedented growth over the past two decades. By manipulating the amplitude, phase, polarization, and dispersion of light wavefronts in spatial, spectral, and temporal domains, viable solutions for the implementation of all-optical analog computation and information processing have been provided. In this review, we summarize the latest developments and emerging trends of computational meta-devices as innovative platforms for spatial optical analog differentiators and information processing. Based on the general concepts of spatial Fourier transform and Green’s function, we analyze the physical mechanisms of meta-devices in the application of amplitude differentiation, phase differentiation, and temporal differentiation and summarize their applications in image edge detection, image edge enhancement, and beam shaping. Finally, we explore the current challenges and potential solutions in optical analog differentiators and provide perspectives on future research directions and possible developments.

Enhancing the sensitivity of biomedical spectroscopy is crucial for advancing medical research and diagnostics. Metasurfaces have emerged as powerful platforms for enhancing the sensitivity of various biomedical spectral detection technologies. This capability arises from their unparalleled ability to improve interactions between light and matter through the localization and enhancement of light fields. In this article, we review representative approaches and recent advances in metasurface-enhanced biomedical spectroscopy. We provide a comprehensive discussion of various biomedical spectral detection technologies enhanced by metasurfaces, including infrared spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and other spectral modalities. We demonstrate the advantages of metasurfaces in improving detection sensitivity, reducing detection limits, and achieving rapid biomolecule detection while discussing the challenges associated with the design, preparation, and stability of metasurfaces in biomedical detection procedures. Finally, we explore future development trends of metasurfaces for enhancing biological detection sensitivity and emphasize their wide-ranging applications.

Guided-mode resonance (GMR) is a key principle for various nanophotonic elements in practice. In parallel, GMR structures offer an efficient experimental platform for fundamental study of novel wave phenomena because of its versatile capability to synthesize complicated potential distributions and analyze deep internal properties conveniently in the optical far-fields. In this paper, we provide a brief review of topological GMR effects as a promising subtopic of the emerging topological photonics. Starting from a conceptually minimal model, we explain basic topological parameters, associated optical properties, experimental realizations, and potential applications. We treat topics of recent interest including topological edge-state resonances, deterministic beam shaping and mode matching, bound states in the continuum, unidirectional resonances, and polarization vortices. We finally address limitations, remaining challenges, and perspective of the topic.

Circular dichroism (CD) spectroscopy is essential for biochemistry, structural biology and pharmaceutical chemistry. While the chiroptical properties of chiral molecules are characterized by the Pasteur parameter κ , it is commonly conceived that the generation of CD is solely attributed to the imaginary part κ ′′. However, since the imaginary part κ ′′ is orders of magnitude smaller than the real part κ ′ for most chiral molecules, the achievable sensitivity of CD spectroscopy is quite limited. Here, we report a recipe for realizing ultrasensitive CD spectroscopy based on the κ ′ component of chiral molecules. Two quasi-bound states in the continuum are coupled by chiral molecules to form two hybridized branches, whose wavelengths and eigenpolarizations are very sensitive to the value of κ ′. Giant CD signals over four orders of magnitude larger than the case without mode coupling are thus produced, paving the way towards chiral structure analysis at the single molecule level.

It has long been desired to enable global structural optimization of organic light-emitting diodes (OLEDs) for maximal light extraction. The most critical obstacles to achieving this goal are time-consuming optical simulations and discrepancies between simulation and experiment. In this work, by leveraging transfer learning, we demonstrate that fast and reliable prediction of OLED optical properties is possible with several times higher data efficiency compared to previously demonstrated surrogate solvers based on artificial neural networks. Once a neural network is trained for a base OLED structure, it can be transferred to predict the properties of modified structures with additional layers with a relatively small number of additional training samples. Moreover, we demonstrate that, with only a few tenths of experimental data sets, a neural network can be trained to accurately predict experimental measurements of OLEDs, which often differ from simulation results due to fabrication and measurement errors. This is enabled by transferring a pre-trained network, built with a large amount of simulated data, to a new network capable of correcting systematic errors in experiment. Our work proposes a practical approach to designing and optimizing OLED structures with a large number of design parameters to achieve high optical efficiency.

The development of novel camouflage technologies is of great significance, exerting an impact on both fundamental science and diverse military and civilian applications. Effective camouflage aims to reduce the recognizability of an object, making it to effortlessly blend with the environment. For infrared camouflage, it necessitates precise control over surface emissivity and temperature to ensure that the target blends effectively with the surrounding infrared background. This study presents a semimetal–dielectric–metal metasurface emitter engineered for the application of infrared camouflage. The metasurface, with a total thickness of only 545 nm, consists of a Bi micro-disk array and a continuous ZnS and Ti film beneath it. Unlike conventional metal-based metasurface design, our approach leverages the unique optical properties of Bi, achieving an average emissivity of 0.91 in the 5–8 μm non-atmospheric transparency window. Experimental results indicate that the metasurface emitter achieves lower radiation and actual temperatures compared to those observed in comparative experiments, highlighting its superior energy dissipation and thermal stability. The metasurface offers advantages such as structural simplicity, cost-effectiveness, angular insensitivity, and deep-subwavelength features, rendering it suitable for a range of applications including military camouflage and anti-counterfeiting, with potential for broad deployment in infrared technologies.

Light behaviours in disordered materials have been of research interest primarily at length scales beyond or comparable to the wavelength of light, because order and disorder are often believed to be almost indistinguishable in the subwavelength regime according to effective medium theory (EMT). However, it was demonstrated that the breakdown of EMT occurs even at deep-subwavelength scales when interface phenomena, such as the Goos–Hänchen effect, dominate light flows. Here we develop the engineering of disordered multilayers at deep-subwavelength scales to achieve angle-selective manipulation of wave localization. To examine the disorder-dependent EMT breakdown, we classify the intermediate regime of microstructural phases between deep-subwavelength crystals and uncorrelated disorder through the concept of stealthy hyperuniformity (SHU). We devise material phase transitions from SHU to uncorrelated disorder for distinct angular responses of wave localization by tailoring the short-range and long-range order in SHU multilayers. The result paves the way to the realization of deep-subwavelength disordered metamaterials, bridging the fields of disordered photonics and metamaterials.

Structural colors generated by optical micro-/nanostructures offer a notable advantage over traditional chemical pigments, including higher purity, greater brightness, resistance to fading, and enhanced environmental friendliness. However, achieving dynamically switchable color displays with high performances and without resorting to complex nanofabrication methods remain a challenge. Here, we present a simple method using grayscale lithography and conformal coating to create Salisbury screen (SS) cavities with variable resonant wavelengths, enabling the formation of tunable colorful patterns. The dynamic color display is achieved through the phase change of vanadium dioxide (VO 2 ) nanostructures under electrothermal effects. At a low actuation voltage of 1.4 V, high performances of color switching such as high sensitivity, fast speed, high repeatability, and wide-view angle are achieved. The tunable structural colors, featuring a simple preparation process and high-speed switching, represent a promising alternative for applications such as thermal sensors, security information encryption, and dynamic full-color displays.

Recent research on non-Abelian phenomena has cast a new perspective on controlling light. In this work, we provide a simple and general approach to induce non-Abelian gauge field to tremble the light beam trajectory. With in-plane duality symmetry relaxed, our theoretical analysis finds that non-Abelian electric field can be synthesized through a simple real-space rotation of any biaxial material. With orthogonal optical modes excited, their interference leads to an oscillation of the propagating optical beam, which is a direct consequence of the emergence of non-Abelian electric field, influencing light in a manner similar with how electric fields act on charged particles. Our microwave experiments provide unambiguous evidence to the observation of such an optical Zitterbewegung effect where excellent agreement can be found between theorical derivation, numerical simulations and experiments. By extending the idea to optical regime using natural material, we here provide another example to shake the general intuition that light travels in straight lines in homogeneous media.

Metasurface-assisted waveguide couplers, or meta-couplers, innovatively link free-space optics with on-chip devices, offering flexibility for polarization and wavelength (de)multiplexing, mode-selective coupling, and guided mode manipulation. However, conventional meta-couplers still face challenges with low coupling efficiency and narrow bandwidth due to critical near-field coupling caused by waveguide constraints and unit-cell–based design approach, which cannot be accurately addressed using traditional design methods. In this paper, quasi-continuous dielectric catenary arrays are first employed to enhance efficiency and bandwidth by addressing adjacent coupling issues of discrete metasurface. Then, diffraction analysis demonstrates that the performance of forward-designed couplers is hindered by spurious diffraction orders and destructive interference. To further enhance performance, an adjoint-based topology optimization algorithm is utilized to customize electric near-field, which can effectively suppress spurious diffraction orders and destructive near-field interference, achieving ultra-high coupling efficiency of 93 % with 16.7 dB extinction ratios at 1,550 nm. Additionally, a broadband meta-coupler exceeds 350 nm bandwidth with 50 % average coupling efficiency across O- to L-bands using multiobjective optimization. These high-performance devices may render them suitable for applications in optical communications, sensing, and nonlinear optics. Moreover, the inverse design method shows potential for improving the performance of various metasurface-integrated on-chip devices.

Plasmonic nanocavities with highly localized fields in their nanogaps significantly enhance light–matter interactions at the nanoscale, surpassing the diffraction limit. Strong coupling between a plasmonic nanocavity and a molecule forms hybrid upper and lower branch states, resulting in Rabi splitting (RS) in optical spectra. However, scattering and absorption spectra often fail to unambiguously distinguish whether the double peaks arise from energy transparency or RS. In contrast, photoluminescence (PL) clearly reveals the quantum state of a molecule coupled with a plasmon by filtering out background fields. This paper presents a theoretical framework based on nonlocal response theory to calculate the PL of a single molecule coupled with arbitrary metallic nanostructures. Our theory provides an analytical approach to design the spatial arrangement of metallic nanostructures and molecular orbitals and to calculate the PL in strongly coupled systems, addressing limitations in previous studies. Using this framework, we investigated a coupled system comprising a gold nanoplate dimer and a planar porphyrin tape. By modifying porphyrin units to modulate coupling strength, we explored the molecular quantum state coupled with the nanocavity through PL analysis. We elucidated the spectral features of absorption, excitation, and PL in weak and strong coupling regimes and evaluated the dependence of coupling strength on the molecular position and orientation within the nanogap. Our results demonstrate that the quantum state of a molecule in an optically forbidden transition can be excited by the highly localized field in the nanogap. This work advances the fundamental understanding of light–matter interactions at the nanoscale and provides a foundation for the development of future nanophotonic devices.

Hadamard matrices, composed of mutually orthogonal vectors, are widely used in various applications due to their orthogonality. In optical imaging, Hadamard microscopy has been applied to achieve optical sectioning by separating scattering and background noise from desired signals. This method involves sequential illumination using Hadamard patterns and subsequent image processing. However, it typically requires costly light modulation devices, such as digital micromirror devices (DMDs) or spatial light modulators (SLMs), to generate multiple illumination patterns. In this study, we present spectral Hadamard microscopy based on a holographic matasurface. We noticed that certain patterns repeat within other Hadamard patterns under specific condition, allowing the entire set to be reproduced from a single pattern. This finding suggests that generating a single pattern is sufficient to implement Hadamard microscopy. To demonstrate this, we designed a metasurface to generate an illumination pattern and conducted imaging simulations. Results showed that holographic metasurface-based Hadamard microscopy effectively suppressed scattering signals, resulting in clear fluorescent images. Furthermore, we demonstrated that hyperspectral imaging can be achieved with Hadamard microscopy using dispersive optical elements, as the orthogonality of the Hadamard pattern enables to resolve spectral information. The reconstructed hyperspectral images displayed a color distribution closely matching the synthetic hyperspectral images used as ground truth. Our findings suggest that optical sectioning and hyperspectral imaging can be accomplished without light modulation devices, a capability typically unattainable with standard wide-field microscopes. We showed that sophisticated metasurfaces have the potential to replace and enhance conventional optical components, and we anticipate that this study will contribute to advancements in metasurface-based optical microscopy.

We theoretically investigate the tunneling of two-dimensional surface polaritons (2DSPs) through plasmonic nanoplates on atomically thin crystals. By developing an analytic model based on coupled-mode theory, we examine the tunneling efficiency as a function of the plate’s width and height. Our analysis reveals that the tunneling is mediated by evanescent waves induced near the plate edges due to interactions between 2DSPs and the plate, enabling the consistent recoupling of 2DSPs across plates of finite width, regardless of their height. Additionally, we clarify that these evanescent waves not only facilitate tunneling but also induce an anomalous phase shift in the 2DSPs reflected from the plate. This phase shift is shown to play a critical role in the resonant tunneling of 2DSPs through two plates, effectively creating a polariton-assisted resonator in the deep subwavelength regime.

Microdisk lasers have emerged as compact on-chip optical sensors due to their small size, simple structure, and efficient lasing capabilities. However, conventional microdisk laser sensors face challenges in enhancing interactions with external analytes, as their energy remains predominantly confined within the laser material. In this study, we present a novel microdisk laser sensor incorporating periodic meta-hole patterning, designed to enhance external interaction while maintaining the integrity of the whispering gallery mode (WGM). Numerical simulations show that in an InGaAsP microdisk laser (5 μm diameter, 250 nm thickness), the WGM remains stable with periodic meta-holes (period a = 340 nm, diameter d < 0.4 a ), achieving a resonant wavelength near 1,500 nm. The inclusion of meta-holes led to a substantial improvement in sensitivity, reaching up to 100.8 nm/RIU – a 2.26-fold increase over nonpatterned microdisks. Experimental validation confirmed lasing in structures with a d / a ratio of 0.32, achieving a maximum sensitivity of 74.5 nm/RIU, which represents a 2.02-fold enhancement compared to nonpatterned designs. This advancement in microdisk laser design not only opens new possibilities for high-performance, miniaturized optical sensors but also holds significant potential for integration into next-generation on-chip sensing technologies.

High resolution imaging represents a relentless pursuit within the field of optical system. Multi-frame super-resolution (SR) is an effective method for enhancing sampling density, while it heavily relies on sub-pixel scale displacement of a bulky camera. Based on the symmetric transformation of quadratic-phase metasurface, we propose scaled transverse translation (STT) utilizing planar optical elements (POEs) to facilitate sub-pixel sampling and remote super-resolution imaging. The STT module composed of a pair of planar optical elements with conjugated quadratic phase profile is fabricated and experimentally verified. By displacing POE within a millimeter-level range, we achieve sub-micron in imaging shift accuracy. Furthermore, the results of SR and SR enhanced Fourier ptychography imaging demonstrate significant compatibility and effectiveness of this module. The resolution improvement in FP imaging increases from 2× to 2.8× by sub-pixel sampling using this module. Moreover, defect reduction and contrast enhancement are obtained. With its advantages of light-weight, simple structure and ease of implementation, this method shows considerable potential for numerous imaging applications.

A new type of optical microscopy based on hyperbolic polariton-coupled emission (HPCE) is demonstrated. By employing hyperbolic metamaterials as the substrate, we show a nearly 6-fold increase in fluorescence intensity in the HPCE microscope compared to total internal reflection fluorescence (TIRF) on glass substrates. Moreover, we achieve precise, time-dependent control of the fluorescence intensity by modulating the incidence angle with a galvo scanner. This tunability offers extensive potential for applications in super-resolution fluorescence microscopy and high-sensitivity sensing, enabling real-time fluorescence intensity adjustment.

Littrow diffraction devices are commonly used in the laser field (e.g., laser resonators and spectrometers), where system integration requires larger incidence angles and perfect broadband efficiency. Compared to traditional diffraction devices, which struggle to manipulate light paths under large-angle incidence, metasurfaces has the potential to enhance the broadband efficiency. Despite quasi three-dimensional metasurfaces effects, only perfect anomalous reflection under normal incidence at limited wavelengths was achieved due to energy flow mismatch in the broadband Littrow configuration. Here, we propose a supercell metasurface capable of regulating broadband non-local responses. The metasurface effectively suppresses non-local responses under Littrow mounting, while providing sufficient non-local responses through strong structural coupling effects when the incidence deviates from the Littrow mounting. A large-angle broadband Littrow diffraction metasurface in the mid-infrared spectrum (3.11 µm ∼ 3.52 µm) has been successfully realized, with 99 % efficiency at Littrow angle of 70°. Our results break through the bandwidth limitations of perfect diffraction, providing robust support for the practical applications of metasurfaces in Littrow diffraction devices.

Complex power, also known as alternating current (AC) power, is a well-established concept in an electric circuit composed of resistive and reactive elements. On the other hand, the role of complex power in optics has been elusive. In this work, we reveal that the complex energy and momentum determine the resonance frequency and the decay rate of open cavity resonance, the so-called quasinormal modes (QNMs), respectively. We also demonstrate the role of the complex energy and momentum in typical open cavities analytically and numerically: the Fabry–Perot cavity, the surface plasmon polaritons (SPPs), the plasmonic nanorod, the nanosphere, and the dielectric supercavity.

An intermediate mirror has been proposed to enhance multijunction solar cell efficiency by selectively reflecting the light beyond higher energy bandgap of top cell, while simultaneously transmitting the rest of lower-energy light. Therefore, it reduces the higher-energy absorption spectral tail of the bottom cell (thermalization loss) and increase the absorption in the top cell. However, its effectiveness has only been theoretically validated in simplified tandem with basic components such as an antireflection coating (ARC) and top/bottom absorbers. In contrast, experimentally optimized tandem cells, such as perovskite (PVK)/silicon (Si) two-terminal configurations, include additional stacked electrodes, ultrathinned intermediate electrode, and random textures to maximize efficiency. Herein, we revisited the role of the intermediate mirror in these advanced tandem cells. Our results show that the incorporation of ideal intermediate mirror (IIM) does not improve efficiency both in textured and flat tandem cells, with its theoretical upper limit of efficiency being similar to or even lower than that of experimentally optimized cells.

In traditional optical imaging, image sensors only record intensity information, and phase information of transparent samples such as cells and semiconductor materials is hard to obtain. Quantitative phase imaging techniques are crucial for obtaining detailed phase information, but current methods often require complex interferometric setups or mechanical adjustments, limiting their practical applicability. Here, we proposed a novel meta-device integrating a PB phase-based meta-lens, a refractive lens, and an electronically tunable lens with a polarization camera to capture multiple defocused images simultaneously for the transport of intensity equation-based phase retrieval algorithm. By leveraging the distinct focus lengths for left-circularly polarized and right-circularly polarized light, the meta-device eliminates the need for multiple shots and mechanical movements. Our approach enables rapid, precise, quantitative phase imaging at different depths. The experiment shows the accuracy of our methods is 98.47 % and with a 2.52 mm depth range of the objects that can be retrieved, making it highly suitable for dynamic and depth-varying samples, such as cells in solution.

Terahertz magneto-plasmonics plays a crucial role in platforms for isolation and sensing applications, operating at terahertz frequencies. In spite of recent efforts to enhance magneto-optic effects using metasurfaces, the mechanism for optimizing these effects remains unclear in the terahertz regime. Here we investigate terahertz magneto-optic effects using 100 nm-thick iron slot antennas with varying widths, ranging from 20 µm to 300 nm. Interestingly, as the width of slot antenna decreases, this enhancement peaks around 1 µm, after which the effect diminishes for smaller widths. Based on the effective medium theory, the slot antennas exhibit a maximum Faraday rotation angle near the epsilon-near-zero region. Although the field enhancements in the slot become stronger with the sub-micron widths, the magneto-optic effect may decrease with increasing effective dielectric constant due to gap plasmon effects in the sub-micron region. Our findings provide essential criteria for designing ferromagnetic metasurfaces with enhanced Faraday rotations at terahertz frequencies.

Structured light is a widely used 3D imaging method with a drawback that it typically requires a long baseline length between the laser projector and the camera sensor, which hinders its utilization in space-constrained scenarios. On the other hand, the application of passive 3D imaging methods, such as depth from depth-dependent point spread functions (PSFs), is impeded by the challenge in measuring textureless scenes. Here, we combine the advantages of both structured light and depth-dependent PSFs and propose a baseline-free structured light 3D imaging system. A metasurface is designed to project a structured dot array and encode depth information in the double-helix pattern of each dot simultaneously. Combined with a straightforward and fast algorithm, we demonstrate accurate 3D point cloud acquisition for various real-world scenarios including multiple cardboard boxes and a living human face. Such a technique may find application in a broad range of areas including consumer electronics and precision metrology.

Large language models (LLMs) have gained significant prominence in language processing, demonstrating remarkable performance across a wide range of tasks. Recently, LLMs have been explored in various scientific fields beyond language-based tasks. However, their application in the design of nanophotonic devices remains less explored. Here, we investigate the capabilities of LLMs to address nanophotonic design problems without requiring domain-specific expertise of the user. Our findings show that an LLM with in-context learning enables nonexpert users to calculate optical responses of multilayer films via numerical simulations. Through conversational interaction and feedback between the LLM and the user, an optimal design of the multilayer films can be also produced for the user-provided target optical properties. Furthermore, we fine-tune the LLM using text-based representations of the structure and properties of optical metasurfaces. We demonstrate that the fine-tuned LLM can generate metasurface designs with target properties by reversing the input and output text. This research highlights the potential of LLMs to expedite the nanophotonic design process and to make it more accessible to a wider audience.

Holography is a highly desired technology in modern photonics, yet setups for traditional generating methods suffer from complexity and bulky sizes. While metasurface-based holography exhibits advantages of compactness and easy-fabrication, most meta-holograms realized so far exhibit only single functionality, with a few multifunctional ones suffering from imbalances of efficiency and device-thickness. Here, we propose a generic approach to design ultra-thin metasurfaces for realization of multiple holographic images with high efficiencies , and experimentally verify the concept in the telecom regime. We first design a series of high-efficiency reflective meta-atoms exhibiting incident-spin-delinked reflection phases governed by geometric and resonant mechanisms, and experimentally characterize their optical properties at wavelengths around 1,064 nm. We next experimentally demonstrate a single-functional meta-hologram as a benchmark test. Finally, we employ the designed meta-atoms to construct a metasurface with the thickness ∼1/4 λ , and experimentally demonstrate its capability of generating two distinct holographic images under illuminations of circularly polarized light beams with different helicities, possessing generation efficiencies ∼48.08 %. Our work provides a highly-efficient and ultra-compact platform to generate multifunctional holographic images, which may inspire numerous applications in integration optics.

Metasurfaces, two-dimensional planar optical devices based on subwavelength-scale structures, have garnered significant attention for their potential to replace conventional optical components in various fields. These devices can manipulate the amplitude, phase, and polarization of light in versatile ways, offering complex functionalities within a single, space-efficient device. However, enhancing their functionality remains a challenge, requiring an expansion in the design flexibility of the structural elements, known as meta-atoms. In this study, we revealed that by varying the two independent lengths of the cross-shaped structure at a wavelength of 980 nm, a phase singularity exists in the region of high reflection. In addition, we found that the phase of transmitted light can be modulated from 0 to 2 π by encircling this singularity. Based on the identified phase singularity, we designed and fabricated a polarization-independent metalens with varying numerical apertures to experimentally validate the feasibility of high-reflectivity transmissive wavefront engineering metasurfaces. The introduced meta-atoms based on a phase singularity are expected to open new avenues for applications, such as those requiring light attenuation and concentration simultaneously or the development of resonant cavity structures capable of beam modulation.

Exceptional points (EPs) in non-Hermitian systems have attracted significant interest due to their unique behaviors, including novel wave propagation and radiation. While EPs have been explored in various photonic systems, their integration into standard photonic platforms can expand their applicability to broader technological domains. In this work, we propose and experimentally demonstrate EPs in an integrated photonic strip waveguide configuration, exhibiting unique deep wave penetration and uniform-intensity radiation profiles. By introducing the second-order grating on one side of the waveguide, forward and backward propagating modes are coupled both directly through second-order coupling and indirectly through first-order coupling via a radiative intermediate mode. To describe the EP behavior in a strip configuration, we introduce modified coupled-mode equations that account for both transverse and longitudinal components. These coupled-mode formulas reveal the formation of EPs in bandgap closure, achieved by numerically optimizing the grating’s duty cycle to manipulate the first- and second-order couplings simultaneously. Experimental observations, consistent with simulations, confirm the EP behavior, with symmetric transmission spectra and constant radiation profiles at the EP wavelength, in contrast to conventional exponential decay observed at detuned wavelengths. These results demonstrate the realization of EPs in a widely applicable strip waveguide configuration, paving the way for advanced EP applications in nonlinear and ultrafast photonics, as well as advanced sensing technologies.