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Abstract Remarkable advancements have been made in the design of optical metasurfaces in recent years, particularly in compact designs. However, for their practical integration into diverse optical systems, there is a pressing need for metasurfaces to transition toward larger areas without compromising their performance. From a design perspective, efforts in the design process must focus on reducing computational costs and enhancing performance in larger areas. In this review, we introduce diverse optical analyses applicable to wide areas, including the modification of boundary conditions, fast multipole methods, coupled mode theory, and neural network–based approaches. In addition, inverse design methods based on the adjoint method or deep learning, which are suitable for large-scale designs, are described. Numerous fast and accurate simulation methods make it possible to assess optical properties over large areas at a low cost, whereas diverse inverse design methods hold promise for high performance. By concurrently addressing both the essential aspects of designing large-area metasurfaces, we comprehensively discuss various approaches to develop metasurfaces with high performance over expansive regions. Finally, we outline additional challenges and prospects for realizing mass-produced high-performance metasurfaces, unlocking their full potential for optical applications.

Metasurfaces have opened the door to next-generation optical devices due to their ability to dramatically modulate electromagnetic waves at will using periodically arranged nanostructures. However, metasurfaces typically have static optical responses with fixed geometries of nanostructures, which poses challenges for implementing transition to technology by replacing conventional optical components. To solve this problem, liquid crystals (LCs) have been actively employed for designing tunable metasurfaces using their adjustable birefringent in real time. Here, we review recent studies on LC-powered tunable metasurfaces, which are categorized as wavefront tuning and spectral tuning. Compared to numerous reviews on tunable metasurfaces, this review intensively explores recent development of LC-integrated metasurfaces. At the end of this review, we briefly introduce the latest research trends on LC-powered metasurfaces and suggest further directions for improving LCs. We hope that this review will accelerate the development of new and innovative LC-powered devices.

Abstract Conventional photonic devices exhibit static optical properties that are design-dependent, including the material’s refractive index and geometrical parameters. However, they still possess attractive optical responses for applications and are already exploited in devices across various fields. Hydrogel photonics has emerged as a promising solution in the field of active photonics by providing primarily deformable geometric parameters in response to external stimuli. Over the past few years, various studies have been undertaken to attain stimuli-responsive photonic devices with tunable optical properties. Herein, we focus on the recent advancements in hydrogel-based photonics and micro/nanofabrication techniques for hydrogels. In particular, fabrication techniques for hydrogel photonic devices are categorized into film growth, photolithography (PL), electron-beam lithography (EBL), and nanoimprint lithography (NIL). Furthermore, we provide insights into future directions and prospects for deformable hydrogel photonics, along with their potential practical applications.

Abstract Nanoimprint lithography (NIL) has attracted attention recently as a promising fabrication method for dielectric metalenses owing to its low cost and high throughput, however, high aspect ratio (HAR) nanostructures are required to manipulate the full 2π phase of light. Conventional NIL using a hard-polydimethylsiloxane (h-PDMS) mold inevitably incurs shear stress on the nanostructures which is inversely proportional to the surface area parallel to the direction of detachment. Therefore, HAR structures are subjected to larger shear stresses, causing structural failure. Herein, we propose a novel wet etching NIL method with no detachment process to fabricate flawless HAR metalenses. The water-soluble replica mold is fabricated with polyvinyl alcohol (PVA) which is simpler than an h-PDMS mold, and the flexibility of the PVA mold is suitable for direct printing as its high tensile modulus allows high-resolution patterning of HAR metalenses. The diffraction-limited focusing of the printed metalenses demonstrates that it operates as an ideal lens in the visible regime. This method can potentially be used for manufacturing various nanophotonic devices that require HAR nanostructures at low cost and high throughput, facilitating commercialization.

Abstract Metasurfaces have been continuously garnering attention in both scientific and industrial fields, owing to their unprecedented wavefront manipulation capabilities using arranged subwavelength artificial structures. To date, research has mainly focused on the full control of electromagnetic characteristics, including polarization, phase, amplitude, and even frequencies. Consequently, versatile possibilities of electromagnetic wave control have been achieved, yielding practical optical components such as metalenses, beam-steerers, metaholograms, and sensors. Current research is now focused on integrating the aforementioned metasurfaces with other standard optical components (e.g., light-emitting diodes, charged-coupled devices, micro-electro-mechanical systems, liquid crystals, heaters, refractive optical elements, planar waveguides, optical fibers, etc.) for commercialization with miniaturization trends of optical devices. Herein, this review describes and classifies metasurface-integrated optical components, and subsequently discusses their promising applications with metasurface-integrated optical platforms including those of augmented/virtual reality, light detection and ranging, and sensors. In conclusion, this review presents several challenges and prospects that are prevalent in the field in order to accelerate the commercialization of metasurfaces-integrated optical platforms.

Abstract Multi‐functional metasurfaces have attracted great attention due to the significant possibilities to realize highly integrated and ultra‐compact meta‐devices. Merging nano‐printing and holographic information multiplexing is one of the effective ways to achieve multi‐functionality, and such a merger can increase the information encoding capacity. However, the current approaches rely on stacking layers and interleaving, where multiple resonators effectively combine different functionalities on the cost of efficiency, design complexity, and challenging fabrication. To address such challenges, a single meta‐nanoresonator‐based tri‐functional metasurface is proposed by combining the geometric phase‐based spin‐decoupling and Malus's law intensity modulation. The proposed strategy effectively improves information capacity owing to the orientation degeneracy of spin‐decoupling rather than layer stacking or super‐cell designs. To validate the proposed strategy, a metasurface demonstrating two helicity‐dependent holographic outputs is presented in far‐field, whereas a continuous nano‐printing image is in near‐field. It is also employed on CMOS‐compatible and cost‐effective hydrogen amorphous silicon providing transparent responses for the whole visible band. As a result, the proposed metasurface has high transmission efficiency in the visible regime and verifies the design strategy without adding extra complexities to conventional nano‐pillar geometry. Therefore, the proposed metasurface opens new avenues in multi‐functional meta‐devices design and has promising applications in anti‐counterfeiting, optical storage and displays.​

In the field of nanophotonics, metasurfaces have come to the forefront in real‐world applications, owing to their accessible exotic optical properties that can be readily designed and fabricated using currently available techniques. The subwavelength dimensions and lightweight characteristics of metasurfaces are attractive qualities for the miniaturization of optical devices and are already exploited in devices that can rival, and sometimes even outperform, conventional bulky optics. Over the past decade, ample research has been undertaken to produce high‐performance metasurfaces with exciting optical properties that cannot be found in nature or achieved with conventional optics. To open the path to widely used devices in our everyday lives, the next obvious step for metasurfaces is the development of tunability. Herein, the techniques and development of applications of tunable metasurfaces are presented through the incorporation of active materials and controllable external stimuli, and the uncovered tunable characteristics are critically analyzed. The review rounds up by proposing the future directions and prospects for actively tunable metasurfaces and their potential practical applications.