Hydrogels for active photonics

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.

Tailoring high-refractive-index nanocomposites for manufacturing of ultraviolet metasurfaces

Nanoimprint lithography(NIL)has been utilized to address the manufacturing challenges of high cost and low throughput for optical metasurfaces.To overcome the limitations inherent in conventional imprint resins characterized by a low refractive index(n),high-n nanocomposites have been introduced to directly serve as meta-atoms.However,comprehensive research on these nanocomposites is notably lacking.In this study,we focus on the composition of high-n zirconium dioxide(ZrO_(2))nanoparticle(NP)concentration and solvents used to produce ultraviolet(UV)metaholograms and quantify the transfer fidelity by the measured conversion efficiency.The utilization of 80 wt%ZrO_(2)NPs in MIBK,MEK,and acetone results in conversion efficiencies of 62.3%,51.4%,and 61.5%,respectively,at a wavelength of 325 nm.The analysis of the solvent composition and NP concentration can further enhance the manufacturing capabilities of high-n nanocomposites in NIL,enabling potential practical use of optical metasurfaces.

Arbitrarily structured quantum emission with a multifunctional metalens

Structuring light emission from single-photon emitters(SPEs)in multiple degrees of freedom is of great importance for quantum information processing towards higher dimensions.However,traditional control of emission from quantum light sources relies on the use of multiple bulky optical elements or nanostructured resonators with limited functionalities,constraining the potential of multi-dimensional tailoring.Here we introduce the use of an ultrathin polarisation-beam-splitting metalens for the arbitrary structuring of quantum emission at room temperature.Owing to the complete and independent polarisation and phase control at the single meta-atom level,the designed metalens enables simultaneous mapping of quantum emission from ultra-bright defects in hexagonal boron nitride and imprinting of an arbitrary wavefront onto orthogonal polarisation states of the sources.The hybrid quantum metalens enables simultaneous manipulation of multiple degrees of freedom of a quantum light source,including directionality,polarisation,and orbital angular momentum.This could unleash the full potential of solid-state SPEs for their use as high-dimensional quantum sources for advanced quantum photonic applications.

One-step printable platform for high-efficiency metasurfaces down to the deep-ultraviolet region

A single-step printable platform for ultraviolet(UV)metasurfaces is introduced to overcome both the scarcity of low-loss UV materials and manufacturing limitations of high cost and low throughput.By dispersing zirconium dioxide(ZrO_(2))nanoparticles in a UV-curable resin,ZrO_(2)nanoparticle-embedded-resin(nano-PER)is developed as a printable material which has a high refractive index and low extinction coefficient from near-UV to deep-UV.In ZrO_(2)nano-PER,the UV-curable resin enables direct pattern transfer and ZrO_(2)nanoparticles increase the refractive index of the composite while maintaining a large bandgap.With this concept,UV metasurfaces can be fabricated in a single step by nanoimprint lithography.As a proof of concept,UV metaholograms operating in near-UV and deep-UV are experimentally demonstrated with vivid and clear holographic images.The proposed method enables repeat and rapid manufacturing of UV metasurfaces,and thus will bring UV metasurfaces more close to real life.

Building an optics and photonics research ecosystem in South Korea:Collaborative innovation between academia and industry

Photonics research is a rapidly growing field that has found applications in various aspects of our lives,ran-ging from communication to medical imaging1-5.Col-laborative innovation between academia and industry has been a driving force behind the significant advances in photonics research in South Korea.In this special issue,we present cutting-edge research in photonics,highlighting the importance of collaboration between academia and industry(Fig.1).

Writing nanometer-scale structures for centimeter-scale color printing

Structural coloration,the production of color with nanoscale structures,has steadily gained attention owing to numerous advantages such as environmental friendliness,long-term durability,and vivid coloration compared to conventional chemical pigments.1–3 In addition,recent de-velopments in the field of nanofabrication have led to an increase in research on structural coloration with sophisticated artificial materials,known as metamaterials.

Electromagnetic chirality:from fundamentals to nontraditional chiroptical phenomena

Chirality arises universally across many different fields.Recent advancements in artificial nanomaterials have demonstrated chiroptical responses that far exceed those found in natural materials.Chiroptical phenomena are complicated processes that involve transitions between states with opposite parities,and solid interpretations of these observations are yet to be clearly provided.In this review,we present a comprehensive overview of the theoretical aspects of chirality in light,nanostructures,and nanosystems and their chiroptical interactions.Descriptions of observed chiroptical phenomena based on these fundamentals are intensively discussed.We start with the strong intrinsic and extrinsic chirality in plasmonic nanoparticle systems,followed by enantioselective sensing and optical manipulation,and then conclude with orbital angular momentum-dependent responses.This review will be helpful for understanding the mechanisms behind chiroptical phenomena based on underlying chiral properties and useful for interpreting chiroptical systems for further studies.

Tunable metasurfaces towards versatilemetalenses and metaholograms:a review

Metasurfaces have attracted great attention due to their ability to manipulate the phase,amplitude,and polarization of light in a compact form.Tunable metasurfaces have been investigated recently through the integration with mechanically moving components and electrically tunable elements.Two interesting applications,in particular,are to vary the focal point of metalenses and to switch between holographic images.We present the recent progress on tunable metasurfaces focused on metalenses and metaholograms,including the basic working principles,advantages,and disadvantages of each working mechanism.We classify the tunable stimuli based on the light source and electrical bias,as well as others such as thermal and mechanical modulation.We conclude by summarizing the recent progress of metalenses and metaholograms,and providing our perspectives for the further development of tunable metasurfaces.

Nanoimprint lithography for high-throughput fabrication of metasurfaces

Metasurfaces are composed of periodic subwavelength nanostructures and exhibit optical properties that are not found in nature.They have been widely investigated for optical applications such as holograms,wavefront shaping,and structural color printing,however,electron-beam lithography is not suitable to produce large-area metasurfaces because of the high fabrication cost and low productivity.Although alternative optical technologies,such as holographic lithography and plasmonic lithography,can overcome these drawbacks,such methods are still constrained by the optical diffraction limit.To break through this fundamental problem,mechanical nanopatteming processes have been actively studied in many fields,with nanoimprint lithography(NIL)coming to the forefront.Since NIL replicates the nanopattem of the mold regardless of the diffraction limit,NIL can achieve sufficiently high productivity and patterning resolution,giving rise to an explosive development in the fabrication of metasurfaces.In this review,we focus on various NIL technologies for the manufacturing of metasurfaces.First,we briefly describe conventional NIL and then present various NIL methods for the scalable fabrication of metasurfaces.We also discuss recent applications of NIL in the realization of metasurfaces.Finally,we conclude with an outlook on each method and suggest perspectives for future research on the high-throughput fabrication of active metasurfaces.

Liquid crystal-powered Mie resonators for electrically tunable photorealistic color gradients and dark blacks

Taking inspiration from beautiful colors in nature,structural colors produced from nanostructured metasurfaces have shown great promise as a platform for bright,highly saturated,and high-resolution colors.Both plasmonic and dielectric materials have been employed to produce static colors that fulfil the required criteria for high-performance color printing,however,for practical applications in dynamic situations,a form of tunability is desirable.Combinations of the additive color palette of red,green,and blue enable the expression of further colors beyond the three primary colors,while the simultaneous intensity modulation allows access to the full color gamut.Here,we demonstrate an electrically tunable metasurface that can represent saturated red,green,and blue pixels that can be dynamically and continuously controlled between on and off states using liquid crystals.We use this to experimentally realize ultrahigh-resolution color printing,active multicolor cryptographic applications,and tunable pixels toward high-performance full-color reflective displays.

Geometric and physical configurations of meta-atoms for advanced metasurface holography

Metasurfaces consisting of subwavelength structures,so-called meta-atoms,have steadily attracted considerable attention for advanced holography due to their advantages in terms of high-resolution holographic images,large field of view,and compact device volume.In contrast to conventional holographic displays using bulky conventional diffractive optical elements,metasurface holography enables arbitrary complex wavefront shaping with a much smaller footprint.In this review,we classify metasurface holography according to the meta-atom design methodologies,which can further expand hologram functionalities.We describe light-matter interactions,particularly in metasurface systems,using the relevant the Jones matrix to rigorously explain modulations of the amplitude,phase,and polarization of light.Six different types of metaatoms are presented,and the corresponding achievable wavefronts that form the holographic images in the far-field are also provided.Such a simple classification will give a straightforward approach to design and further realize advanced metasurface holographic devices.

Computation at the speed of light:metamaterials for all-optical calculations and neural networks

The explosion in the amount of information that is being processed is prompting the need for new computing systems beyond existing electronic computers.Photonic computing is emerging as an attractive alternative due to performing calculations at the speed of light,the change for massive parallelism,and also extremely low energy consumption.We review the physical implementation of basic optical calculations,such as differentiation and integration,using metamaterials,and introduce the realization of all-optical artificial neural networks.We start with concise introductions of the mathematical principles behind such optical computation methods and present the advantages,current problems that need to be overcome,and the potential future directions in the field.We expect that our review will be useful for both novice and experienced researchers in the field of all-optical computing platforms using metamaterials.

Optical spin-symmetry breaking for high-efficiency directional helicity-multiplexed metaholograms

Helicity-multiplexed metasurfaces based on symmetric spin–orbit interactions (SOIs) have practical limits because they cannot provide central-symmetric holographic imaging. Asymmetric SOIs can effectively address such limitations, with several exciting applications in various fields ranging from asymmetric data inscription in communications to dual side displays in smart mobile devices. Low-loss dielectric materials provide an excellent platform for realizing such exotic phenomena efficiently. In this paper, we demonstrate an asymmetric SOI-dependent transmission-type metasurface in the visible domain using hydrogenated amorphous silicon (a-Si:H) nanoresonators. The proposed design approach is equipped with an additional degree of freedom in designing bi-directional helicity-multiplexed metasurfaces by breaking the conventional limit imposed by the symmetric SOI in half employment of metasurfaces for one circular handedness. Two on-axis, distinct wavefronts are produced with high transmission efficiencies, demonstrating the concept of asymmetric wavefront generation in two antiparallel directions. Additionally, the CMOS compatibility of a-Si:H makes it a cost-effective alternative to gallium nitride (GaN) and titanium dioxide (TiO2) for visible light. The cost-effective fabrication and simplicity of the proposed design technique provide an excellent candidate for high-efficiency, multifunctional, and chip-integrated demonstration of various phenomena.

Structural color switching with a doped indium-gallium-zinc-oxide semiconductor

Structural coloration techniques have improved display science due to their high durability in terms of resistance to bleaching and abrasion,and low energy consumption.Here,we propose and demonstrate an all-solid-state,large-area,lithography-free color filter that can switch structural color based on a doped semiconductor.Particularly,an indium-gallium-zinc-oxide(IGZO)thin film is used as a passive index-changing layer.The refractive index of the IGZO layer is tuned by controlling the charge carrier concentration;a hydrogen plasma treatment is used to control the conductivity of the IGZO layer.In this paper,we verify the color modulation using finite difference time domain simulations and experiments.The IGZO-based color filter technology proposed in this study will pave the way for charge-controlled tunable color filters displaying a wide gamut of colors on demand.

Giant chiro-optical responses in multipolar-resonances-based single-layer dielectric metasurfaces

Chiro-optical effects offer a wide range of potential applications in nanophotonics,such as advanced imaging and molecular sensing and separation.Flat single-layer metasurfaces composed of subwavelength meta-atoms have gained significant attention due to their exceptional characteristics in light–matter interactions.Although metasurface-based devices have manipulated electromagnetic waves,the compact on-chip realization of giant chiro-optical effects remains a challenge at optical frequencies.In this work,we experimentally and numerically demonstrate an all-dielectric metasurface to realize large chiro-optical effects in the visible regime.Notably,the proposed strategy of utilizing achiral nanofins instead of conventional chiral structures provides an extra degree of design freedom.The mutual coupling between carefully engineered nanofins produces constructive and destructive interference,leading to the asymmetric transmission of 70%and average circular dichroism exceeding 60%.We investigate the underlying mechanism behind the chiro-optical effects using the theory of multipolar decomposition.The proposed design mechanism maximizes the chiro-optical response through a single-layer metasurface with potential applications in high-efficiency integrated ultrathin polarization rotators and shapers,chiral polarizers for optical displays,chiral beam splitters,and chiral sensors.

Hyperbolic metamaterials: fusing artificial structures to natural 2D materials

Optical metamaterials have presented an innovative method of manipulating light.Hyperbolic metamaterials have an extremely high anisotropy with a hyperbolic dispersion relation.They are able to support high-k modes and exhibit a high density of states which produce distinctive properties that have been exploited in various applications,such as super-resolution imaging,negative refraction,and enhanced emission control.Here,state-of-the-art hyperbolic metamaterials are reviewed,starting from the fundamental principles to applications of artificially structured hyperbolic media to suggest ways to fuse natural two-dimensional hyperbolic materials.The review concludes by indicating the current challenges and our vision for future applications of hyperbolic metamaterials.

High numerical aperture RGB achromatic metalens in the visible

We theoretically and experimentally demonstrate an RGB achromatic metalens that operates concurrently at three visible wavelengths(λ=450, 532, and 700 nm) with a high numerical aperture of 0.87. The RGB metalens is designed by simple integration of metalens components with the spatial interleaving method. The simulated spatial interleaving metalens shows RGB achromatic operation with focusing efficiencies of 25.2%, 58.7%, and66.4% at the wavelengths of 450, 532, and 700 nm, respectively. A 450 μm diameter metalens operating at three designated wavelengths is fabricated with low-loss hydrogenated amorphous silicon. The fabricated metalens has the measured focusing efficiencies of 5.9%, 11.3%, and 13.6% at λ = 450, 532, and 700 nm, respectively. The Strehl ratios of 0.89, 0.88, and 0.82 are obtained at given wavelengths, which show a capability of diffractionlimited operation.

Thermally-curable nanocomposite printing for the scalable manufacturing of dielectric metasurfaces

Metasurfaces consisting of artificially designed meta-atoms have been popularized recently due to their advantages of amplitude and phase of light control.However,the electron beam lithography method for metasurface fabrication has high cost and low throughput,which results in a limitation for the fabrication of metasurfaces.In this study,nanocomposite printing technology is used to fabricate high-efficiency metasurfaces with low cost.To demonstrate the efficiency of the proposed fabrication method,a metahologram is designed and fabricated using a nanocomposite.The metahologram exhibits conversion efficiencies of 48%and 35%at wavelengths of 532 and 635 nm,respectively.The nanocomposite is composed of polymers with nanoparticles,so durability tests are also performed to evaluate the effects of temperature and humidity on the metasurfaces.The test verifies that at temperatures below the glass transition temperature of the base resin,the nanostructures do not collapse,so the efficiency of the metasurfaces remains almost the same.The surrounding humidity does not affect the nanostructures at all.Hence,the durability of the nanocomposite metasurfaces can be further enhanced by replacing the base resin,and this nanocomposite printing method will facilitate practical metasurface use at low cost.

Unlocking the future of optical security with metasurfaces

The complex degrees of freedom of light,such as amplitude,phase,polarization,and orbital angular momentum,make it a prime candidate for use in optical security and encryption.By exploiting the unique characteristics of metasurfaces,exciting new optical security platforms have been demonstrated.