Liquid crystal-integrated metasurfaces for an active photonic platform

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 LCpowered 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.

Switchable diurnal radiative cooling by doped VO_(2)

This paper presents design and simulation of a switchable radiative cooler that exploits phase transition in vanadium di-oxide to turn on and off in response to temperature.The cooler consists of an emitter and a solar reflector separated by a spacer.The emitter and the reflector play a role of emitting energy in mid-infrared and blocking incoming solar energy in ultraviolet to near-infrared regime,respectively.Because of the phase transition of doped vanadium dioxide at room tem-perature,the emitter radiates its thermal energy only when the temperature is above the phase transition temperature.The feasibility of cooling is simulated using real outdoor conditions.We confirme that the switchable cooler can keep a desired temperature,despite change in environmental conditions.

Integrated metasurfaces for re-envisioning a near-future disruptive optical platform

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.

Emerging low-cost,large-scale photonic platforms with soft lithography and self-assembly

Advancements in micro/nanofabrication have enabled the realization of practical micro/nanoscale photonic devices such as absorbers,solar cells,metalenses,and metaholograms.Although the performance of these photonic devices has been improved by enhancing the design flexibility of structural materials through advanced fabrication methods,achieving large-area and high-throughput fabrication of tiny structural materials remains a challenge.In this aspect,various technologies have been investigated for realizing the mass production of practical devices consisting of micro/nanostructural materials.This review describes the recent advancements in soft lithography,colloidal self-assembly,and block copolymer self-assembly,which are promising methods suitable for commercialization of photonic applications.In addition,we introduce low-cost and large-scale techniques realizing micro/nano devices with specific examples such as display technology and sensors.The inferences presented in this review are expected to function as a guide for promising methods of accelerating the mass production of various sub-wavelength-scale photonic devices.

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.

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.

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.

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.

On-demand design of spectrally sensitive multiband absorbers using an artificial neural network

We report an approach assisted by deep learning to design spectrally sensitive multiband absorbers that work in the visible range.We propose a five-layered metal-insulator-metal grating structure composed of aluminum and silicon dioxide,and we design its structural parameters by using an artificial neural network(ANN).For a spectrally sensitive design,spectral information of resonant wavelengths is additionally provided as input as well as the reflection spectrum.The ANN facilitates highly robust design of a grating structure that has an average mean squared error(MSE)of 0.023.The optical properties of the designed structures are validated using electromagnetic simulations and experiments.Analysis of design results for gradually changing target wavelengths of input shows that the trained ANN can learn physical knowledge from data.We also propose a method to reduce the size of the ANN by exploiting observations of the trained ANN for practical applications.Our design method can also be applied to design various nanophotonic structures that are particularly sensitive to resonant wavelengths,such as spectroscopic detection and multi-color applications.

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.

Capillary-force-induced collapse lithography for controlled plasmonic nanogap structures

The capillary force effect is one of the most important fabrication parameters that must be considered at the micro/nanoscale because it is strong enough to deform micro/nanostructures.However,the deformation of micro/nanostructures due to such capillary forces(e.g.,stiction and collapse)has been regarded as an undesirable and uncontrollable obstacle to be avoided during fabrication.Here,we present a capillary-force-induced collapse lithography(CCL)technique,which exploits the capillary force to precisely control the collapse of micro/nanostructures.CCL uses electron-beam lithography,so nanopillars with various shapes can be fabricated by precisely controlling the capillary-force-dominant cohesion process and the nanopillar-geometry-dominant collapse process by adjusting the fabrication parameters such as the development time,electron dose,and shape of the nanopillars.CCL aims to achieve sub-10-nm plasmonic nanogap structures that promote extremely strong focusing of light.CCL is a simple and straightforward method to realize such nanogap structures that are needed for further research such as on plasmonic nanosensors.