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.

Special issue on digital and intelligent optics

Innovations in artificial intelligence have revolutionized various areas,especially optics.The rapid development of novel optoelectronic devices at nanoscale has exhibited the multiple functionalities,high integration,compactness,fast modulation and scalability,showcasing new breakthroughs of digital optics and optoelectronic processors for intelligent computations.

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.

High Refractive Index Ti3O5 Films for Dielectric Metasurfaces

Ti33O55 films are deposited with the help of an electron beam evaporator for their applications in metasurfaces. The film of subwavelength （632nm） thickness is deposited on a silicon substrate and annealed at 400℃. The ellipsometry result shows a high refractive index above 2.5 with the minimum absorption coefficient in the visible region, which is necessary for high efficiency of transparent metasurfaces. Atomic force microscopy analysis is employed to measure the roughness of the as-deposited films. It is seen from micrographs that the deposited films are very smooth with the minimum roughness to prevent scattering and absorption losses for metasurface devices. The absence of grains and cracks can be seen by scanning electron microscope analysis, which is favorable for electron beam lithography. Fourier transform infrared spectroscopy reveals the transmission and reflection obtained from the film deposited on glass substrates. The as-deposited film shows high transmission above 60%, which is in good agreement with metasurfaces.

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.

Iso-propagation vortices:a breakthrough in optical communication technology

The rapid advance of optical communication technology requires improvements in data transmission capacity and efficiency.Conventional methods of increasing the transmission capacity such as polarization,wavelength,and spatial mode division multiplexing have shown significant promise,while spatial mode division multiplexing(SDM)offers a new approach through the use of orthogonal spatial modes as distinct communication channels.

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.

Azimuthal rotation-controlled nanoinscribing for continuous patterning of period-and shapetunable asymmetric nanogratings

We present an azimuthal-rotation-controlled dynamic nanoinscribing(ARC-DNI)process for continuous and scalable fabrication of asymmetric nanograting structures with tunable periods and shape profiles.A sliced edge of a nanograting mold,which typically has a rectangular grating profile,slides over a polymeric substrate to induce its burrfree plastic deformation into a linear nanopattern.During this continuous nanoinscribing process,the“azimuthal angle,”that is,the angle between the moving direction of the polymeric substrate and the mold’s grating line orientation,can be controlled to tailor the period,geometrical shape,and profile of the inscribed nanopatterns.By modulating the azimuthal angle,along with other important ARC-DNI parameters such as temperature,force,and inscribing speed,we demonstrate that the mold-opening profile and temperature-and time-dependent viscoelastic polymer reflow can be controlled to fabricate asymmetric,blazed,and slanted nanogratings that have diverse geometrical profiles such as trapezoidal,triangular,and parallelogrammatic.Finally,period-and profile-tunable ARC-DNI can be utilized for the practical fabrication of diverse optical devices,as is exemplified by asymmetric diffractive optical elements in this study.

Structural coloration:advancements and challenges

Structural coloration,a phenomenon where color is created by micro-or nano-structures rather than chemical pigment,has gained traction due to its eco-friendliness,robustness to fading over time,high-resolution capabilities,and tunability[1].As interest grows in next-generation displays like mini-light-emitting diodes(LEDs),micro-LEDs,and high-resolution near-eye displays,structural coloration emerges as a promising solution to meet their demanding specifications.

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.

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.

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.

Optical Orbital Angular Momentum: Thirty Years and Counting

In 1992,Professor Allen’s paper published in Physical Review A indi-cated that laser light with a Laguerre-Gaussian amplitude distribution has an orbital angular momentum(OAM).Since then,conceptual stud-ies on OAM of photons have been attracting continual attention in physics and optics communities.In recent years,we have witnessed the birth and fast development of many break-through technologies driven by the concept of OAM,from advanced light-field manipulation to super-resolution imaging,classical and quantum optical communi-cations,and many others.

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.

Advent of torsional optomechanics from Beth’s legacy

Torsional optomechanics,which involves the transfer of angular mo-mentum from light to matter,has been a vibrant research area since Beth’s pioneering contribution in 1935.Beth proposed a method to measure the transfer of spin angular momentum using a torsional pen-dulum,1 laying the foundation for classical and quantum optomechanics related to particle levitation,trapping,and cooling in modern optics.

Recent advances in 2D,3D and higher-order topological photonics

Over the past decade,topology has emerged as a major branch in broad areas of physics,from atomic lattices to condensed matter.In particular,topology has received significant attention in photonics because light waves can serve as a platform to investigate nontrivial bulk and edge physics with the aid of carefully engineered photonic crystals and metamaterials.Simultaneously,photonics provides enriched physics that arises from spin-1 vectorial electromagnetic fields.Here,we review recent progress in the growing field of topological photonics in three parts.The first part is dedicated to the basics of topological band theory and introduces various two-dimensional topological phases.The second part reviews three-dimensional topological phases and numerous approaches to achieve them in photonics.Last,we present recently emerging fields in topological photonics that have not yet been reviewed.This part includes topological degeneracies in nonzero dimensions,unidirectional Maxwellian spin waves,higher-order photonic topological phases,and stacking of photonic crystals to attain layer pseudospin.In addition to the various approaches for realizing photonic topological phases,we also discuss the interaction between light and topological matter and the efforts towards practical applications of topological photonics.

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.