Mechanically Tunable Dielectric Resonator Metasurfaces at Visible Frequencies

A terahertz meta-sensor array for 2D strain mapping

Large-scale stretchable strain sensor arrays capable of mapping two-dimensional strain distributions have gained interest for applications as wearable devices and relating to the Internet of Things. However, existing strain sensor arrays are usually unable to achieve accurate directional recognition and experience a trade-off between high sensing resolution and large area detection. Here, based on classical Mie resonance, we report a flexible meta-sensor array that can detect the in-plane direction and magnitude of preloaded strains by referencing a dynamically transmitted terahertz (THz) signal. By building a one-to-one correspondence between the intrinsic electrical/magnetic dipole resonance frequency and the horizontal/perpendicular tension level, arbitrary strain information across the meta-sensor array is accurately detected and quantified using a THz scanning setup. Particularly, with a simple preparation process of micro template-assisted assembly, this meta-sensor array offers ultrahigh sensor density (~11.1 cm-2) and has been seamlessly extended to a record-breaking size (110 × 130 mm2), demonstrating its promise in real-life applications.

Multifunctional Metasurface Tuning by Liquid Crystals in Three Dimensions

Optical metasurfaces present remarkable opportunities for manipulating wave propagation in unconventional ways, surpassing the capabilities of traditional optical devices. In this work, we introduce and demonstrate a multifunctional dynamic tuning of dielectric metasurfaces containing liquid crystals (LCs) through an effective three-dimensional (3D) control of the molecular orientation. We theoretically and experimentally study the spectral tuning of the electric and magnetic resonances of dielectric metasurfaces, which was enabled by rotating an external magnetic field in 3D. Our approach allows for the independent control of the electric and magnetic resonances of a metasurface, enabling multifunctional operation. The magnetic field tuning approach eliminates the need for the pre-alignment of LCs and is not limited by a finite set of directions in which the LC molecules can be oriented. Our results open new pathways for realizing dynamically reconfigurable metadevices and observing novel physical effects without the usual limitations imposed by the boundary conditions of LC cells and the external voltage.

Continuously Tunable Optical Modulation Using Vanadium Dioxide Huygens Metasurfaces

Efficient and dynamic light manipulation at small scale is highly desirable for many photonics applications. Active optical metasurfaces represent a useful way of achieving this due to their creative design potential, compact footprint, and low power consumption, paving the way toward the realization of chip-scale photonic devices with tunable optical functionality on demand. Here, we demonstrate a dynamically tunable, dual-function metasurface based on dielectric resonances in vanadium dioxide that is capable of independent active amplitude and phase control without the use of mechanical parts. Significant developments in the nanofabrication of vanadium dioxide have been shown to enable this metasurface. Gradual thermal control of the metasurface yields a computationally predicted continuously tuned amplitude modulation of 19 dB with negligible phase modulation and a continuously tuned phase modulation of 228° with negligible amplitude modulation, both at near-infrared wavelengths. Experimentally, a maximum continuously tuned amplitude modulation of 9.6 dB and phase modulation of 120° are shown, along with demonstration of stable intermediate states and repeated modulation without degradation. Reprogrammable optical functionality can thus be achieved in precisely engineered nanoantenna arrays for adaptive modulation of amplitude and phase of light for applications such as tunable holograms, lenses, and beam deflectors.

Polarization-dependent reconfigurable light field manipulation by liquid-immersion metasurface

Traditional grating lenses can accumulate phase for adjusting wavefronts, and plasmonic resonances can be excited in metasurfaces with discrete structures for optical field modulation. Diffractive and plasma optics have been developing in parallel, with easy processing, small size, and dynamic control advantages. Due to theoretical hybridization, structural design can combine advantages and show great potential value. Changing the shape and size of the flat metasurface can easily produce light field reflections, but changes in height are rarely cross-explored. We propose a graded metasurface with a single-structure periodic arrangement, which can mix the effects of plasmonic resonance and grating diffraction. As for solvents of different polarities, strong polarization-dependent beam reflections are produced, enabling versatile beam convergence and deflection. Dielectric/metal nanostructures with selective hydrophobic/hydrophilic properties can be arranged by the structural material specification to selectively settle the location of the solution in a liquid environment. Furthermore, the wetted metasurface is actively triggered to achieve spectral control and initiate polarization-dependent beam steering in the broadband visible light region. Actively reconfigurable polarization-dependent beam steering has potential applications in tunable optical displays, directional emission, beam manipulation and processing, and sensing technologies.

Tunable ultraviolet polarized light switch based on all-dielectric metasurfaces on a stretchable substrate

Most ultraviolet (UV) passive optics are currently non-tunable and lack external modulation methods because of the poor tunability of wide-bandgap semiconductor materials in UV working media. This study investigates the excitation of magnetic dipole resonances in the solar-blind UV region by hafnium oxide metasurfaces using elastic dielectric polydimethylsiloxane (PDMS). The near-field interactions between the resonant dielectric elements can be modulated by the mechanical strain of the PDMS substrate, which can flatten the structure's resonant peak beyond the solar-blind UV wavelength range, thereby turning on or off the optical switch in the solar-blind UV region. The device has a facile design and can be used in various applications, such as UV polarization modulation, optical communications, and spectroscopy.

Scalable Hydrogel-Based Nanocavities for Switchable Meta-Holography with Dynamic Color Printing

Devices used for meta-optics display are currently undergoing a revolutionary transition from static to dynamic. Despite various tuning strategy demonstrations such as mechanical, electrical, optical, and thermal tunings, a longstanding challenge for their practical application has been the achievement of a conveniently accessible real-life tuning scheme for realizing versatile functionality dynamics outside the laboratory. In this study, we demonstrate a practical tuning strategy to realize a dynamic color printing with a switchable meta-holography exhibition based on hydrogel-based nanocavities. On the basis of the inflation sensitivity of a hydrogel to humidity alteration, its transmissive color was notably tuned from 450 to 750 nm. More intriguingly, by controlling the sample dry/immersed states in real time, we successfully enabled dual-channel switchable meta-holography. With the advantages of facile architecture, daily stimulus with large-area modulation, and high chromaticity, our proposed hydrogel-based nanocavities provide a promising path toward tunable display/encryption, optical sensors, and next-generation display technology.

Flexible Metamaterial Electronics

Over the last decade, extensive efforts have been made on utilizing advanced materials and structures to improve the properties and functionalities of flexible electronics. While the conventional ways are approaching their natural limits, a revolutionary strategy, namely metamaterials, is emerging toward engineering structural materials to break the existing fetters. Metamaterials exhibit supernatural physical behaviors, in aspects of mechanical, optical, thermal, acoustic, and electronic properties that are inaccessible in natural materials, such as tunable stiffness or Poisson's ratio, manipulating electromagnetic or elastic waves, and topological and programmable morphability. These salient merits motivate metamaterials as a brand-new research direction and have inspired extensive innovative applications in flexible electronics. Here, such a groundbreaking interdisciplinary field is first coined as "flexible metamaterial electronics," focusing on enhancing and innovating functionalities of flexible electronics via the design of metamaterials. Herein, the latest progress and trends in this infant field are reviewed while highlighting their potential value. First, a brief overview starts with introducing the combination of metamaterials and flexible electronics. Then, the developed applications are discussed, such as self-adaptive deformability, ultrahigh sensitivity, and multidisciplinary functionality, followed by the discussion of potential prospects. Finally, the challenges and opportunities facing flexible metamaterial electronics to advance this cutting-edge field are summarized.

High-efficiency, four-channel beam splitter based on a fishnet-shaped continuous metasurface

Beam splitters play important roles in several optical systems. Due to the growing demand for the miniaturization of optical systems, it is necessary to design beam splitters with nanoscale dimensions to miniaturize the essential components for integrated optical circuits. In this work, we propose and numerically demonstrate a broadband, high efficient, and four-channel beam splitter based on a fishnet-shaped metasurface. The proposed structure is constructed of cruciform AlSb nanoantennas on the PDMS substrate. The simple design can split a beam of light into four beams with equal intensity, it achieves a conversion efficiency above 83%, and an anomalous transmission intensity exceeding 0.8 for the wavelength range of 761-835 nm. In this wavelength range, the beam splitting angle changes from 46.45° to 53.68°. Moreover, the four-channel beam splitter is tunable when the metasurface is designed as a discrete structure. At the wavelength of 874 nm, the beam splitting angle can be adjusted from 56.34° to 46.39° as the period increases from 1050 nm to 1207 nm by stretching the substrate. The presented metasurface might enable promising applications in integrated optical devices, owing to its advantages of multi-channel, wide broadband, high efficiency, and large beam split angle.

Recent Progress in Reconfigurable and Intelligent Metasurfaces: A Comprehensive Review of Tuning Mechanisms, Hardware Designs, and Applications

Intelligent metasurfaces have gained significant importance in recent years due to their ability to dynamically manipulate electromagnetic (EM) waves. Their multifunctional characteristics, realized by incorporating active elements into the metasurface designs, have huge potential in numerous novel devices and exciting applications. In this article, recent progress in the field of intelligent metasurfaces are reviewed, focusing particularly on tuning mechanisms, hardware designs, and applications. Reconfigurable and programmable metasurfaces, classified as space gradient, time modulated, and space-time modulated metasurfaces, are discussed. Then, reconfigurable intelligent surfaces (RISs) that can alter their wireless environments, and are considered as a promising technology for sixth-generation communication networks, are explored. Next, the recent progress made in simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) that can achieve full-space EM wave control are summarized. Finally, the perspective on the challenges and future directions of intelligent metasurfaces are presented.

Active metasurface in the near-infrared region by gating ultrathin TiN films

Ultrathin titanium nitride (TiN) films have become a novel material flatform for constructing active metasurfaces in the near-infrared region. In this Letter, we numerically achieved the dual functions of switchable linear dichroism (LD) and tunable perfect absorption in a G-shape gold resonators/TiN film hybrid metasurface by gating ultrathin TiN films. As the carrier density of TiN decreases, the modulation depth for LD strength is about 70% at 1211 nm. Meanwhile, the response wavelength of perfect absorption (∼1) shifts to the blue by around 130 nm with a change of carrier density of 12%. Our proposed active metasurface with the capability of strength-switchable LD and wavelength-tunable perfect absorption has considerable potential in dynamic electro-optic modulation and flat photonic devices with reconfigurable functionalities.

Recent Advances in Tunable Metasurfaces: Materials, Design, and Applications

Metasurfaces, a two-dimensional (2D) form of metamaterials constituted by planar meta-atoms, exhibit exotic abilities to tailor electromagnetic (EM) waves freely. Over the past decade, tremendous efforts have been made to develop various active materials and incorporate them into functional devices for practical applications, pushing the research of tunable metasurfaces to the forefront of nanophotonics. Those active materials include phase change materials (PCMs), semiconductors, transparent conducting oxides (TCOs), ferroelectrics, liquid crystals (LCs), atomically thin material, etc., and enable intriguing performances such as fast switching speed, large modulation depth, ultracompactness, and significant contrast of optical properties under external stimuli. Integration of such materials offers substantial tunability to the conventional passive nanophotonic platforms. Tunable metasurfaces with multifunctionalities triggered by various external stimuli bring in rich degrees of freedom in terms of material choices and device designs to dynamically manipulate and control EM waves on demand. This field has recently flourished with the burgeoning development of physics and design methodologies, particularly those assisted by the emerging machine learning (ML) algorithms. This review outlines recent advances in tunable metasurfaces in terms of the active materials and tuning mechanisms, design methodologies, and practical applications. We conclude this review paper by providing future perspectives in this vibrant and fast-growing research field.

Electro-Optical Modulation in High Q Metasurface Enhanced with Liquid Crystal Integration

Electro-optical tuning metasurfaces are particularly attractive since they open up routes for dynamic reconfiguration. The electro-optic (EO) modulation strength essentially depends on the sensitivity to the EO-induced refractive index changes. In this paper, lithium niobate (LiNbO₃) metasurfaces integrated with liquid crystals (LCs) are theoretically investigated. Cylinder arrays are proposed to support quasi-bound states in the continuum (quasi-BICs). The quasi-BIC resonances can significantly enhance the lifetime of photons and the local field, contributing to the EO-refractive index changes. By integrating metasurfaces with LCs, the combined influence of the LC reorientation and the Pockels electro-optic effect of LiNbO₃ is leveraged to tune the transmitted wavelength and phase spectrum around the quasi-BIC wavelength, resulting in an outstanding tuning sensitivity up to Δλ/ΔV ≈ 0.6 nm/V and relieving the need of high voltage. Furthermore, the proposed structure can alleviate the negative influence of sidewall tilt on device performance. The results presented in this work can foster wide application and prospects for the implementation of tunable displays, light detection and ranging (LiDAR), and spatial light modulators (SLMs).

Enabling Active Nanotechnologies by Phase Transition: From Electronics, Photonics to Thermotics

Phase transitions can occur in certain materials such as transition metal oxides (TMOs) and chalcogenides when there is a change in external conditions such as temperature and pressure. Along with phase transitions in these phase change materials (PCMs) come dramatic contrasts in various physical properties, which can be engineered to manipulate electrons, photons, polaritons, and phonons at the nanoscale, offering new opportunities for reconfigurable, active nanodevices. In this review, we particularly discuss phase-transition-enabled active nanotechnologies in nonvolatile electrical memory, tunable metamaterials, and metasurfaces for manipulation of both free-space photons and in-plane polaritons, and multifunctional emissivity control in the infrared (IR) spectrum. The fundamentals of PCMs are first introduced to explain the origins and principles of phase transitions. Thereafter, we discuss multiphysical nanodevices for electronic, photonic, and thermal management, attesting to the broad applications and exciting promises of PCMs. Emerging trends and valuable applications in all-optical neuromorphic devices, thermal data storage, and encryption are outlined in the end.

Design of Multifunctional Tunable Metasurface Assisted by Elastic Substrate

Metasurfaces with both multifunctionality and tunability hold great application potential in next-generation optical devices. In this paper, we propose a stretchable metasurface composed of arrays of identical dielectric rectangular resonators embedded in the polydimethylsiloxane (PDMS) substrate. It is shown that the metasurface possesses three functions at the operating wavelength of 532 nm. The switching of functions can be implemented by changing the period Px of the metasurface, induced by stretching the PDMS substrate along the x-direction. When the period Px is less than the operating wavelength of 532 nm, the behavior of metasurface can switch between transmissive window and reflective mirror. When the period Px of the metasurface varies from 532 nm to 700 nm, the metasurface act as a dynamic equal-power beam splitter with conversion efficiency higher than 90%, and the corresponding splitting angle can be adjusted from 90° to around 49.5°. Moreover, we achieve the switching of transmissive window/reflective mirror/split-ratio-variable splitter based on the metasurface consisting of arrays of identical L-shaped resonators embedded in the PDMS substrate.

Recent Advancement in Optical Metasurface: Fundament to Application

Metasurfaces have gained growing interest in recent years due to their simplicity in manufacturing and lower insertion losses. Meanwhile, they can provide unprecedented control over the spatial distribution of transmitted and reflected optical fields in a compact form. The metasurfaces are a kind of planar array of resonant subwavelength components that, depending on the intended optical wavefronts to be sculpted, can be strictly periodic or quasi-periodic, or even aperiodic. For instance, gradient metasurfaces, a subtype of metasurfaces, are designed to exhibit spatially changing optical responses, which result in spatially varying amplitudes of scattered fields and the associated polarization of these fields. This paper starts off by presenting concepts of anomalous reflection and refraction, followed by a brief discussion on the Pancharatanm-Berry Phase (PB) and Huygens' metasurfaces. As an introduction to wavefront manipulation, we next present their key applications. These include planar metalens, cascaded meta-systems, tunable metasurfaces, spectrometer retroreflectors, vortex beams, and holography. The review concludes with a summary, preceded by a perspective outlining our expectations for potential future research work and applications.

Second-harmonic generation tuning by stretching arrays of GaAs nanowires

We present a wearable device with III-V nanowires in a flexible polymer, which is used for active mechanical tuning of the second-harmonic generation intensity. An array of vertical GaAs nanowires was grown with metalorganic vapour-phase epitaxy, then embedded in polydimethylsiloxane and detached from the rigid substrate with mechanical peel off. Experimental results show a tunability of the second-harmonic generation intensity by a factor of two for 30% stretching which matches the simulations including the distribution of sizes. We studied the impact of different parameters on the band dispersion and tunability of the second-harmonic generation, such as the pitch, the length, and the diameter. We predict at least three orders of magnitude active mechanical tuning of the nonlinear signal intensity for nanowire arrays. The flexibility of the array together with the resonant wavelength engineering make such structures perspective platforms for future bendable or stretchable nanophotonic devices as light sources or sensors.

Actively Tunable Metasurfaces via Plasmonic Nanogap Cavities with Sub-10-nm VO2 Films

Actively tunable optical materials integrated with engineered subwavelength structures could enable novel optoelectronic devices, including reconfigurable light sources and tunable on-chip spectral filters. The phase-change material vanadium dioxide (VO₂) provides a promising solid-state solution for dynamic tuning; however, previous demonstrations have been limited to thicker and often rough VO₂ films or require a lattice-matched substrate for growth. Here, sub-10-nm-thick VO₂ films are realized by atomic layer deposition (ALD) and integrated with plasmonic nanogap cavities to demonstrate tunable, spectrally selective absorption across 1200 nm in the near-infrared (NIR). Upon inducing the phase transition via heating, the absorption resonance is blue-shifted by as much as 60 nm. This process is reversible upon cooling and repeatable over more than ten temperature cycles. Dynamic, ultrathin VO₂ films deposited by ALD, as demonstrated here, open up new potential architectures and applications where VO₂ can be utilized to provide reconfigurability including three-dimensional, flexible and large-area structures.

Metasurface-empowered spectral and spatial light modulation for disruptive holographic displays

The holographic display, one of the most realistic ways to reconstruct optical images in three-dimensional (3D) space, has gained a lot of attention as a next-generation display platform for providing deeper immersive experiences to users. So far, diffractive optical elements (DOEs) and spatial light modulators (SLMs) have been used to generate holographic images by modulating electromagnetic waves at each pixel. However, such architectures suffer from limitations in terms of having a resolution of only a few microns and the bulkiness of the entire optical system. In this review, we describe novel metasurfaces-based nanophotonic platforms that have shown exceptional control of electromagnetic waves at the subwavelength scale as promising candidates to overcome existing restrictions, while realizing flat optical devices. After introducing the fundamentals of metasurfaces in terms of spatial and spectral wavefront modulation, we present a variety of multiplexing approaches for high-capacity and full-color metaholograms exploiting the multiple properties of light as an information carrier. We then review tunable metaholograms using active materials modulated by several external stimuli. Afterward, we discuss the integration of metasurfaces with other optical elements required for future 3D display platforms in augmented/virtual reality (AR/VR) displays such as lenses, beam splitters, diffusers, and eye-tracking sensors. Finally, we address the challenges of conventional nanofabrication methods and introduce scalable preparation techniques that can be applied to metasurface-based nanophotonic technologies towards commercially and ergonomically viable future holographic displays.

Active optical metasurfaces: comprehensive review on physics, mechanisms, and prospective applications

Optical metasurfaces with subwavelength thickness hold considerable promise for future advances in fundamental optics and novel optical applications due to their unprecedented ability to control the phase, amplitude, and polarization of transmitted, reflected, and diffracted light. Introducing active functionalities to optical metasurfaces is an essential step to the development of next-generation flat optical components and devices. During the last few years, many attempts have been made to develop tunable optical metasurfaces with dynamic control of optical properties (e.g., amplitude, phase, polarization, spatial/spectral/temporal responses) and early-stage device functions (e.g., beam steering, tunable focusing, tunable color filters/absorber, dynamic hologram, etc) based on a variety of novel active materials and tunable mechanisms. These recently-developed active metasurfaces show significant promise for practical applications, but significant challenges still remain. In this review, a comprehensive overview of recently-reported tunable metasurfaces is provided which focuses on the ten major tunable metasurface mechanisms. For each type of mechanism, the performance metrics on the reported tunable metasurface are outlined, and the capabilities/limitations of each mechanism and its potential for various photonic applications are compared and summarized. This review concludes with discussion of several prospective applications, emerging technologies, and research directions based on the use of tunable optical metasurfaces. We anticipate significant new advances when the tunable mechanisms are further developed in the coming years.

Metasurface-Driven Optically Variable Devices

Optically variable devices (OVDs) are in tremendous demand as optical indicators against the increasing threat of counterfeiting. Conventional OVDs are exposed to the danger of fraudulent replication with advances in printing technology and widespread copying methods of security features. Metasurfaces, two-dimensional arrays of subwavelength structures known as meta-atoms, have been nominated as a candidate for a new generation of OVDs as they exhibit exceptional behaviors that can provide a more robust solution for optical anti-counterfeiting. Unlike conventional OVDs, metasurface-driven OVDs (mOVDs) can contain multiple optical responses in a single device, making them difficult to reverse engineered. Well-known examples of mOVDs include ultrahigh-resolution structural color printing, various types of holography, and polarization encoding. In this review, we discuss the new generation of mOVDs. The fundamentals of plasmonic and dielectric metasurfaces are presented to explain how the optical responses of metasurfaces can be manipulated. Then, examples of monofunctional, tunable, and multifunctional mOVDs are discussed. We follow up with a discussion of the fabrication methods needed to realize these mOVDs, classified into prototyping and manufacturing techniques. Finally, we provide an outlook and classification of mOVDs with respect to their capacity and security level. We believe this newly proposed concept of OVDs may bring about a new era of optical anticounterfeit technology leveraging the novel concepts of nano-optics and nanotechnology.

Quasi-Guided Modes in Titanium Dioxide Arrays Fabricated via Soft Nanoimprint Lithography

Reversible quasi-guided modes (QGMs) are observed in titanium dioxide (TiO2) metasurface arrays fabricated via soft nanoimprint lithography. A TiO2 layer between the nanopillar array and the substrate can facilitate the propagation of QGMs. This layer is porous, allowing for the tuning of the layer properties by incorporating another material. The presence of the QGMs is strongly dependent on the refractive index of the TiO2 layer. QGMs are not supported if the refractive index of the porous TiO2 is too low. It is demonstrated that after depositing R6G on the array QGMs can be observed as very strong and narrow reflectance peaks and transmittance dips. Furthermore, as the second material can penetrate through the pores into the layer it can experience the regions of high field enhancement associated with the QGMs. These results are of interest for a wide range of applications including but not limited to sensing, nonlinear optics, and emission control.

Mechanical modulation of multifunctional responses in three-dimensional terahertz metamaterials

Reconfigurable metamaterials have attracted a surge of attention for their formidable capability to dynamically manipulate the electromagnetic wave. Among the multifarious modulation methods, mechanical deformation is widely adopted to tune the electromagnetic response of the stereotype metamaterial owing to its straightforward and continuous controllability on the metamaterial structure. However, previous morphologic reconfigurations of metamaterials are typically confined in planar deformation that renders limited tunable functionalities. Here we have proposed a novel concept of out-of-plane deformation to broaden the functionalities of mechanically reconfigurable metamaterials via introducing a cross-shaped metamaterial. Our results show that the out-of-plane mechanical modulation dramatically enhances the magnetic response of the pristine metamaterial. Furthermore, by uncrossing the bars of cross-shaped meta-atoms, a L-shaped metamaterial is proposed to verify the effectiveness of such a mechanical method on the handedness switching via changing mechanical loading-paths. More importantly, the differential transmission for circularly polarized incidences can be continuously modulated from -0.45 to 0.45, and the polarization states of the transmission wave can be dynamically manipulated under the linearly polarized illumination. Our proposed mechanical modulation principle might open a novel avenue toward the three-dimensional reconfigurable metamaterials and shows their ample applications in the areas of chiroptical control, tunable polarization rotator and converter.

Continuous-zoom bifocal metalens by mutual motion of cascaded bilayer metasurfaces in the visible

Metalens, a subcategory of metasurfaces, has been widely investigated by virtue of its miniature and ultrathin characteristics as well as versatile functionalities. In this study, a tunable bifocal metalens with two continuous-zoom foci is proposed and numerically verified. This design utilizes two cascaded layers of metasurfaces, and different phase profiles for incidences of opposite helicities are imparted on each layer by the combination of geometric phase and propagation phase. When two layers of metasurfaces are actuated laterally, focal lengths of both foci are tuned continuously, with the difference of both focal lengths increasing or decreasing. Additionally, the zoom range for each focus can be designed at will, and the relative intensity of both foci can be modulated by altering the ellipticity of incidence, with the focusing efficiency of the bifocal metalens varying from 19.8% to 32.7% for numerical apertures in a range from 0.53 to 0.78. The proposed device is anticipated to find applications in multi-plane imaging, optical tomography technique, optical data storage, and so on.

Stretching-Tunable High-Frequency Magnetic Properties of Wrinkled CoFeB Films Grown on PDMS

We demonstrated a convenient method via applying uniaxial tensile strains to continuously tune the high-frequency properties of flexible magnetic films. CoFeB films were magnetron sputtered onto prestretched polydimethylsiloxane (PDMS) membranes. They exhibit a self-assembled periodic wrinkling surface structure because of the large mismatch of Young's moduli between the elastomeric PDMS substrates and the metal layers. The wrinkling morphology and the residual tensile stress caused by the Poisson effect can be continuously tuned by a uniaxial stretching strain less than the growth prestrain, which consequently results in changes in high-frequency performance. The initial permeability and the ferromagnetic resonance frequency of flexible CoFeB thin films can be monotonously tuned in wide ranges of about hundreds and 1 GHz, respectively. A good repeatability over thousands of stretching-relaxing cycles has been demonstrated without any obvious reduced high-frequency properties. This flexible CoFeB films with excellent stretching-tunable high-frequency performances are promising for application in flexible and tunable microwave devices.

All-Dielectric Nanostructures with a Thermoresponsible Dynamic Polymer Shell

We suggest a new strategy for creating stimuli-responsive bio-integrated optical nanostructures based on Mie-resonant silicon nanoparticles covered by an ensemble of similarity negatively charged polyelectrolytes (heparin and sodium polystyrene sulfonate). The dynamic tuning of the nanostructures' optical response is due to light-induced heating of the nanoparticles and swelling of the polyelectrolyte shell. The resulting hydrophilic/hydrophobic transitions significantly change the shell thickness and reversible shift of the scattering spectra for individual nanoparticles up to 60 nm. Our findings bring novel opportunities for the application of smart nanomaterials in nanomedicine and bio-integrated nanophotonics.

Nanophotonics for light detection and ranging technology

Light detection and ranging (LiDAR) technology, a laser-based imaging technique for accurate distance measurement, is considered one of the most crucial sensor technologies for autonomous vehicles, artificially intelligent robots and unmanned aerial vehicle reconnaissance. Until recently, LiDAR has relied on light sources and detectors mounted on multiple mechanically rotating optical transmitters and receivers to cover an entire scene. Such an architecture gives rise to limitations in terms of the imaging frame rate and resolution. In this Review, we examine how novel nanophotonic platforms could overcome the hardware restrictions of existing LiDAR technologies. After briefly introducing the basic principles of LiDAR, we present the device specifications required by the industrial sector. We then review a variety of LiDAR-relevant nanophotonic approaches such as integrated photonic circuits, optical phased antenna arrays and flat optical devices based on metasurfaces. The latter have already demonstrated exceptional functional beam manipulation properties, such as active beam deflection, point-cloud generation and device integration using scalable manufacturing methods, and are expected to disrupt modern optical technologies. In the outlook, we address the upcoming physics and engineering challenges that must be overcome from the viewpoint of incorporating nanophotonic technologies into commercially viable, fast, ultrathin and lightweight LiDAR systems.

Large-area all-dielectric metasurface fabricated by an anodized aluminum oxide template

Transmissive metasurfaces formed by high-index dielectric materials have received great attention due to its potential in holograms, deflectors, beam converters, and flat lenses. However, a key challenge of all-dielectric metasurfaces is the limited scale and high cost in fabrication, such as electron beam lithography (EBL) and focused ion beam (FIB) lithography. In this paper, for the first time to our knowledge, an anodized aluminum oxide (AAO) template is combined with titanium dioxide (TiO₂) metasurface fabrication with advantages of large area (>2cm²) and low cost. Using the ordered anodized aluminum oxide (AAO) as an evaporation mask, a TiO₂ nanocylinder array is deposited through the AAO mask onto the SiO₂ substrate. Electric and magnetic dipole resonances of TiO₂ metasurface appear in the visible spectrum. Furthermore, we demonstrate the interaction of the CsPbBr_1.5I_1.5 quantum dot (QD) emission with magnetic dipole (MD) resonance of TiO₂ metasurface. Our results reveal that the metasurface exhibits remarkable photoluminescence (PL) enhancement of 25%. Up to now, a TiO₂ metasurface with 2.25-cm²-large area using AAO template method has never been attempted. Different from the metasurfaces fabricated by FIB and EBL, our method offers great ease for large-area metasurface fabrication, which is convenient for metasurface researchers and avoids costly facilities.

Dielectric Resonance-Based Optical Metasurfaces: From Fundamentals to Applications

Optical metasurface as a booming research field has put forward profound progress in optics and photonics. Compared with metallic-based components, which suffer from significant thermal loss and low efficiency, high-index all-dielectric nanostructures can readily combine electric and magnetic Mie resonances together, leading to efficient manipulation of optical properties such as amplitude, phase, polarization, chirality, and anisotropy. These advances have enabled tremendous developments in practical photonic devices that can confine and guide light at the nanoscale. Here we review the recent development of local electromagnetic resonances such as Mie-type scattering, bound states in the continuum, Fano resonances, and anapole resonances in dielectric metasurfaces and summarize the fundamental principles of dielectric resonances. We discuss the recent research frontiers in dielectric metasurfaces including wavefront-shaping, metalenses, multifunctional and computational approaches. We review the strategies and methods to realize the dynamic tuning of dielectric metasurfaces. Finally, we conclude with an outlook on the challenges and prospects of dielectric metasurfaces.

All-Dielectric Silicon Nanoring Metasurface for Full-Color Printing

Structural color has been particularly attractive as it provides a highly promising approach for next-generation color printing. Plasmonic nanostructures have been intensively investigated for color printing, while suffering from intrinsic loss that degrades the quality of the coloration. Dielectric materials have emerged as an alternative because of their high refractive index that enables highly confined optical modes within the nanostructure at the diffraction limit. Here, we demonstrate an all-dielectric nanoring metasurface for coloration. By harnessing the intrinsic nanoring structure design, vivid structural color has been achieved in the visible range. The color gamut is expected to occupy 115% of the standard color space (sRGB) on the chromaticity diagram of the International Commission on Illumination (CIE) 1931 in theory. Our structure can be applied to various complex devices and materials and find potential applications such as displays, information, and art works.

Tunable beam splitter using bilayer geometric metasurfaces in the visible spectrum

Metasurfaces have been widely investigated for their capabilities of manipulating wavefront versatilely and miniaturizing traditional optical elements into ultrathin devices. In this study, a nanoscale tunable beam splitter utilizing a bilayer of geometric metasurfaces in the visible spectrum is proposed and numerically examined. Inspired by the diffractive Alvarez lens and multilayer geometric metasurfaces, opposite quadratic phase distributions are imparted on both layers, and a varying linear phase gradient will arise through relatively lateral displacement between two layers, generating tunable angles of deflection. In addition, such geometric metasurfaces offer opposite directions of phase gradients for orthogonal circularly polarized incidences, leading to effective polarization beam splitting. Results prove that the splitting angles can be tuned precisely, and the energy split ratio can be effectively changed according to the ellipticity of the polarized incidence. This design could find significant applications in optical communication, measurement, display, and so on.

Information Metamaterial Systems

Metamaterials have great capabilities and flexibilities in controlling electromagnetic (EM) waves because their subwavelength meta-atoms can be designed and tailored in desired ways. However, once the structure-only metamaterials (i.e., passive metamaterials) are fabricated, their functions will be fixed. To control the EM waves dynamically, active devices are integrated into the meta-atoms, yielding active metamaterials. Traditionally, the active metamaterials include tunable metamaterials and reconfigurable metamaterials, which have either small-range tunability or a few numbers of reconfigurability. Recently, a special kind of active metamaterials, digital coding and programmable metamaterials, have been presented, which can realize a large number of distinct functionalities and switch them in real time with the aid of field programmable gate array (FPGA). More importantly, the digital coding representations of metamaterials make it possible to bridge the digital world and physical world using the metamaterial platform and make the metamaterials process digital information directly, resulting in information metamaterials. In this review article, we firstly introduce the evolution of metamaterials and then present the concepts and basic principles of digital coding metamaterials and information metamaterials. With more details, we discuss a series of information metamaterial systems, including the programmable metamaterial systems, software metamaterial systems, intelligent metamaterial systems, and space-time-coding metamaterial systems. Finally, we introduce the current progress and predict the future trends of information metamaterials.

Dynamic beam splitter employing an all-dielectric metasurface based on an elastic substrate

Beam splitters are an indispensable part of optical measurements and applications. We propose a dynamic beam splitter incorporating all-dielectric metasurface in an elastic substrate under external mechanical stimulus of stretching. The optical behavior at 720 nm wavelength shows that it can be changed from a pure optical-diode-like behavior to a dynamic beam splitter. Although the structure is designed running at 720 nm, the design approach with appropriate materials can be used at any wavelength. Various cases, including wavelength and polarization dependencies, are thoroughly investigated to demonstrate the principles of operating conditions of two different regimes of the designed metasurface.

Stretchable Biaxial and Shear Strain Sensors Using Diffractive Structural Colors

Structural colors that can be changed dynamically, using either plasmonic nanostructures or photonic crystals, are rapidly emerging research areas for stretchable sensors. Despite the wide applications of various techniques to achieve strain-responsive structural colors, important factors in the feasibility of strain sensors-such as their sensing mechanism, stability, and reproducibility-have not yet been explored. Here, we introduce a stretchable, diffractive, color-based wireless strain sensor that can measure strain using the entire visible spectrum, based on an array of cone-shaped nanostructures on the surface of an elastomeric substrate. By stretching or compressing the substrate, the diffractive color can be tuned according to the changing grating pitch. Using the proposed method, we designed three types of strain-sensing modes: large-deformation (maximum 100%) tensile strain, biaxial 2D strain, and shear strain (maximum 78%). The strain sensors were fabricated, and applicability to strain-sensing was evaluated.

Soft optical metamaterials

Optical metamaterials consist of artificially engineered structures exhibiting unprecedented optical properties beyond natural materials. Optical metamaterials offer many novel functionalities, such as super-resolution imaging, negative refraction and invisibility cloaking. However, most optical metamaterials are comprised of rigid materials that lack tunability and flexibility, which hinder their practical applications. This limitation can be overcome by integrating soft matters within the metamaterials or designing responsive metamaterial structures. In addition, soft metamaterials can be reconfigured via optical, electrical, thermal and mechanical stimuli, thus enabling new optical properties and functionalities. This paper reviews different types of soft and reconfigurable optical metamaterials and their fabrication methods, highlighting their exotic properties. Future directions to employ soft optical metamaterials in next-generation metamaterial devices are identified.

Frequency conversion and parametric amplification using a virtually rotating metasurface

We analyze the scattering of circularly polarized electromagnetic waves from a time-varying metasurface having a time-dependent surface susceptibility that locally mimics a rotating, anisotropic surface. Such virtually rotating metasurfaces (VRM) can be realized by means of electronically tunable surface elements and reach microwave-range rotation frequencies. It is shown that the scattered field contains the incident tone, as well as a single up-or down converted tone which differs by twice the rotation frequency of the surface. A simple full frequency converter is then proposed by augmenting the VRM with a metal screen separated by a proper distance. It is shown that after reflection from this system, the incident tone is fully converted to a single down- or up-converted tone, and shows amplification in the case of up conversion. The analysis of these time-rotating scenarios is carried out by switching to a rotating frame for the fields, leading to time-invariant equations, and thus using common phasor-representation. All results are also validated against an in-house 1D-FDTD code showing excellent agreement. A lumped element model using a 2D periodic metal mesh grid loaded with time-varying capacitive nodes is also presented that enables the VRM concept. This model is then further used to design a 3D realization, verified with static full-wave simulations for different values of the capacitor arrangement. Furthermore, the effect of piece-wise constant changes of surface susceptibility in a general virtually rotating metasurface is studied and it is shown to operate with acceptable results, which is of practical importance. The results of this paper can open new ways for realization of frequency conversion and amplification, in a magnetless and linear time-varying system.

Stretchable All-Dielectric Metasurfaces with Polarization-Insensitive and Full-Spectrum Response

Mechanical stretching has been an effective way to achieve widely tunable optical response in artificial nanostructures. However, the typical stretchable optical devices produce exactly the reverse effects for two orthogonal linear polarizations, significantly hindering their practical applications in many emerging systems. Herein, we demonstrate an approach for a mechanically tunable all-dielectric metasurface with polarization insensitivity and full-spectrum response in the visible range from 450 to 650 nm. By embedding a TiO₂ metasurface in a polydimethylsiloxane substrate and stretching it in one direction, we find that the distinct reflection colors of two orthogonal linear polarizations can be tuned across the entire visible spectrum simultaneously. Encryption and display of information have also been realized with the same technique. The corresponding calculations show that the spectral responses of light with polarizations perpendicular and parallel to the strain are determined by two different mechanisms, that is, the near-field mutual interaction and the grating effects. This research shall shed light on stretchable and wearable photonics.

Mie Resonant Structural Colors

Structural colors refer to colors produced by the interference of light scattered by judiciously arranged nano- or microscopic structures. In this Forum Article, we discuss the use of Mie resonant scattering in structural colors with dielectric and metal-dielectric hybrid structures to achieve notable figures of merit in pixel size and gamut range. Compared with plasmonic structures, resonant dielectric and hybrid structures are subjected to less loss while providing strong field confinement and large scattering cross sections, making them appealing for realizing vibrant colors at ultrahigh resolutions. We outline the basic principles behind Mie resonances in analytically solvable structures and highlight the relation between these resonances and color with demonstrations in dielectric metasurfaces. Mie resonant colors occurring in nonplanar designs including disordered systems are also explored. We review recent advances in dynamic and reversibly tunable Mie resonant colors and conclude by providing an outlook for future research directions.

Spatiotemporal light control with active metasurfaces

Optical metasurfaces have provided us with extraordinary ways to control light by spatially structuring materials. The space-time duality in Maxwell's equations suggests that additional structuring of metasurfaces in the time domain can even further expand their impact on the field of optics. Advances toward this goal critically rely on the development of new materials and nanostructures that exhibit very large and fast changes in their optical properties in response to external stimuli. New physics is also emerging as ultrafast tuning of metasurfaces is becoming possible, including wavelength shifts that emulate the Doppler effect, Lorentz nonreciprocity, time-reversed optical behavior, and negative refraction. The large-scale manufacturing of dynamic flat optics has the potential to revolutionize many emerging technologies that require active wavefront shaping with lightweight, compact, and power-efficient components.

Shape-Dependent, Chiro-optical Response of UV-Active, Nanohelix Metamaterials

We harness a synergy between morphology and the electromagnetic response of semiconducting material to engineer the chiro-optical properties of metamaterials that are active at ultraviolet (UV) wavelengths. Chiral metamaterials have recently ushered in new research directions in fundamental light-matter interactions, while simultaneously opening a range of promising photonics-based applications from polarization control to improved biosensing methods. Despite these recent advances, to date, very little attention has been focused upon engineered large UV-chiro-optical activity, where naturally occurring molecular optical activity bands are most typically encountered. Here, we systematically alter the morphology of titanium dioxide nanohelices, which make up the elements of the chiral metamaterials, to investigate how the nanoparticle shape affects chiro-optical activity across the UV spectrum. When the nanoscale critical dimensions fall within a particular size range, giant chiro-optical activity is observed, which is on the order of the strongest demonstrated in the UV to date and can be tuned by slight alterations of the nanohelices' morphology.

Mechanochromic Reconfigurable Metasurfaces

The change of optical properties that some usually natural compounds or polymeric materials show upon the application of external stress is named mechanochromism. Herein, an artificial nanomechanical metasurface formed by a subwavelength nanowire array made of molybdenum disulfide, molybdenum oxide, and silicon nitride changes color upon mechanical deformation. The aforementioned deformation induces reversible changes in the optical transmission (relative transmission change of 197% at 654 nm), thus demonstrating a giant mechanochromic effect. Moreover, these types of metasurfaces can exist in two nonvolatile states presenting a difference in optical transmission of 45% at 678 nm, when they are forced to bend rapidly. The wide optical tunability that photonic nanomechanical metasurfaces, such as the one presented here, possess by design, can provide a valuable platform for mechanochromic and bistable responses across the visible and near infrared regime and form a new family of smart materials with applications in reconfigurable, multifunctional photonic filters, switches, and stress sensors.

Controlling the degrees of freedom in metasurface designs for multi-functional optical devices

This review focuses on the control over the degrees of freedom (DOF) in metasurfaces, which include the input DOF (the polarization, wavelength and incident angle of the input light and some dynamic controls), parameter DOF (the complex geometric design of metasurfaces) and output DOF (the phase, polarization and amplitude of the output light). This framework could clearly show us the development process of metasurfaces, from single-functional to multi-functional ones. Advantages of the multi-functional metasurfaces are discussed in the context of various applications, including 3D holography, broadband achromatic metalenses and multi-dimensional encoded information. By combining all the input and output DOF together, we can realize ideal optical meta-devices with deep subwavelength thickness and striking functions beyond the reach of traditional optical components. Moreover, new research directions may emerge when merging different DOF in metasurfaces with other important concepts, such as parity-time symmetry and topology, so that we can have the complete control of light waves in a prescribed manner.

Broadband near-infrared TiO2 dielectric metamaterial absorbers

Metamaterial absorbers (MAs) have drawn increasing attention due to their prospects in many fields such as sensing, thermal emission, solar energy harvesting, etc. However, it remains challenging to realize broadband MAs with a simple structure. Here, we propose a broadband, polarization-insensitive, and omnidirectional MA working in the near-infrared range with simple structure, which is composed of titanium dioxide (TiO₂) cylinder nano-antenna arrays on the top of a vanadium (V) film deposited on a silicon substrate. This device demonstrates broadband absorption spectra from 820 to 1440 nm with the absorption above 90%, with high absorption up to the incident angle of ∼50°. The broadband absorption of the designed MA is mainly attributed to the interaction both of dielectric cavity resonance and electric dipole resonance. The electric and magnetic field intensity distribution of the MA are analyzed to better understand its absorption mechanism. In addition, the effects of the geometrical parameters on absorption are discussed. The demonstrated MA is relatively easy to fabricate and can be realized with other proper materials to work in other wavelength bands. The design is useful for applications such as solar energy harvesting, sensing, and camouflage.

Gate-tunable optical filter based on conducting oxide metasurface heterostructure

A gate-tunable plasmonic optical filter incorporating a subwavelength patterned metal-insulator-metal metasurface heterostructure is proposed. An additional thin transparent conducting oxide (TCO) layer is embedded in the insulator layer to form a double metal-oxide-semiconductor configuration. Heavily n-doped indium tin oxide (ITO) is employed as the TCO material, whose optical property can be electrically tuned by the formation of a thin active epsilon-near-zero layer at the ITO-oxide interfaces. Full-wave electromagnetic simulations show that amplitude modulation and shift of transmission peak are achievable with 3-5 V applied bias, depending on the application. Moreover, the modulation strength and transmission peak shift increase with a thinner ITO layer. This work is an essential step toward a realization of next-generation compact photonic/plasmonic integrated devices.

Flexible plasmonic modulators induced by the thermomechanical effect

Reconfigurable plasmon-based flexible devices, composed of artificial plasmonic nanostructures on stretchable substrates, show great promise for dynamic functionalities such as tunability, switching and modulation of electromagnetic waves. Here, we theoretically proposed and experimentally demonstrated a simple and efficient flexible plasmonic modulator based on an array of gold nanostructures on a poly(dimethylsiloxane) (PDMS) substrate. Arising from the current-induced local Joule heat, the local expansion of the PDMS substrate widens the gap distances between the neighboring gold wires, which results in a spectral shift of the plasmon resonance. The experimental results show that the plasmon resonance has a blue-shift of 39 nm under a total power consumption of only 10.5 mW, which results in a high modulation depth of up to 30.5% for the modulator. Such a low power consumption can be ascribed to the small active area and excellent thermal isolation of the PDMS. The optical and thermomechanical responses were confirmed and understood by the electromagnetic and thermomechanical co-simulations based on the finite-difference time-domain and finite-element methods. This novel mechanism to manipulate light provides new opportunities for active optical components and integrated circuits.

Efficient visible light modulation based on electrically tunable all dielectric metasurfaces embedded in thin-layer nematic liquid crystals

All-dielectric metasurfaces have attracted attention for highly efficient visible light manipulation. So far, however, they are mostly passive devices, while those allowing dynamic control remain a challenge. A highly efficient tuning mechanism is immersing the metasurface in a birefringent liquid crystal (LC), whose refractive index can be electrically controlled. Here, an all-dielectric tunable metasurface is demonstrated based on this concept, operating at visible frequencies and based on TiO₂ nanodisks embedded in a thin LC layer. Small driving voltages from 3~5 V are sufficient to tune the metasurface resonances, with an associated transmission modulation of more than 65%. The metasurface optical responses, including the observed electric and magnetic dipole resonance shifts as well as the interfacial anchoring effect of the LC induced by the presence of the nanostructures, are systematically discussed. The dynamic tuning observed in the transmission spectra can pave the way to dynamically tunable metasurface devices for efficient visible light modulation applications.

Mechanically tunable focusing metamirror in the visible

A compact, flat lens with dynamically tunable focal length will be an essential component in advanced reconfigurable optical systems. One approach to realize a flat tunable lens is by utilizing metasurfaces, which are two-dimensional nanostructures capable of tailoring the wavefront of incident light. When a metasurface with a hyperboloidal phase profile, i.e., a metalens, is fabricated on a substrate that can be actuated, its focal length can be adjusted dynamically. Here, we design and realize the first reflection type, tunable metalens (i.e., metamirror) operating in the visible regime (670 nm). It is shown that the focal length can be continuously adjusted by up to 45% with a 0% to 20% lateral stretching of the substrate, while maintaining diffraction-limited focusing and high focusing efficiency. Our design as a flat optics element has potential in widespread applications, such as wearable mixed reality systems, biomedical instruments and integrated optics devices.

One-Dimensional Programmable Polymeric Microfiber Waveguide with Optically Reconfigurable Photonic Functions

Programmable materials and reconfigurable photonic components, which can change their physicochemical properties and functionalities upon external stimuli, are a major topic of interest in modern science. However, most conventional reconfigurable photonic components rely heavily on mechanical deformation, restricting their application. Herein, a novel strategy based on a dynamically tunable fluorescence resonance energy transfer process to design and fabricate programmable fluorescent micropatterns within single polymer microfiber is proposed. A set of reconfigurable photonic components (including optical switchable waveguide systems, photonic analogies of diodes and transistors, as well as one-dimensional (1D) optical encoding) can be achieved within a single polymeric waveguide microfiber straightforwardly, in which such photonic components can be written, erased, and rewritten as 1D binary patterns with tailored external optical stimuli. These results might be of great fundamental value for the rational design of novel reconfigurable photonic devices with numerous potential applications in highly integrated optical communication components and optical computing devices.