Active Tuning of Spontaneous Emission by Mie-Resonant Dielectric Metasurfaces

Tunable emission from H-type supramolecular polymers in optical nanocavities

H-type supramolecular polymers with preferred helicity and highly efficient emission have been prepared from the self-assembly of chiral tetraphenylene-based monomers. Implementation of the one-dimensional fibers into dielectric nanoparticle arrays allows for a significant reshaping of fluorescence due to weak light-matter coupling.

Switchable unidirectional emissions from hydrogel gratings with integrated carbon quantum dots

Directional emission of photoluminescence despite its incoherence is an attractive technique for light-emitting fields and nanophotonics. Optical metasurfaces provide a promising route for wavefront engineering at the subwavelength scale, enabling the feasibility of unidirectional emission. However, current directional emission strategies are mostly based on static metasurfaces, and it remains a challenge to achieve unidirectional emissions tuning with high performance. Here, we demonstrate quantum dots-hydrogel integrated gratings for actively switchable unidirectional emission with simultaneously a narrow divergence angle less than 1.5° and a large diffraction angle greater than 45°. We further demonstrate that the grating efficiency alteration leads to a more than 7-fold tuning of emission intensity at diffraction order due to the variation of hydrogel morphology subject to change in ambient humidity. Our proposed switchable emission strategy can promote technologies of active light-emitting devices for radiation control and optical imaging.

Interactions of Fundamental Mie Modes with Thin Epsilon-near-Zero Substrates

Extensive research has focused on Mie modes in dielectric nanoresonators, enabling the creation of thin optical devices surpassing their bulk counterparts. This study investigates the interactions between two fundamental Mie modes, electric and magnetic dipoles, and the epsilon-near-zero (ENZ) mode. Analytical, simulation, and experimental analyses reveal that the presence of the ENZ substrate significantly modifies these modes despite a large size mismatch. Electric and magnetic dipole modes, both with ∼12 THz line widths, exhibit 21 and 26 THz anticrossings, respectively, when coupled to the ENZ mode, indicating strong coupling. We also demonstrate that this strongly coupled system yields notably large subpicosecond nonlinear responses. Our results establish a solid foundation for designing functional, nonlinear, dynamic dielectric metasurfaces with ENZ materials.

Hybrid silicon-organic Huygens' metasurface for phase modulation

Spatial light modulators have desirable applications in sensing and free space communication because they create an interface between the optical and electronic realms. Electro-optic modulators allow for high-speed intensity manipulation of an electromagnetic wavefront. However, most surfaces of this sort pose limitations due to their ability to modulate intensity rather than phase. Here we investigate an electro-optic modulator formed from a silicon-organic Huygens' metasurface. In a simulation-based study, we discover a metasurface design immersed in high-performance electro-optic molecules that can achieve near-full resonant transmission with phase coverage over the full 2π range. Through the electro-optic effect, we show 140 ∘ (0.79π) modulation over a range of -100 to 100 V at 1330 nm while maintaining near-constant transmitted field intensity (between 0.66 and 0.8). These results potentiate the fabrication of a high-speed spatial light modulator with the resolved parameters.

Electrically Tunable Reflective Metasurfaces with Continuous and Full-Phase Modulation for High-Efficiency Wavefront Control at Visible Frequencies

All-dielectric optical metasurfaces can locally control the amplitude and phase of light at the nanoscale, enabling arbitrary wavefront shaping. However, lack of postfabrication tunability has limited the true potential of metasurfaces for many applications. Here, we utilize a thin liquid crystal (LC) layer as a tunable medium surrounding the metasurface to achieve a phase-only spatial light modulator (SLM) with high reflection in the visible frequency, exhibiting active and continuous resonance tuning with associated 2π phase control and uncoupled amplitude. Dynamic wavefront shaping is demonstrated by programming 96 individually addressable electrodes with a small pixel pitch of ∼1 μm. The small pixel size is facilitated by the reduced LC thickness, strongly suppressing cross-talk among pixels. This device is used to demonstrate dynamic beam steering with a wide field-of-view and high absolute diffraction efficiencies. We believe that our demonstration may help realize next-generation, high-resolution SLMs, with wide applications in dynamic holography, tunable optics, and light detection and ranging (LiDAR), to mention a few.

Programmable Wavefront Control in the Visible Spectrum Using Low-Loss Chalcogenide Phase-Change Metasurfaces

All-dielectric metasurfaces provide unique solutions for advanced wavefront manipulation of light with complete control of amplitude and phase at sub-wavelength scales. One limitation, however, for most of these devices is the lack of any post-fabrication tunability of their response. To break this limit, a promising approach is employing phase-change materials (PCMs), which provide fast, low energy, and non-volatile means to endow metasurfaces with a switching mechanism. In this regard, great advancements have been done in the mid-infrared and near-infrared spectrum using different chalcogenides. In the visible spectral range, however, very few devices have demonstrated full phase manipulation, high efficiencies, and reversible optical modulation. In this work, a programmable all-dielectric Huygens' metasurface made of antimony sulfide (Sb2 S3 ) PCM is experimentally demonstrated, a low loss and high-index material in the visible spectral range with a large contrast (≈0.5) between its amorphous and crystalline states. ≈2π phase modulation is shown with high associated transmittance and it is used to create programmable beam-steering devices. These novel chalcogenide PCM metasurfaces have the potential to emerge as a platform for next-generation spatial light modulators and to impact application areas such as programmable and adaptive flat optics, light detection and ranging (LiDAR), and many more.

Angular Control of Circularly Polarized Emission from Achiral Molecules via Magnetic Dipoles Sustained in a Chiral Metamirror

Circularly polarized emission (CPE) plays an important role in the designs of advanced displays and photonic integrated circuits. Unfortunately, the control of CPE handedness is limited by the chiral metasurfaces employed to emit chiral light. Particularly, the switching of the handedness with chiral metasurfaces relies on flipping the metasurfaces, which adds some constraints to practical applications. Herein, we propose an angle-sensitive chiral metamirror with Mie resonators to realize handedness switching. The Mie resonator supports a magnetic dipole having large field enhancement. This chiral metamirror is applied to excite CPEs with opposite handedness at emission angles within 10°. In contrast to the conventional methods, this work proposes a more efficient approach to manipulate the handedness of CPE.

Recent Advances in Metaphotonic Biosensors

Metaphotonic devices, which enable light manipulation at a subwavelength scale and enhance light-matter interactions, have been emerging as a critical pillar in biosensing. Researchers have been attracted to metaphotonic biosensors, as they solve the limitations of the existing bioanalytical techniques, including the sensitivity, selectivity, and detection limit. Here, we briefly introduce types of metasurfaces utilized in various metaphotonic biomolecular sensing domains such as refractometry, surface-enhanced fluorescence, vibrational spectroscopy, and chiral sensing. Further, we list the prevalent working mechanisms of those metaphotonic bio-detection schemes. Furthermore, we summarize the recent progress in chip integration for metaphotonic biosensing to enable innovative point-of-care devices in healthcare. Finally, we discuss the impediments in metaphotonic biosensing, such as its cost effectiveness and treatment for intricate biospecimens, and present a prospect for potential directions for materializing these device strategies, significantly influencing clinical diagnostics in health and safety.

Precision Tailoring Quasi-BIC Resonance of a-Si:H Metasurfaces

The capability of tailoring the resonance wavelength of metasurfaces is important as it can alleviate the manufacturing precision required to produce the exact structure according to the design of the nanoresonators. Tuning of Fano resonances by applying heat has been theoretically predicted in the case of silicon metasurfaces. Here, we experimentally demonstrate the permanent tailoring of quasi-bound states in the continuum (quasi-BIC) resonance wavelength in an a-Si:H metasurface and quantitatively analyze the modification in the Q-factor with gradual heating. A gradual increment in temperature leads to a spectral shift in the resonance wavelength. With the support of ellipsometry measurements, the spectral shift resulting from the short-duration (ten minutes) heating is identified to be due to refractive index variations in the material rather than a geometric effect or amorphous/polycrystalline phase transition. In the case of quasi-BIC modes in the near-infrared, resonance wavelength could be adjusted from T = 350 °C to T = 550 °C without affecting the Q-factor considerably. Apart from the temperature-induced resonance trimming, large Q-factors can be attained at the highest analyzed temperature (T = 700 °C) in the near-infrared quasi-BIC modes. Resonance tailoring is just one of the possible applications of our results. We expect that our study is also insightful in the design of a-Si:H metasurfaces where large Q-factors are required at high temperatures.

Recent Advances in Reconfigurable Metasurfaces: Principle and Applications

Metasurfaces have shown their great capability to manipulate electromagnetic waves. As a new concept, reconfigurable metasurfaces attract researchers' attention. There are many kinds of reconfigurable components, devices and materials that can be loaded on metasurfaces. When cooperating with reconfigurable structures, dynamic control of the responses of metasurfaces are realized under external excitations, offering new opportunities to manipulate electromagnetic waves dynamically. This review introduces some common methods to design reconfigurable metasurfaces classified by the techniques they use, such as special materials, semiconductor components and mechanical devices. Specifically, this review provides a comparison among all the methods mentioned and discusses their pros and cons. Finally, based on the unsolved problems in the designs and applications, the challenges and possible developments in the future are discussed.

Quantum Dot Photoluminescence Enhancement in GaAs Nanopillar Oligomers Driven by Collective Magnetic Modes

Single photon sources based on semiconductor quantum dots are one of the most prospective elements for optical quantum computing and cryptography. Such systems are often based on Bragg resonators, which provide several ways to control the emission of quantum dots. However, the fabrication of periodic structures with many thin layers is difficult. On the other hand, the coupling of single-photon sources with resonant nanoclusters made of high-index dielectric materials is known as a promising way for emission control. Our experiments and calculations show that the excitation of magnetic Mie-type resonance by linearly polarized light in a GaAs nanopillar oligomer with embedded InAs quantum dots leads to quantum emitters absorption efficiency enhancement. Moreover, the nanoresonator at the wavelength of magnetic dipole resonance also acts as a nanoantenna for a generated signal, allowing control over its radiation spatial profile. We experimentally demonstrated an order of magnitude emission enhancement and numerically reached forty times gain in comparison with unstructured film. These findings highlight the potential of quantum dots coupling with Mie-resonant oligomers collective modes for nanoscale single-photon sources development.

Tunable Polarization-Multiplexed Achromatic Dielectric Metalens

Tunable metasurfaces provide a compact and efficient strategy for optical active wavefront shaping. Varifocal metalens is one of the most important applications. However, the existing tunable metalens rarely serves broadband wavelengths restricting their applications in broadband imaging and color display due to chromatic aberration. Herein, an electrically tunable polarization-multiplexed achromatic metalens integrated with twisted nematic liquid crystals (TNLCs) in the visible region is demonstrated. The phase profiles at different wavelengths under two orthogonal polarization channels are customized by the particle swarm optimization algorithm and matched with the dielectric metaunits database to achieve polarization-multiplexed achromatic performance. By combining the broadband linear polarization conversion ability of TNLC, the tunability of varifocal achromatic metalens is realized by applying different voltages. Further, the electrically tunable customized dispersion-manipulated metalens and switchable color metaholograms are demonstrated. The proposed devices will accelerate the application of metasurfaces in broadband zoom imaging, AR/VR displays and spectral detection.

Near-infrared toroidal dipole response supported by silicon metasurfaces

The dielectric metasurfaces supporting non-radiative toroidal dipole resonances play important roles in nanophotonics. In this paper, toroidal dipole resonances using a double-axe nanostructure array in the near-infrared region are theoretically investigated by the characterization of the near-field distribution and far-field scattering. An experimental quality factor of 261 is obtained at the resonant wavelength of 1498 nm.

Humidity-Controlled Tunable Emission in a Dye-Incorporated Metal-Hydrogel-Metal Cavity

Actively controllable photoluminescence is potent for a wide variety of applications from biosensing and imaging to optoelectronic components. Traditionally, methods to achieve active emission control are limited due to complex fabrication processes or irreversible tuning. Here, we demonstrate active emission tuning, achieved by changing the ambient humidity in a fluorescent dye-containing hydrogel integrated into a metal-insulator-metal (MIM) system. Altering the overlapping region of the MIM cavity resonance and the absorption and emission spectra of the dye used is the underlying principle to achieving tunability of the emission. We first verify this by passive tuning of cavity resonance and further experimentally demonstrate active tuning in both air and aqueous environments. The proposed approach is reversible, easy to integrate, and spectrally scalable, thus providing opportunities for developing tunable photonic devices.

Light-Matter Interactions in Hybrid Material Metasurfaces

This Review focuses on the integration of plasmonic and dielectric metasurfaces with emissive or stimuli-responsive materials for manipulating light-matter interactions at the nanoscale. Metasurfaces, engineered planar structures with rationally designed building blocks, can change the local phase and intensity of electromagnetic waves at the subwavelength unit level and offers more degrees of freedom to control the flow of light. A combination of metasurfaces and nanoscale emitters facilitates access to weak and strong coupling regimes for enhanced photoluminescence, nanoscale lasing, controlled quantum emission, and formation of exciton-polaritons. In addition to emissive materials, functional materials that respond to external stimuli can be combined with metasurfaces to engineer tunable nanophotonic devices. Emerging metasurface designs including surface-functionalized, chemically tunable, and multilayer hybrid metasurfaces open prospects for diverse applications, including photocatalysis, sensing, displays, and quantum information.

High resolution multispectral spatial light modulators based on tunable Fabry-Perot nanocavities

Spatial light modulators (SLMs) are the most relevant technology for dynamic wavefront manipulation. They find diverse applications ranging from novel displays to optical and quantum communications. Among commercial SLMs for phase modulation, Liquid Crystal on Silicon (LCoS) offers the smallest pixel size and, thus, the most precise phase mapping and largest field of view (FOV). Further pixel miniaturization, however, is not possible in these devices due to inter-pixel cross-talks, which follow from the high driving voltages needed to modulate the thick liquid crystal (LC) cells that are necessary for full phase control. Newly introduced metasurface-based SLMs provide means for pixel miniaturization by modulating the phase via resonance tuning. These devices, however, are intrinsically monochromatic, limiting their use in applications requiring multi-wavelength operation. Here, we introduce a novel design allowing small pixel and multi-spectral operation. Based on LC-tunable Fabry-Perot nanocavities engineered to support multiple resonances across the visible range (including red, green and blue wavelengths), our design provides continuous 2π phase modulation with high reflectance at each of the operating wavelengths. Experimentally, we realize a device with 96 pixels (~1 μm pitch) that can be individually addressed by electrical biases. Using it, we first demonstrate multi-spectral programmable beam steering with FOV~18° and absolute efficiencies exceeding 40%. Then, we reprogram the device to achieve multi-spectral lensing with tunable focal distance and efficiencies ~27%. Our design paves the way towards a new class of SLM for future applications in displays, optical computing and beyond.

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.

Tunable dual-band metamaterial absorber in the infrared range based on split-ring-groove array

In this paper, we present a tunable dual-band perfect metamaterial absorber working in the infrared band by integrating a metallic split-ring-groove resonator array with a liquid crystal (LC) layer atop a metal substrate. By varying the height of the central nanodisks, the absorptivity of the dual-band absorption peaks can be simultaneously adjusted. The dual-band resonance frequencies of the proposed absorber exhibit continuous tunability by adjusting the refractive index of the LC, which can be controlled by applying external voltage. The mechanism of the perfect absorption is attributed to the gap plasmonic resonance coupling regime. The presented absorber exhibits good tolerance to incidence angles up to 60° and shows polarization dependent performance, which may offer promising applications in sensing, modulator, and optical absorption switching in the infrared regime.

Spontaneous emission in inertial and dissipative nematic liquid crystals: the role of critical phenomena

We develop a rigorous, field-theoretical approach to the study of spontaneous emission in inertial and dissipative nematic liquid crystals (LCs), disclosing an alternative application of the massive Stückelberg gauge theory to describe critical phenomena in these systems. This approach allows one not only to unveil the role of phase transitions in the spontaneous emission in LCs but also to make quantitative predictions for quantum emission in realistic nematics of current scientific and technological interest in the field of metamaterials. Specifically, we predict that one can switch on and off quantum emission in LCs by varying the temperature in the vicinities of the crystalline-to-nematic phase transition, for both the inertial and dissipative cases. We also predict from first principles the value of the critical exponent that characterizes such a transition, which we show not only to be independent of the inertial or dissipative dynamics, but also to be in good agreement with experiments. We determine the orientation of the dipole moment of the emitter relative to the nematic director that inhibits spontaneous emission, paving the way to achieve directionality of the emitted radiation, a result that could be applied in tuneable photonic devices such as metasurfaces and tuneable light sources.

Active magnetic dipole emission by the Ge2Sb2Te5 nanodisk

Active light manipulation plays a critical role in nanophotonics. In this Letter, we investigate the modulation properties of magnetic dipole (MD) emission based on the phase change material Ge₂Sb₂Te₅ hollow nanodisk (GST-HND). The results show that the amorphous GST-HND supports a strong MD response with a radiative decay enhancement of 282 times and quantum efficiency of 100%. More importantly, by tuning the crystallization rate of GST, the active manipulation of MD radiation is achieved with a quantum efficiency modulation depth of up to 95% at a specific wavelength. Our work may provide significant instruction for the active tuning of optical nanodevices.

Electrically Tunable Optical Metasurfaces for Dynamic Polarization Conversion

Dynamic control over the polarization of light is highly desirable in many optical applications, including optical communications, laser science, three-dimensional displays, among others. Conventional methods for polarization control are often based on bulky optical elements. To achieve highly integrated optical devices, metasurfaces, which have been intensively studied in recent years, hold great promises to replace conventional optical elements for a variety of optical functions. In this work, we demonstrate electrically tunable optical metasurfaces for dynamic polarization conversion at visible frequencies. By exploring both the geometric and propagation phase tuning capabilities, rapid and reversible polarization rotation up to 90° is achieved for linearly polarized light. The dynamic functionality is imparted by liquid crystals, which serve as a thin surrounding medium with electrically tunable refractive indices for the metasurface antennas. Furthermore, we expand our concept to demonstrate electrically tunable metasurfaces for dynamic holography and holographic information generation with independently controlled multiple pixels.

Piston-Type Optical Modulator for Dynamic Thermal Radiation Tuning Applications

This study introduces a movable piston-like structure that provides a simple and cost-effective avenue for dynamically tuning thermal radiation. This structure leverages two materials with dissimilar optical responses-graphite and aluminum-to modulate from a state of high reflectance to a state of high absorptance. A cavity is created in the graphite to house an aluminum cylinder, which is displaced to actuate the device. In its raised state, the large aluminum surface area promotes a highly reflective response, while in its lowered state, the expanded graphite surface area and blackbody cavity-like interactions significantly enhance absorptance. By optimizing the area ratio, reflectance tunability of over 30% is achieved for nearly the entire ultraviolet, visible, and near-infrared wavelength regions. Furthermore, a theoretical analysis postulates wavelength-dependent effectivenesses as high as 0.70 for this method, indicating that tunabilities approaching 70% can be achieved by exploiting near-ideal absorbers and reflectors. The analog nature of this control method allows for an infinitely variable optical response between the upper and lower bounds of the device. These valuable characteristics would enable this material structure to serve practical applications, such as reducing cost and energy requirements for environmental temperature management operations.

Space-Time Quantum Metasurfaces

Metasurfaces have recently entered the realm of quantum photonics, enabling manipulation of quantum light using a compact nanophotonic platform. Realizing the full potential of metasurfaces at the deepest quantum level requires the ability to tune coherent light-matter interactions continuously in space and time. Here, we introduce the concept of space-time quantum metasurfaces for arbitrary control of the spectral, spatial, and spin properties of nonclassical light using a compact photonic platform. We show that space-time quantum metasurfaces allow on-demand tailoring of entanglement among all degrees of freedom of a single photon. We also show that spatiotemporal modulation induces asymmetry at the fundamental level of quantum fluctuations, resulting in the generation of steered and vortex photon pairs out of vacuum. Space-time quantum metasurfaces have the potential to enable novel photonic functionalities, such as encoding quantum information into high-dimensional color qudits using designer modulation protocols, sculpting multispectral and multispatial modes in spontaneous emission, and generating reconfigurable hyperentanglement for high-capacity quantum communications.

Silicon Nanoantenna Mix Arrays for a Trifecta of Quantum Emitter Enhancements

Dielectric nanostructures have demonstrated optical antenna effects due to Mie resonances. Previous work has exhibited enhancements in absorption, emission rates and directionality with practical limitations. In this paper, we present a Si mix antenna array to achieve a trifecta enhancement of ∼1200-fold with a Purcell factor of ∼47. The antenna design incorporates ∼10 nm gaps, within which fluorescent molecules strongly absorb the pump laser energy through a resonant mode. In the emission process, the antenna array increases the radiative decay rates of the fluorescence molecules via a Purcell effect and provides directional emission through a separate mode. This work could lead to novel CMOS-compatible platforms to enhance fluorescence for biological and chemical applications.

Electrically Tunable Multifunctional Polarization-Dependent Metasurfaces Integrated with Liquid Crystals in the Visible Region

Metasurfaces open up new avenues for designing planar optics, enabling compact dynamic metadevices. Numerous dynamic strategies have been proposed, among which liquid crystal (LC) based metasurfaces are expected due to the maturity of LC materials. However, existing schemes rarely exploit the polarization manipulation capabilities of metasurfaces and the limited performance hinders the development of practical addressable devices. Here, we demonstrate an electrically tunable multifunctional polarization-dependent metasurface integrated with LCs in the visible range. By a combination of the helicity-dependent metasurface and the birefringent LCs, continuous intensity tuning and switching of two helicity channels are realized. Electrically tunable mono- and multicolor switchable metaholograms and dynamic varifocal metalenses are demonstrated with a simple and performance-enhancing integration scheme. Further, electrically addressable dynamic metasurfaces are achieved. The proposed modulation and integration schemes pave the way for addressable dynamic metasurface devices in various applications, such as space light modulators, light detection and ranging systems, and holographic displays.

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.

Liquid crystal integrated metadevice for reconfigurable hologram displays and optical encryption

The ultimate goal of metasurface research in recent years is to apply metasurface to reality applications and improve the performance compared to its counterpart, namely conventional optical elements with the same function. Inspired by the application of electrically addressing spatial light modulator (EA-SLM) and based on the binary holographic algorithm, here we propose a reconfigurable metadevice integrated with the nematic liquid crystal (NLC). The smart metadevice directly uses the subwavelength antennas as the main contributor to the phase accumulation instead of the NLC layer. By applying different electrical modulation patterns on the NLC, the metadevice can realize the function of dynamic holographic display as traditional SLMs but features in smaller size, higher resolution and lager field of view. In addition, we improved the existing computer-generated hologram algorithm to generate three holograms with quantitative correlation and also propose a new optical encryption method based on our metadevice. The encryption method needs four elements in total to decrypt and can fully meets the requirements of the various encrypted content. We believe such metadevice paves the way for the new generation of micro-optical display and optical encryption devices.

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.

Suppressing material loss in the visible and near-infrared range for functional nanophotonics using bandgap engineering

All-dielectric nanostructures have recently opened exciting opportunities for functional nanophotonics, owing to their strong optical resonances along with low material loss in the near-infrared range. Pushing these concepts to the visible range is hindered by their larger absorption coefficient, thus encouraging the search for alternative dielectrics for nanophotonics. Here, we employ bandgap engineering to synthesize hydrogenated amorphous Si nanoparticles (a-Si:H NPs) offering ideal features for functional nanophotonics. We observe significant material loss suppression in a-Si:H NPs in the visible range caused by hydrogenation-induced bandgap renormalization, producing strong higher-order resonant modes in single NPs with Q factors up to ~100 in the visible and near-IR range. We also realize highly tunable all-dielectric meta-atoms by coupling a-Si:H NPs to photochromic spiropyran molecules. ~70% reversible all-optical tuning of light scattering at the higher-order resonant mode under a low incident light intensity is demonstrated. Our results promote the development of high-efficiency visible nanophotonic devices.

Large absorption of high-index semiconductors has hindered the application of all dielectric nanostructures in the visible range. Here, the authors present bandgap-engineered hydrogenated amorphous Si nanoparticles with Q-factors up to 100 and their integration with photochromic molecules as tunable meta-atoms.

Polarization tunable color filters based on all-dielectric metasurfaces on a flexible substrate

Structural color filters based on all-dielectric materials are considered to be promising alternatives to metal nanostructures due to significant advantages, such as high-quality resonance effects and low losses of Ohmic effects. We demonstrate a polarization tunable color filter based on all-dielectric metasurfaces, which is based on the arrays of asymmetric monocrystalline silicon nanoblocks on the flexible substrate. By adjusting the physical dimensions of nanoblocks, the filter can exhibit a variety of bright transmission colors. Furthermore, the designed dielectric metasurfaces are sensitive to the linear polarization direction of the incident light, thus a wide range of color images can be created by changing the polarization angles. All of the color filter including the dielectric silicon nanoblocks, the overcladding, and the flexible substrate can be delaminated from the handler substrates and the optical property is reconfigurable, which will find applications in the functional color display, polarization detection and imaging, and secured optical tag.

Topologically-tuned spin Hall shift around Fano resonance

The topological magnetoelectric effect is associated with the photonic spin-orbit interaction. However, due to the proportionate fine structure constant of the topological term, the topological magnetoelectric effect is usually weak. In this paper, we demonstrate that the axion term enables manipulation of the spin Hall shift of light around Fano resonance. And, the excited surface plasmon near the nanoparticle's interface could enhance the topological magnetoelectric effect for several orders. Numerical simulation of near field and far-field scattering confirms our theoretical results. Our work may pave the way to exploit the topological magnetoelectric effect in practical applications, such as optical sensing and nanoprobing.

Liquid-Crystal-Mediated Geometric Phase: From Transmissive to Broadband Reflective Planar Optics

Planar optical elements that can manipulate the multidimensional physical parameters of light efficiently and compactly are highly sought after in modern optics and nanophotonics. In recent years, the geometric phase, induced by the photonic spin-orbit interaction, has attracted extensive attention for planar optics due to its powerful beam shaping capability. The geometric phase can usually be generated via inhomogeneous anisotropic materials, among which liquid crystals (LCs) have been a focus. Their pronounced optical properties and controllable and stimuli-responsive self-assembly behavior introduce new possibilities for LCs beyond traditional panel displays. Recent advances in LC-mediated geometric phase planar optics are briefly reviewed. First, several recently developed photopatterning techniques are presented, enabling the accurate fabrication of complicated LC microstructures. Subsequently, nematic LC-based transmissive planar optical elements and chiral LC-based broadband reflective elements are reviewed systematically. Versatile functionalities are revealed, from conventional beam steering and focusing, to advanced structuring. Combining the geometric phase with structured LC materials offers a satisfactory platform for planar optics with desired functionalities and drastically extends exceptional applications of ordered soft matter. Some prospects on this rapidly advancing field are also provided.

Generalized Maxwell projections for multi-mode network Photonics

The design of optical resonant systems for controlling light at the nanoscale is an exciting field of research in nanophotonics. While describing the dynamics of few resonances is a relatively well understood problem, controlling the behavior of systems with many overlapping states is considerably more difficult. In this work, we use the theory of generalized operators to formulate an exact form of spatio-temporal coupled mode theory, which retains the simplicity of traditional coupled mode theory developed for optical waveguides. We developed a fast computational method that extracts all the characteristics of optical resonators, including the full density of states, the modes quality factors, and the mode resonances and linewidths, by employing a single first principle simulation. This approach can facilitate the analytical and numerical study of complex dynamics arising from the interactions of many overlapping resonances, defined in ensembles of resonators of any geometrical shape and in materials with arbitrary responses.

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.

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.

Phase Modulation with Electrically Tunable Vanadium Dioxide Phase-Change Metasurfaces

We report a dynamically tunable reflectarray metasurface that continuously modulates the phase of reflected light in the near-infrared wavelength range under active electrical control of the phase transition from semiconducting to semimetallic states. We integrate a vanadium dioxide (VO₂) active layer into the dielectric gap of antenna elements in a reflectarray metasurface, which undergoes an insulator-to-metal transition upon resistive heating of the metallic patch antenna. The induced phase transition in the VO₂ film strongly perturbs the magnetic dipole resonance supported by the metasurface. By carefully controlling the volume fractions of coexisting metallic and dielectric regions of the VO₂ film, we observe a continuous shift of the phase of the reflected light, with a maximal achievable phase shift as high as 250°. We also observe a reflectance modulation of 23.5% as well as a spectral shift of the resonance position by 175 nm. The metasurface phase modulation is fairly broadband, yielding large phase shifts at multiple operation wavelengths.

Toroidal metasurface resonances in microwave waveguides

We theoretically investigate the possibility to load microwave waveguides with dielectric particle arrays that emulate the properties of infinite, two-dimensional, all-dielectric metasurfaces. First, we study the scattering properties and the electric and magnetic multipole modes of dielectric cuboids and identify the conditions for the excitation of the so-called anapole state. Based on the obtained results, we design metasurfaces composed of a square lattice of dielectric cuboids, which exhibit strong toroidal resonances. Then, three standard microwave waveguide types, namely parallel-plate waveguides, rectangular waveguides, and microstrip lines, loaded with dielectric cuboids are designed, in such a way that they exhibit the same resonant features as the equivalent dielectric metasurface. The analysis shows that parallel-plate and rectangular waveguides can almost perfectly reproduce the metasurface properties at the resonant frequency. The main attributes of such resonances are also observed in the case of a standard impedance-matched microstrip line, which is loaded with only a small number of dielectric particles. The results demonstrate the potential for a novel paradigm in the design of "metasurface-loaded" microwave waveguides, either as functional elements in microwave circuitry, or as a platform for the experimental study of the properties of dielectric metasurfaces.

Manipulation of Magnetic Dipole Emission from Eu3+ with Mie-Resonant Dielectric Metasurfaces

Mie-resonant high-index dielectric nanoparticles and metasurfaces have been suggested as a viable platform for enhancing both electric and magnetic dipole transitions of fluorescent emitters. While the enhancement of the electric dipole transitions by such dielectric nanoparticles has been demonstrated experimentally, the case of magnetic-dipole transitions remains largely unexplored. Here, we study the enhancement of spontaneous emission of Eu³⁺ ions, featuring both electric and magnetic-dominated dipole transitions, by dielectric metasurfaces composed of Mie-resonant silicon nanocylinders. By coating the metasurfaces with a layer of an Eu³⁺ doped polymer, we observe an enhancement of the Eu³⁺ emission associated with the electric (at 610 nm) and magnetic-dominated (at 590 nm) dipole transitions. The enhancement factor depends systematically on the spectral proximity of the atomic transitions to the Mie resonances as well as their multipolar order, both controlled by the nanocylinder size. Importantly, the branching ratio of emission via the electric or magnetic transition channel can be modified by carefully designing the metasurface, where the magnetic dipole transition is enhanced more than the electric transition for cylinders with radii of about 130 nm. We confirm our observations by numerical simulations based on the reciprocity principle. Our results open new opportunities for bright nanoscale light sources based on magnetic transitions.

Active control of anapole states by structuring the phase-change alloy Ge2Sb2Te5

High-index dielectric nanoparticles supporting a distinct series of Mie resonances have enabled a new class of optical antennas with unprecedented functionalities. The great wealth of multipolar responses has not only brought in new physical insight but also spurred practical applications. However, how to make such a colorful resonance palette actively tunable is still elusive. Here, we demonstrate that the structured phase-change alloy Ge2Sb2Te5 (GST) can support a diverse set of multipolar Mie resonances with active tunability. By harnessing the dramatic optical contrast of GST, we realize broadband (Δλ/λ ~ 15%) mode shifting between an electric dipole resonance and an anapole state. Active control of higher-order anapoles and multimodal tuning are also investigated, which make the structured GST serve as a multispectral optical switch with high extinction contrasts (>6 dB). With all these findings, our study provides a new direction for realizing active nanophotonic devices.

Active tuning of the Fano resonance from a Si nanosphere dimer by the substrate effect

All-dielectric materials have aroused great interest for their unique light scattering and lower losses compared with plasmonics. Generally, optical properties made by all-dielectric materials can be passively controlled by varying the geometry, size and refractive index at the design stage. Therefore, the realization of active tuning in the field of nanophotonics is important to improve the practicality and achieve light-on-chip technology in the future. Herein, we combine the high refractive index of Si and the phase transition of VO₂ to form an active tuning hybrid nanostructure with higher quality factor by depositing Si nanospheres on the VO₂ layer with an Al₂O₃ substrate. As the temperature goes up, the refractive index of the VO₂ layer switches from high to low. The scattering intensity of the magnetic dipole resonance of Si nanospheres decreases differently depending on their size, while the intensity of the electric dipole resonance remains almost unchanged. Meanwhile, Fano resonances are observed in the Si nanosphere dimers with a continuous variable Fano lineshape when adjusting the temperature. Mie theory and substrate-induced resonant magneto-electric effects are used to analyze and explain these phenomena. Tuning of the Fano resonance is attributed to the substrate effect from the interaction between Si nanospheres and phase transition of the VO₂ layer with temperature. These light scattering properties of such a hybrid nanostructure make it promising for temperature sensing or as a light source at the nanometer scale.

Light-Emitting Metasurfaces: Simultaneous Control of Spontaneous Emission and Far-Field Radiation

Light-emitting sources and devices permeate every aspect of our lives and are used in lighting, communications, transportation, computing, and medicine. Advances in multifunctional and "smart lighting" would require revolutionary concepts in the control of emission spectra and directionality. Such control might be possible with new schemes and regimes of light-matter interaction paired with developments in light-emitting materials. Here we show that all-dielectric metasurfaces made from III-V semiconductors with embedded emitters have the potential to provide revolutionary lighting concepts and devices, with new functionality that goes far beyond what is available in existing technologies. Specifically, we use Mie-resonant metasurfaces made from semiconductor heterostructures containing epitaxial quantum dots. By controlling the symmetry of the resonant modes, their overlap with the emission spectra, and other structural parameters, we can enhance the brightness by 2 orders of magnitude, as well as reduce its far-field divergence significantly.

Recent Advances in Tunable and Reconfigurable Metamaterials

Metamaterials are composed of nanostructures, called artificial atoms, which can give metamaterials extraordinary properties that cannot be found in natural materials. The nanostructures themselves and their arrangements determine the metamaterials’ properties. However, a conventional metamaterial has fixed properties in general, which limit their use. Thus, real-world applications of metamaterials require the development of tunability. This paper reviews studies that realized tunable and reconfigurable metamaterials that are categorized by the mechanisms that cause the change: inducing temperature changes, illuminating light, inducing mechanical deformation, and applying electromagnetic fields. We then provide the advantages and disadvantages of each mechanism and explain the results or effects of tuning. We also introduce studies that overcome the disadvantages or strengthen the advantages of each classified tunable metamaterial.

Optical metasurfaces: new generation building blocks for multi-functional optics

Optical metasurfaces (OMs) have emerged as promising candidates to solve the bottleneck of bulky optical elements. OMs offer a fundamentally new method of light manipulation based on scattering from resonant nanostructures rather than conventional refraction and propagation, thus offering efficient phase, polarization, and emission control. This perspective highlights state of the art OMs and provides a roadmap for future applications, including active generation, manipulation and detection of light for quantum technologies, holography and sensing.