Using low-loss phase-change materials for mid-infrared antenna resonance tuning

Beyond the Surface: Interconnection of Viscosity, Crystal Growth, and Diffusion in Ge25Se75 Glass-Former

The knowledge of viscosity behavior, crystal growth phenomenon, and diffusion is important in producing, processing, and practical applications of amorphous solids prepared in different forms (bulk glasses and thin films). This work uses microscopy to study volume crystal growth in Ge25Se75 bulk glasses and thermally evaporated thin films. The collected growth data measured over a wide temperature range show a significant increase in crystal growth rates in thin films. The crystal growth is analyzed using near-surface viscosities obtained in bulks and thin films using nanoindentation and melt viscosities measured by a pressure-assisted melt filling technique. The crystal growth analysis provides information on the size of the structural units incorporated into the growing crystals, essential for estimating the diffusion coefficients and explaining the difference in crystal growth rates in bulk and thin films. The crystal growth analysis also reveals the decoupling between diffusion and viscous flow described by the Stokes-Einstein-Eyring relation. Moreover, to the authors' best knowledge, the manuscript provides the first evaluation estimation of the effective self-diffusion coefficient directly from growth data in chalcogenide glass-formers. The present data show a similar relation between diffusion coefficients (D) and crystal growth rates (u): u ≈ D0.87, which is found in several molecular glasses.

Optoelectronic tuning of plasmon resonances via optically modulated hot electrons

Fast optical modulation of nanoplasmonics is fundamental for on-chip integration of all-optical devices. Although various strategies have been proposed for dynamic modulation of surface plasmons, critical issues of device compatibility and extremely low efficiency in the visible spectrum hamper the application of optoplasmonic nanochips. Here we establish an optoplasmonic system based on Au@Cu2-xS hybrid core-shell nanoparticles. The optical excitation of hot electrons and their charge transfer to the semiconductor coating (Cu2-xS) lead to lowered electron density of Au, which results in the red shift of the localized surface plasmon resonance. The hot electrons can also transport through the Cu2-xS layer to the metal substrate, which increases the conductance of the nanogap. As such, the coupled gap plasmon blue-shifts with a magnitude of up to ∼15 nm, depending on the excitation power and the thickness of the coatings, which agrees with numerical simulations. All of this optoelectronic tuning process is highly reversible, controllable and fast with a modulated laser beam, which is highly compatible and sufficiently useful for on-chip integration of nanophotonic devices.

Multispectral camouflage and radiative cooling using dynamically tunable metasurface

With the increasing demand for privacy, multispectral camouflage devices that utilize metasurface designs in combination with mature detection technologies have become effective. However, these early designs face challenges in realizing multispectral camouflage with a single metasurface and restricted modes. Therefore, this paper proposes a dynamically tunable metasurface. The metasurface consists of gold (Au), antimony selenide (Sb2Se3), and aluminum (Al), which enables radiative cooling, light detection and ranging (LiDAR) and infrared camouflage. In the amorphous phase of Sb2Se3, the thermal radiation reduction rate in the mid wave infrared range (MWIR) is up to 98.2%. The echo signal reduction rate for the 1064 nm LiDAR can reach 96.3%. In the crystalline phase of Sb2Se3, the highest cooling power is 65.5 Wm-2. Hence the metasurface can reduce the surface temperature and achieve efficient infrared camouflage. This metasurface design provides a new strategy for making devices compatible with multispectral camouflage and radiative cooling.

Nonlocal, Flat-Band Meta-Optics for Monolithic, High-Efficiency, Compact Photodetectors

Miniaturized photodetectors are becoming increasingly sought-after components for next-generation technologies, such as autonomous vehicles, integrated wearable devices, or gadgets embedded on the Internet of Things. A major challenge, however, lies in shrinking the device footprint while maintaining high efficiency. This conundrum can be solved by realizing a nontrivial relation between the energy and momentum of photons, such as dispersion-free devices, known as flat bands. Here, we leverage flat-band meta-optics to simultaneously achieve critical absorption over a wide range of incidence angles. For a monolithic silicon meta-optical photodiode, we achieved an ∼10-fold enhancement in the photon-to-electron conversion efficiency. Such enhancement over a large angular range of ∼36° allows incoming light to be collected via a large-aperture lens and focused on a compact photodiode, potentially enabling high-speed and low-light operation. Our research unveils new possibilities for creating compact and efficient optoelectronic devices with far-reaching impact on various applications, including augmented reality and light detection and ranging.

Versatile spaceborne photonics with chalcogenide phase-change materials

Recent growth in space systems has seen increasing capabilities packed into smaller and lighter Earth observation and deep space mission spacecraft. Phase-change materials (PCMs) are nonvolatile, reconfigurable, fast-switching, and have recently shown a high degree of space radiation tolerance, thereby making them an attractive materials platform for spaceborne photonics applications. They promise robust, lightweight, and energy-efficient reconfigurable optical systems whose functions can be dynamically defined on-demand and on-orbit to deliver enhanced science or mission support in harsh environments on lean power budgets. This comment aims to discuss the recent advances in rapidly growing PCM research and its potential to transition from conventional terrestrial optoelectronics materials platforms to versatile spaceborne photonic materials platforms for current and next-generation space and science missions. Materials International Space Station Experiment-14 (MISSE-14) mission-flown PCMs outside of the International Space Station (ISS) and key results and NASA examples are highlighted to provide strong evidence of the applicability of spaceborne photonics.

Tunable parity-time symmetry vortex laser from a phase change material-based microcavity

Traditional light sources cannot emit an electromagnetic (EM) field with an orbital angular momentum (OAM), limiting their applications in modern optics. The recent development of the OAM laser, mainly based on micro- and nanostructures, can satisfy the increasing requirements for on-chip photonics and information capacities. Nevertheless, the photonic structures have fixed parameters that prevent these OAM lasers from being dynamically tuned. Here, we propose tunable vortex lasing from a microring cavity integrated by a phase change material, Ge2Sb2Te5 (GST225). By modulating the complex refractive index to create an exceptional point (EP) to break the degeneracy of whispering gallery modes with opposite orientations, the microlaser working at the EP can impart an artificial angular momentum, thus emitting vortex beams with well-defined OAM. The grating scatter on the edge of the microring can provide efficient vertical radiation. The vortex laser wavelength from the GST225/InGaAsP dual-layered microring cavity can be dynamically tuned by switching the state of GST225 between amorphous and crystalline without changing the microring geometry. We construct an electric-thermal model to show the tuning range of operating wavelengths (EPs) from 1544.5 to 1565.9 nm in ~25 ns. Our study on high-speed tunable PT-symmetry vortex lasers facilitates the next generation of integrated optoelectronic devices for optical computing and communications in both classical and quantum regions.

Roadmap for phase change materials in photonics and beyond

Phase Change Materials (PCMs) have demonstrated tremendous potential as a platform for achieving diverse functionalities in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum, ranging from terahertz to visible frequencies. This comprehensive roadmap reviews the material and device aspects of PCMs, and their diverse applications in active and reconfigurable micro-nanophotonic devices across the electromagnetic spectrum. It discusses various device configurations and optimization techniques, including deep learning-based metasurface design. The integration of PCMs with Photonic Integrated Circuits and advanced electric-driven PCMs are explored. PCMs hold great promise for multifunctional device development, including applications in non-volatile memory, optical data storage, photonics, energy harvesting, biomedical technology, neuromorphic computing, thermal management, and flexible electronics.

Review: tunable nanophotonic metastructures

Tunable nanophotonic metastructures offer new capabilities in computing, networking, and imaging by providing reconfigurability in computer interconnect topologies, new optical information processing capabilities, optical network switching, and image processing. Depending on the materials and the nanostructures employed in the nanophotonic metastructure devices, various tuning mechanisms can be employed. They include thermo-optical, electro-optical (e.g. Pockels and Kerr effects), magneto-optical, ionic-optical, piezo-optical, mechano-optical (deformation in MEMS or NEMS), and phase-change mechanisms. Such mechanisms can alter the real and/or imaginary parts of the optical susceptibility tensors, leading to tuning of the optical characteristics. In particular, tunable nanophotonic metastructures with relatively large tuning strengths (e.g. large changes in the refractive index) can lead to particularly useful device applications. This paper reviews various tunable nanophotonic metastructures' tuning mechanisms, tuning characteristics, tuning speeds, and non-volatility. Among the reviewed tunable nanophotonic metastructures, some of the phase-change-mechanisms offer relatively large index change magnitude while offering non-volatility. In particular, Ge-Sb-Se-Te (GSST) and vanadium dioxide (VO2) materials are popular for this reason. Mechanically tunable nanophotonic metastructures offer relatively small changes in the optical losses while offering large index changes. Electro-optically tunable nanophotonic metastructures offer relatively fast tuning speeds while achieving relatively small index changes. Thermo-optically tunable nanophotonic metastructures offer nearly zero changes in optical losses while realizing modest changes in optical index at the expense of relatively large power consumption. Magneto-optically tunable nanophotonic metastructures offer non-reciprocal optical index changes that can be induced by changing the magnetic field strengths or directions. Tunable nanophotonic metastructures can find a very wide range of applications including imaging, computing, communications, and sensing. Practical commercial deployments of these technologies will require scalable, repeatable, and high-yield manufacturing. Most of these technology demonstrations required specialized nanofabrication tools such as e-beam lithography on relatively small fractional areas of semiconductor wafers, however, with advanced CMOS fabrication and heterogeneous integration techniques deployed for photonics, scalable and practical wafer-scale fabrication of tunable nanophotonic metastructures should be on the horizon, driven by strong interests from multiple application areas.

Colloidal Ternary Telluride Quantum Dots for Tunable Phase Change Optics in the Visible and Near-Infrared

A structural change between amorphous and crystalline phase provides a basis for reliable and modular photonic and electronic devices, such as nonvolatile memory, beam steerers, solid-state reflective displays, or mid-IR antennas. In this paper, we leverage the benefits of liquid-based synthesis to access phase-change memory tellurides in the form of colloidally stable quantum dots. We report a library of ternary M_xGe_1-xTe colloids (where M is Sn, Bi, Pb, In, Co, Ag) and then showcase the phase, composition, and size tunability for Sn-Ge-Te quantum dots. Full chemical control of Sn-Ge-Te quantum dots permits a systematic study of structural and optical properties of this phase-change nanomaterial. Specifically, we report composition-dependent crystallization temperature for Sn-Ge-Te quantum dots, which is notably higher compared to bulk thin films. This gives the synergistic benefit of tailoring dopant and material dimension to combine the superior aging properties and ultrafast crystallization kinetics of bulk Sn-Ge-Te, while improving memory data retention due to nanoscale size effects. Furthermore, we discover a large reflectivity contrast between amorphous and crystalline Sn-Ge-Te thin films, exceeding 0.7 in the near-IR spectrum region. We utilize these excellent phase-change optical properties of Sn-Ge-Te quantum dots along with liquid-based processability for nonvolatile multicolor images and electro-optical phase-change devices. Our colloidal approach for phase-change applications offers higher customizability of materials, simpler fabrication, and further miniaturization to the sub-10 nm phase-change devices.

Phase-change materials based on amorphous equichalcogenides

Phase-change materials, demonstrating a rapid switching between two distinct states with a sharp contrast in electrical, optical or magnetic properties, are vital for modern photonic and electronic devices. To date, this effect is observed in chalcogenide compounds based on Se, Te or both, and most recently in stoichiometric Sb₂S₃ composition. Yet, to achieve best integrability into modern photonics and electronics, the mixed S/Se/Te phase change medium is needed, which would allow a wide tuning range for such important physical properties as vitreous phase stability, radiation and photo-sensitivity, optical gap, electrical and thermal conductivity, non-linear optical effects, as well as the possibility of structural modification at nanoscale. In this work, a thermally-induced high-to-low resistivity switching below 200 °C is demonstrated in Sb-rich equichalcogenides (containing S, Se and Te in equal proportions). The nanoscale mechanism is associated with interchange between tetrahedral and octahedral coordination of Ge and Sb atoms, substitution of Te in the nearest Ge environment by S or Se, and Sb-Ge/Sb bonds formation upon further annealing. The material can be integrated into chalcogenide-based multifunctional platforms, neuromorphic computational systems, photonic devices and sensors.

Fast modulation of surface plasmons based on the photothermal effect of nonvolatile solid thin films

Nonvolatile phase change materials owing to their robust stability and reversibility have shown significant potential in nanophotonic switches and memory devices. However, their performances deteriorate as the thickness decreases below 10 nm due to the local deformation induced by the phase change, which makes them less compatible with plasmonic nanogaps. Here, we address this issue by photothermally modulating the refractive index of germanium antimony telluride (GST) placed in plasmonic nanogaps, which tunes plasmon resonances in the visible region below the melting point of GST, making such optical switching highly reversible at a rate of up to hundreds of ∼kHz. They are also demonstrated to modulate the waveguiding efficiency of propagating surface plasmons, which is based on the photothermal modulation of plasmons with the assistance of GST. Such hybrid nanoplasmonic system with cost-effective fabrication and efficient operation method provides a promising route towards integrated nanophotonic chips.

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.

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.

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.

Extremely Narrow and Actively Tunable Mie Surface Lattice Resonances in GeSbTe Metasurfaces: Study

Mie surface lattice resonances (SLRs) supported by periodic all-dielectric nanoparticles emerge from the radiative coupling of localized Mie resonances in individual nanoparticles through Rayleigh anomaly diffraction. To date, it remains challenging to achieve narrow bandwidth and active tuning simultaneously. In this work, we report extremely narrow and actively tunable electric dipole SLRs (ED-SLRs) in Ge₂Se₂Te₅ (GST) metasurfaces. Simulation results show that, under oblique incidence with TE polarization, ED-SLRs with extremely narrow linewidth down to 12 nm and high quality factor up to 409 can be excited in the mid-infrared regime. By varying the incidence angle, the ED-SLR can be tuned over an extremely large spectral region covering almost the entire mid-infrared regime. We further numerically show that, by changing the GST crystalline fraction, the ED-SLR can be actively tuned, leading to nonvolatile, reconfigurable, and narrowband filtering, all-optical multilevel modulation, or all-optical switching with high performance. We expect that this work will advance the engineering of Mie SLRs and will find intriguing applications in optical telecommunication, networks, and microsystems.

Far-Infrared Near-Field Optical Imaging and Kelvin Probe Force Microscopy of Laser-Crystallized and -Amorphized Phase Change Material Ge3Sb2Te6

Chalcogenide phase change materials reversibly switch between non-volatile states with vastly different optical properties, enabling novel active nanophotonic devices. However, a fundamental understanding of their laser-switching behavior is lacking and the resulting local optical properties are unclear at the nanoscale. Here, we combine infrared scattering-type scanning near-field optical microscopy (SNOM) and Kelvin probe force microscopy (KPFM) to investigate four states of laser-switched Ge₃Sb₂Te₆ (as-deposited amorphous, crystallized, reamorphized, and recrystallized) with nanometer lateral resolution. We find SNOM to be especially sensitive to differences between crystalline and amorphous states, while KPFM has higher sensitivity to changes introduced by melt-quenching. Using illumination from a free-electron laser, we use the higher sensitivity to free charge carriers of far-infrared (THz) SNOM compared to mid-infrared SNOM and find evidence that the local conductivity of crystalline states depends on the switching process. This insight into the local switching of optical properties is essential for developing active nanophotonic devices.

Quantum Plasmonics: Energy Transport Through Plasmonic Gap

At the interfaces of metal and dielectric materials, strong light-matter interactions excite surface plasmons; this allows electromagnetic field confinement and enhancement on the sub-wavelength scale. Such phenomena have attracted considerable interest in the field of exotic material-based nanophotonic research, with potential applications including nonlinear spectroscopies, information processing, single-molecule sensing, organic-molecule devices, and plasmon chemistry. These innovative plasmonics-based technologies can meet the ever-increasing demands for speed and capacity in nanoscale devices, offering ultrasensitive detection capabilities and low-power operations. Size scaling from the nanometer to sub-nanometer ranges is consistently researched; as a result, the quantum behavior of localized surface plasmons, as well as those of matter, nonlocality, and quantum electron tunneling is investigated using an innovative nanofabrication and chemical functionalization approach, thereby opening a new era of quantum plasmonics. This new field enables the ultimate miniaturization of photonic components and provides extreme limits on light-matter interactions, permitting energy transport across the extremely small plasmonic gap. In this review, a comprehensive overview of the recent developments of quantum plasmonic resonators with particular focus on novel materials is presented. By exploring the novel gap materials in quantum regime, the potential quantum technology applications are also searched for and mapped out.

Non-Layered Gold-Silicon and All-Silicon Frequency-Selective Metasurfaces for Potential Mid-Infrared Sensing Applications

We report simulations on the spectral behavior of non-layered gold-silicon and all-silicon frequency-selective metasurfaces in an asymmetric element configuration in the mid-infrared spectral window of 5-5.8 μm. The non-layered layout is experimentally feasible due to recent technological advances such as nano-imprint and nano-stencil lithography, and the spectral window was chosen due to the multitude of applications in sensing and imaging. The architecture exhibits significant resonance in the window of interest as well as extended tunability by means of variation of cell element sizes and relative coordinates. The results indicate that the proposed metasurface architecture is a viable candidate for mid-infrared absorbers, sensors and imaging systems.

Stimuli-Responsive Phase Change Materials: Optical and Optoelectronic Applications

Stimuli-responsive materials offer a large variety of possibilities in fabrication of solid- state devices. Phase change materials (PCMs) undergo rapid and drastic changes of their optical properties upon switching from one crystallographic phase to another one. This peculiarity makes PCMs ideal candidates for a number of applications including sensors, active displays, photonic volatile and non-volatile memories for information storage and computer science and optoelectronic devices. This review analyzes different examples of PCMs, in particular germanium–antimonium tellurides and vanadium dioxide (VO₂) and their applications in the above-mentioned fields, with a detailed discussion on potential, limitations and challenges.

Electrical tuning of phase-change antennas and metasurfaces

The success of semiconductor electronics is built on the creation of compact, low-power switching elements that offer routing, logic and memory functions. The availability of nanoscale optical switches could have a similarly transformative impact on the development of dynamic and programmable metasurfaces, optical neural networks and quantum information processing. Phase-change materials are uniquely suited to enable their creation as they offer high-speed electrical switching between amorphous and crystalline states with notably different optical properties. Their high refractive index has already been harnessed to fashion them into compact optical antennas. Here, we take the next important step, by showing electrically-switchable phase-change antennas and metasurfaces that offer strong, reversible, non-volatile, multi-phase switching and spectral tuning of light scattering in the visible and near-infrared spectral ranges. Their successful implementation relies on a careful joint thermal and optical optimization of the antenna elements that comprise a silver strip that simultaneously serves as a plasmonic resonator and a miniature heating stage. Our metasurface affords electrical modulation of the reflectance by more than fourfold at 755 nm.

Dynamically Switchable Polarization-Independent Triple-Band Perfect Metamaterial Absorber Using a Phase-Change Material in the Mid-Infrared (MIR) Region

A tunable metamaterial absorber (MMA) by reversible phase transitions in a mid-infrared regime is theoretically investigated. The absorber is composed of a molybdenum (Mo)-germanium-antimony-tellurium (Ge₂Sb₂Te₅, GST)-Mo nanodisk structure superimposed on the GST-Al₂O₃ (aluminum oxide)-Mo film. Studies have shown that the combination of the inlaid metal-medium dielectric waveguide mode and the resonant cavity mode and the excitation of the propagating surface plasmon mode are the main reasons for the formation of the triple-band high absorption. Additionally, through the reversible phase change, the transition from high absorption to high reflection in the mid-infrared region is realized. The symmetry of the absorber eliminates the polarization dependence, and the near unity absorption efficiency can be maintained by incidence angles up to 60°. The presented method will enhance the functionality of the absorber and has the potential for the applications that require active control over light absorption.

Active terahertz spin Hall effect in vanadium dioxide metasurfaces

Insulator-to-metal transition induces large material property variations in vanadium dioxide (VO₂) over a broad frequency band. VO₂, therefore, has been introduced into metallic resonating structures to realize reconfigurable metadevices from microwave to optical wavelengths. Beyond enabling metal/VO₂ hybrid meta-atoms, in the THz regime metallic-phase VO₂ micro-structures can support strong electromagnetic resonances, offering great potential in active manipulation of THz radiation. In this paper, we show that VO₂ dipole antennas can be used to realize geometric phase coded metasurfaces for wave-front shaping and polarization rotation of THz waves. Moreover, we demonstrate that the corresponding efficiency of the THz spin Hall effect is closely related to VO₂'s THz electrical conductivity. In light of the dispersionless nature of the geometric phase, our study indicates that metasurfaces constructed by VO₂ subwavelength resonators are good candidates for active control of broadband THz radiation.

Phase Change Metasurfaces by Continuous or Quasi-Continuous Atoms for Active Optoelectronic Integration

In recent decades, metasurfaces have emerged as an exotic and appealing group of nanophotonic devices for versatile wave regulation with deep subwavelength thickness facilitating compact integration. However, the ability to dynamically control the wave-matter interaction with external stimulus is highly desirable especially in such scenarios as integrated photonics and optoelectronics, since their performance in amplitude and phase control settle down once manufactured. Currently, available routes to construct active photonic devices include micro-electromechanical system (MEMS), semiconductors, liquid crystal, and phase change materials (PCMs)-integrated hybrid devices, etc. For the sake of compact integration and good compatibility with the mainstream complementary metal oxide semiconductor (CMOS) process for nanofabrication and device integration, the PCMs-based scheme stands out as a viable and promising candidate. Therefore, this review focuses on recent progresses on phase change metasurfaces with dynamic wave control (amplitude and phase or wavefront), and especially outlines those with continuous or quasi-continuous atoms in favor of optoelectronic integration.

Tunable wave plates based on phase-change metasurfaces

Wave plates based on metasurfaces have attracted intensive attention over the past decade owing to their compactness and design flexibility. Although various wave plates have been designed, their working wavelengths are fixed once they are made. Here we present a study on tunable wave plates based on phase-change metasurfaces made of Ge₂Sb₂Te₅ nanopillar structures. The Ge₂Sb₂Te₅ nanopillars can work as a high-efficiency transmissive half- or quarter-wave plate depending on their structural parameters. The working wavelength of wave plate can be tuned via the phase transition of Ge₂Sb₂Te₅. Moreover, the polarization state of the transmitted light at a fixed wavelength can be modified by changing the crystallinity of Ge₂Sb₂Te₅. The features suggest that tunable wave plates may have applications in optical modulators, molecular detection, and polarimetric imaging.

In3SbTe2 as a programmable nanophotonics material platform for the infrared

The high dielectric optical contrast between the amorphous and crystalline structural phases of non-volatile phase-change materials (PCMs) provides a promising route towards tuneable nanophotonic devices. Here, we employ the next-generation PCM In₃SbTe₂ (IST) whose optical properties change from dielectric to metallic upon crystallization in the whole infrared spectral range. This distinguishes IST as a switchable infrared plasmonic PCM and enables a programmable nanophotonics material platform. We show how resonant metallic nanostructures can be directly written, modified and erased on and below the meta-atom level in an IST thin film by a pulsed switching laser, facilitating direct laser writing lithography without need for cumbersome multi-step nanofabrication. With this technology, we demonstrate large resonance shifts of nanoantennas of more than 4 µm, a tuneable mid-infrared absorber with nearly 90% absorptance as well as screening and nanoscale “soldering” of metallic nanoantennas. Our concepts can empower improved designs of programmable nanophotonic devices for telecommunications, (bio)sensing and infrared optics, e.g. programmable infrared detectors, emitters and reconfigurable holograms.

Here, the authors introduce In3SbTe2 (IST) as a programmable material platform for plasmonics and nanophotonics in the infrared. They demonstrate direct optical writing, modifying and erasing of metallic crystalline IST nanoantennas, tuning their resonances, as well as nanoscale screening and soldering.

Nonvolatile, Reconfigurable and Narrowband Mid-Infrared Filter Based on Surface Lattice Resonance in Phase-Change Ge2Sb2Te5

We propose a nonvolatile, reconfigurable, and narrowband mid-infrared bandpass filter based on surface lattice resonance in phase-change material Ge2Sb2Te5. The proposed filter is composed of a two-dimensional gold nanorod array embedded in a thick Ge2Sb2Te5 film. Results show that when Ge2Sb2Te5 transits from the amorphous state to the crystalline state, the narrowband reflection spectrum of the proposed filter is tuned from 3.197 μm to 4.795 μm, covering the majority of the mid-infrared regime, the peak reflectance decreases from 72.6% to 25.8%, and the corresponding quality factor decreases from 19.6 to 10.3. We show that the spectral tuning range can be adjusted by varying the incidence angle or the lattice period. By properly designing the gold nanorod sizes, we also show that the quality factor can be greatly increased to 70 at the cost of relatively smaller peak reflection efficiencies, and that the peak reflection efficiency can be further increased to 80% at the cost of relatively smaller quality factors. We expect that this work will advance the engineering of Ge2Sb2Te5-based nonvalatile tunable surface lattice resonances and will promote their applications especially in reconfigurable narrowband filters.

VO2-based active tunable emittance thermochromic flexible coatings

This contribution reports, for what we believe is the first time, on VO₂-based thin-film coatings on flexible Al substrates exhibiting a tunable positive emittance-switching Δε=(ε_H-ε_L)>0. More precisely, a layered stack of a-Si:H/SiO₂/VO₂ on flexible Al sheets presents minimum and maximum values of emissivity of about 0.18 and 0.57 at 40ºC and 83ºC, respectively, and hence allows an emittance-switching Δε of 0.39 and a relative variation Δε/ε_Λ of ∼217%. Such variations fit with the potential applications of such coatings as smart radiation devices in small satellites and spacecraft.

Nanosecond switchable localized surface plasmons through resettable contact angle behavior in silver nanoparticles

In this article, we show nanosecond switchable localized surface plasmon resonance (LSPR) dipole and quadrupole modes from silver (Ag) nanoparticles on fused quartz substrates. Near-spherical Ag nanoparticles (contact angle of 166^°± 9 ) were synthesized by Ultra Violet (UV) laser dewetting of Ag thin films under a glycerol fluid environment. Under a single 9 nanosecond laser pulse irradiation of the particles in air, the particles were changed into a near-hemispherical shape (with contact angle of 103^°± 7 ). The resulting changes in particle contact area and volume fraction in the dielectric media resulted in substantial shift in the wavelength and intensity of the dipolar and quadrupolar LSPR modes to the violet side of visible spectrum. This switching of the plasmon resonance wavelength position could be repeated over multiple cycles by resetting the contact angle by laser re-irradiation under glycerol. This reusable nanoparticle system with reversible plasmonic properties within nanosecond time scales could prove a cost-effective way of designing high speed plasmonic devices in desired wavelength regions.

Tuning of Classical Electromagnetically Induced Reflectance in Babinet Chalcogenide Metamaterials

Metamaterials analog of electromagnetically induced reflectance (EIR) has attracted intense attentions since they can provide various applications for novel photonic devices such as optical detectors with a high sensitivity and slow-light devices with a low loss. The development of dynamic photonic devices desires a tunable EIR feature in metamaterials. However, most metamaterials-induced EIR is not spectrally controllable particularly for the near-infrared (NIR) region. Herein, a tuning of EIR is illustrated in Babinet chalcogenide metamaterials in the NIR region. The EIR response is created by weak hybridization of two dipolar (bright) modes of the paired Au slots. Such a mode interference can be engineered through non-volatile phase transition to the refractive index of the Ge₂Sb₂Te₅ (GST), resulting in an active controlling of the reflection window. A 15% spectral tuning of the reflectance peak is observed experimentally in the NIR region as switching the GST state between amorphous and crystalline.

Supersymmetry in the time domain and its applications in optics

Supersymmetry is a conjectured symmetry between bosons and fermions aiming at solving fundamental questions in string and quantum field theory. Its subsequent application to quantum mechanics led to a ground-breaking analysis and design machinery, later fruitfully extrapolated to photonics. In all cases, the algebraic transformations of quantum-mechanical supersymmetry were conceived in the space realm. Here, we demonstrate that Maxwell's equations, as well as the acoustic and elastic wave equations, also possess an underlying supersymmetry in the time domain. We explore the consequences of this property in the field of optics, obtaining a simple analytic relation between the scattering coefficients of numerous time-varying systems, and uncovering a wide class of reflectionless, three dimensional, all-dielectric, isotropic, omnidirectional, polarisation-independent, non-complex media. Temporal supersymmetry is also shown to arise in dispersive media supporting temporal bound states, which allows engineering their momentum spectra and dispersive properties. These unprecedented features may enable the creation of novel reconfigurable devices, including invisible materials, frequency shifters, isolators, and pulse-shape transformers.

Near-field imaging of the multi-resonant mode induced broadband tunable metamaterial absorber

Metamaterial absorbers with tunability have broad prospects for mid-infrared absorption applications. While various methods have been proposed to control absorption, how to analyse and present the physical image of absorption mechanism in depth is still expected and meaningful. Here, we present experimental spatial near-field distributions of a multi-resonant mode induced broadband tunable metamaterial absorber by using near-field optical microscopy. The absorber is constructed by a metal double-sized unit cell and a metallic mirror separated by a thin Ge₂Sb₂Te₅ (GST) spacer. To clearly obtain the physical images, we used a hybrid unit cell consisting of four square resonators to produce two absorption peaks at 7.8 μm and 8.3 μm. The resonance central-wavelength exhibits a redshift while switching the GST thin film from amorphous to crystalline phase. The near-field amplitude and phase optical responses of the absorber are directly observed at absorption frequencies when GST is in both phases, respectively. This work will pave the way for the fundamental science field and inspire potential applications in optical tunable absorption control.

Metamaterial absorbers with tunability have broad prospects for mid-infrared absorption applications.

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.

Chalcogenide-gold dual-layers coupled to gold nanoparticles for reconfigurable perfect absorption

Recently, tunable high absorptance from various nanophotonic structures has been demonstrated. However, most of these structures require nano-lithography, which is expensive and slow. Lithography-free tuneable absorbers are rarely explored for tuneable visible and near-infrared photonics. Herein, we demonstrate a gold (Au)/chalcogenide dual-layer that is resonantly coupled to Au nanoparticles (NPs). The structure exhibits angle and polarisation-independent high absorptance. At resonance, waveguide cavity-like modes are excited between the film and NPs whilst gap plasmon modes are excited between the NPs. Coalescence of the waveguide cavity-like modes, the gap plasmon modes, and the highly absorbing chalcogenide semiconductor not only leads to perfect absorptance but also a reconfigurable response via reversible structural phase transitions in the chalcogenide film. In the amorphous state, the design provides nearly perfect absorptance for both p- and s-polarisation states at an incident angle of 20°. However, after switching to the crystalline state, the peak absorptance spectrally broadens and redshifts from 980 to 1520 nm. This experimental observation was theoretically validated by the finite element method. Thermal-electric modeling was performed to show that the structural transition from crystalline to amorphous states is possible in just 5 ns, thus allowing high-speed reconfigurable perfect absorbers.

Machine Learning Interatomic Potentials as Emerging Tools for Materials Science

Atomic-scale modeling and understanding of materials have made remarkable progress, but they are still fundamentally limited by the large computational cost of explicit electronic-structure methods such as density-functional theory. This Progress Report shows how machine learning (ML) is currently enabling a new degree of realism in materials modeling: by "learning" electronic-structure data, ML-based interatomic potentials give access to atomistic simulations that reach similar accuracy levels but are orders of magnitude faster. A brief introduction to the new tools is given, and then, applications to some select problems in materials science are highlighted: phase-change materials for memory devices; nanoparticle catalysts; and carbon-based electrodes for chemical sensing, supercapacitors, and batteries. It is hoped that the present work will inspire the development and wider use of ML-based interatomic potentials in diverse areas of materials research.

Design and modeling of a transmission and reflection switchable micro-focusing Fresnel device based on phase-change materials

In this paper, a switchable micro-focusing Fresnel device based on phase-change materials (PCMs) is proposed, which can selectively display the functions of transmission and reflection without the use of mechanical adjustment on micro scale. The switchable function is realized by combining Fresnel structure with PCM. A four-level switchable Fresnel device consisting of a typical PCM Ge₃Sb₂Te₆ (GST-326) is designed to focus light into a focal length of 30 µm at wavelength of 3.1 µm. The optical performance of the switchable device has been analyzed by using finite-difference time-domain (FDTD) method, showing bright convergence point near pre-designed focal length with focusing efficiencies larger than 18%, depth of focus (DOF) less than 4.65 µm and the full width at half-maximum (FWHM) not larger than 1.30 µm. Furthermore, by precisely manipulating the variation of PCM thickness, we also obtain a device that possesses the characteristics of a transmission-reflection focusing beam splitter. The devices show good potential for the combination of traditional binary optical devices and PCM to produce new functions, and provides a promising innovative approach for miniature focal length switching device.

Fabrication and Thermal Properties of Capric Acid/Calcinated Iron Tailings/Carbon Nanotubes Composite as Form-Stable Phase Change Materials for Thermal Energy Storage

In this study, a novel form-stable phase change material (FSPCM) consisting of calcination iron tailings (CIT), capric acid (CA), and carbon nanotubes (CNT) was prepared using a simple direct melt impregnation method, and a series of tests have been carried out to investigate its properties. The leakage tests showed that CA can be retained in CIT with a mass fraction of about 20 wt.% without liquid leakage during the phase change process. Moreover, the morphology, chemical structure, and thermal properties of the fabricated composite samples were investigated. Scanning electron microscope (SEM) micrographs confirmed that CIT had a certain porous structure to confine CA in composites. According to the Fourier transformation infrared spectroscope (FTIR) results, the CA/CIT/CNT FSPCM had good chemical compatibility. The melting temperature and latent heat of CA/CIT/CNT by differential scanning calorimeter (DSC) were determined as 29.70 °C and 22.69 J/g, respectively, in which the mass fraction of CIT and CNT was about 80 wt.% and 5 wt.%, respectively. The thermal gravity analysis (TGA) revealed that the CA/CIT/CNT FSPCM showed excellent thermal stability above its working temperature. Furthermore, the melting and freezing time of CA/CIT/CNT FSPCM doped with 5 wt.% CNT reduced by 42.86% and 54.55% than those of pure CA, and it showed better heat transfer efficiency. Therefore, based on the above analyses, the prepared CA/CIT/CNT FSPCM is not only a promising candidate material for the application of thermal energy storage in buildings, but it also provides a new approach for recycling utilization of iron tailings.

Tunable beam manipulation based on phase-change metasurfaces

A metasurface combined with phase-change material Ge₂Sb₂Te₅ (GST) is proposed to act as a switchable wave plate to adjust spin-orbit interactions (SOIs), so that the polarization and phase of the reflected light are simultaneously manipulated. A converter, which could act as a quarter-wave plate or three-quarter-wave plate when the GST layer is in the amorphous or crystalline state, and a switch, which could act as a mirror (corresponding to the "OFF" state of SOIs) or half-wave plate (corresponding to the "ON" state of SOIs) when the GST layer is in the amorphous or crystalline state, are designed, respectively. Consequently, a convertible vectorial beams converter, which could generate radial or azimuthal polarization, is designed when the GST layer is in the amorphous or crystalline state. In addition, a switchable vortex beam generator could realize orbital angular momentum with topological charge l=±2 when the GST layer changes from amorphous to the crystalline state. The designed metasurface could offer a promising route for high-efficiency reconfigurable devices and encrypted optical communications.

Polariton nanophotonics using phase-change materials

Polaritons formed by the coupling of light and material excitations enable light-matter interactions at the nanoscale beyond what is currently possible with conventional optics. However, novel techniques are required to control the propagation of polaritons at the nanoscale and to implement the first practical devices. Here we report the experimental realization of polariton refractive and meta-optics in the mid-infrared by exploiting the properties of low-loss phonon polaritons in isotopically pure hexagonal boron nitride interacting with the surrounding dielectric environment comprising the low-loss phase change material Ge₃Sb₂Te₆. We demonstrate rewritable waveguides, refractive optical elements such as lenses, prisms, and metalenses, which allow for polariton wavefront engineering and sub-wavelength focusing. This method will enable the realization of programmable miniaturized integrated optoelectronic devices and on-demand biosensors based on high quality phonon resonators.

Here, the authors experimentally demonstrate a platform for tunable polariton refractive and meta-optics based on hexagonal boron nitride and phase change Ge₃Sb₂Te₆. This combination has the advantage of the long-lived phonon-polariton with switchable refractive index of the phase change material.

Broadband transparent optical phase change materials for high-performance nonvolatile photonics

Optical phase change materials (O-PCMs), a unique group of materials featuring exceptional optical property contrast upon a solid-state phase transition, have found widespread adoption in photonic applications such as switches, routers and reconfigurable meta-optics. Current O-PCMs, such as Ge–Sb–Te (GST), exhibit large contrast of both refractive index (Δn) and optical loss (Δk), simultaneously. The coupling of both optical properties fundamentally limits the performance of many applications. Here we introduce a new class of O-PCMs based on Ge–Sb–Se–Te (GSST) which breaks this traditional coupling. The optimized alloy, Ge₂Sb₂Se₄Te₁, combines broadband transparency (1–18.5 μm), large optical contrast (Δn = 2.0), and significantly improved glass forming ability, enabling an entirely new range of infrared and thermal photonic devices. We further demonstrate nonvolatile integrated optical switches with record low loss and large contrast ratio and an electrically-addressed spatial light modulator pixel, thereby validating its promise as a material for scalable nonvolatile photonics.

Here, the authors introduce optical phase change materials based on Ge-Sb-Se-Te which breaks the coupling between refractive index and optical loss allowing low-loss performance benefits. They demonstrate low losses in nonvolatile photonic circuits and electrical pixelated switching have been demonstrated.

All-optical switch and logic gates based on hybrid silicon-Ge2Sb2Te5 metasurfaces

We numerically propose a hybrid metasurface (MS) to realize all-optical switch and logic gates in the shortwave infrared (SWIR) band. Such MS consists of one silicon rod and one Ge₂Sb₂Te₅ (GST) rod pair. Utilizing the transition from an amorphous state to a crystalline state of GST, such MS can produce an electromagnetically induced transparency (EIT) analogue with active control. Based on this, we realize all-optical switching at 1770 nm with a modulation depth of 84%. Besides, three different logic gates, NOT, NOR and OR, can also be achieved in this metadevice simultaneously. Thanks to the reversible and fast phase transition process of GST, this device possesses reconfigurable ability as well as fast response time, and has potential applications in future optical networks.

Integrated metamaterial with functionalities of absorption and electromagnetically induced transparency

A switchable metamaterial with bifunctionality of absorption and electromagnetically induced transparency is proposed based on the phase-transition characteristic of phase change material-vanadium dioxide. When vanadium dioxide is in the metallic state, an isotropic narrow absorber is obtained in the terahertz region, which consists of a top metallic cross, a middle dielectric layer, and a bottom vanadium dioxide film. By adjusting structure parameters, perfect absorption is realized at the frequency of 0.498 THz. This designed narrow absorber is insensitive to polarization and incident angle. Absorptance can still reach 75% for transverse electric polarization and transverse magnetic polarization at the incident angle of 65^∘. When vanadium dioxide is in the insulating state, the top metallic cross will interact with the bottom split ring resonator, and the interaction between them will lead to the appearance of electromagnetically induced transparency. The behavior of electromagnetically induced transparency works well for transverse electric polarization and transverse magnetic polarization at the small incident angle. The designed hybrid metamaterial opens possible avenues for achieving switchable functionalities in a single device.

Reconfigurable parity-time symmetry transition in phase change metamaterials

Although the conceptually remarkable, experimental illustration of parity-time (PT) symmetric quantum systems stays unexplored, their photonics analogues are explored by taking advantage of the similarity between the Helmholtz and Schrödinger equations. However, the static nature of the constitutive parameters of photonics structure inherently limits an achievement of dynamic PT-symmetry transition. We propose mid-infrared metamaterials (MMs) consisting of two orthogonally orientated metallic meta-atoms with one resonator integrated by a chalcogenide glass, GeTe. The PT-symmetry breaking can be actively transited to PT-symmetry by varying the structural state of GeTe between amorphous and crystalline while fixing the MM geometry. An electric-thermal model is constructed to illustrate that reversible switching between PT-symmetry and PT-symmetry breaking can be obtained in ∼270 ns. Our theoretical work lays the basis for designing high-speed reconfigurable PT-symmetry photonics systems that provide a promising approach for dynamically engineering non-Hermitian quantum symmetry.

Advanced Optical Programming of Individual Meta-Atoms Beyond the Effective Medium Approach

Nanometer-thick active metasurfaces (MSs) based on phase-change materials (PCMs) enable compact photonic components, offering adjustable functionalities for the manipulation of light, such as polarization filtering, lensing, and beam steering. Commonly, they feature multiple operation states by switching the whole PCM fully between two states of drastically different optical properties. Intermediate states of the PCM are also exploited to obtain gradual resonance shifts, which are usually uniform over the whole MS and described by effective medium response. For programmable MSs, however, the ability to selectively address and switch the PCM in individual meta-atoms is required. Here, simultaneous control of size, position, and crystallization depth of the switched phase-change material (PCM) volume within each meta-atom in a proof-of-principle MS consisting of a PCM-covered Al-nanorod antenna array is demonstrated. By modifying optical properties locally, amplitude and light phase can be programmed at the meta-atom scale. As this goes beyond previous effective medium concepts, it will enable small adaptive corrections to external aberrations and fabrication errors or multiple complex functionalities programmable on the same MS.

Highly Confined and Switchable Mid-Infrared Surface Phonon Polariton Resonances of Planar Circular Cavities with a Phase Change Material

Mid-infrared (MIR) photonics demands highly confined optical fields to obtain efficient interaction between long-wavelength light and nanomaterials. Surface polaritons excited on polar semiconductor and metallic material interfaces exhibit near-fields localized on subwavelength scales. However, realizing a stronger field concentration in a cavity with a high quality ( Q) factor and a small mode volume is still challenging in the MIR region. This study reports MIR field concentration of surface phonon polaritons (SPhPs) using planar circular cavities with a high Q factor of ∼150. The cavities are fabricated on a thin film of the phase change material Ge₃Sb₂Te₆ (GST) deposited on a silicon carbide (SiC) substrate. Scattering-type scanning near-field optical microscopy visualizes the near-field distribution on the samples and confirms directly that the SPhP field is strongly concentrated at the center of the centrosymmetric cavities. The smallest concentrated field size is 220 nm in diameter which corresponds to 1/50 of the wavelength of the incident light that is far below the diffraction limit. The thin GST film enhances the SPhP confinement, and it is used to switch the confinement off by tuning the cavity resonance induced by the phase change from the amorphous to the crystalline phase. This subwavelength and switchable field concentration within a high- Q polariton cavity has the potential to greatly enhance the light-matter interaction for molecular sensing and emission enhancement in MIR systems.

Efficient unidirectional and broadband vertical-emitting optical coupler assisted by aperture-coupled nanopatch antenna array

Vertical-emitting optical couplers that convert in-plane guided light to out-of-plane emission are crucial elements for future photonic integrated circuits. However, traditional vertical-coupling elements, such as grating couplers, by default radiate light in both upward and downward directions, leading to a significant reduction of device efficiency. In this paper, we propose to solve this problem using a novel nanopatch antenna array, inspired by patch antenna theories commonly deployed in microwave circuits. The proposed nanopatch array features an up-to-down emission directionality up to 12.91 dBc and a wide operating bandwidth of over 400 nm simultaneously. Compared with a typical waveguide grating antenna, our design shows a significantly higher free-space gain of 24.27 dBi. The unidirectional, efficient, and broadband antenna arrays presented here are promising for a range of integrated photonics applications, including inter-chip photonic interconnects, light ranging and detection, optical communications, and biological imaging.

Stoichiometric Engineering of Chalcogenide Semiconductor Alloys for Nanophotonic Applications

A variety of alternative plasmonic and dielectric material platforms-among them nitrides, semiconductors, and conductive oxides-have come to prominence in recent years as means to address the shortcomings of noble metals (including Joule losses, cost, and passive character) in certain nanophotonic and optical-frequency metamaterial applications. Here, it is shown that chalcogenide semiconductor alloys offer a uniquely broad pallet of optical properties, complementary to those of existing material platforms, which can be controlled by stoichiometric design. Using combinatorial high-throughput techniques, the extraordinary epsilon-near-zero, plasmonic, and low/high-index characteristics of Bi:Sb:Te alloys are explored. Depending upon composition they can, for example, have plasmonic figures of merit higher than conductive oxides and nitrides across the entire UV-NIR range, and higher than gold below 550 nm; present dielectric figures of merit better than conductive oxides at near-infrared telecommunications wavelengths; and exhibit record-breaking refractive indices as low as 0.7 and as high as 11.5.

Optofluidic system based on electrowetting technology for dynamically tunable spectrum absorber

An optofluidic system that uses the electrowetting technology to dynamically control the local surface plasmon resonance of the silver nanoparticle is invented. The silver nanoparticle is initially suspended at the interface of the polar liquid and the non-polar liquid. As the interface morphology changes with the applied voltage, the media distribution surrounding particle is changed accordingly, thus realizing the resonance absorption peak's modulation. The investigation result shows that a wide range of the spectral colors from red to blue can be selectively absorbed just by a single device. Specifically, when the radius of the particle is 50 nm, the wavelength of the absorption peak can be dynamically modulated from 460 nm to 607 nm. This proposed method can be used to design and prepare rapidly adjustable optical elements.

Active control of plasmonic colors: emerging display technologies

In recent years there has been a growing interest in the use of plasmonic nanostructures for color generation, a technology that dates back to ancient times. Plasmonic structural colors have several attractive features but once the structures are prepared the colors are normally fixed. Lately, several concepts have emerged for actively tuning the colors, which opens up for many new potential applications, the most obvious being novel color displays. In this review we summarize recent progress in active control of plasmonic colors and evaluate them with respect to performance criteria for color displays. It is suggested that actively controlled plasmonic colors are generally less interesting for emissive displays but could be useful for new types of electrochromic devices relying on ambient light (electronic paper). Furthermore, there are several other potential applications such as images to be revealed on demand and colorimetric sensors.