Abstract:Metasurfaces offer unparalleled functionalities for controlling the propagation and properties of electromagnetic waves. But to transfer these functions to technological applications, it is critical to render them tunable and to enable fast control by external stimuli. In most cases, this has been realized by utilizing tunable materials combined with a top-down nanostructuring process, which is often complicated and time intensive. Here we present a novel strategy for fabricating a tunable metasurface comprisi… Show more
“…This reversible phase transition is the result of an interplay between Mott‐ and Peierls‐type mechanisms, and can result in carrier‐density changes of up to four orders of magnitude across the IMT . The dramatic change in optical properties of VO 2 that accompanies the carrier‐density change has advanced a variety of applications, including optical limiting, nonlinear isolation, switching, and thermal‐emission engineering . Depending on the VO 2 film quality, grain size, the concentration and type of potential impurities, and the degree of strain, the phase transition can vary from abrupt to gradual .…”
The insulator-to-metal transition (IMT) in vanadium dioxide (VO 2 ) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO 2 for optics, the wavelength-dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO 2 thin films across the IMT is characterized for free-space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective-medium theory. VO 2 films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol-gel). While there are differences in the optical properties of VO 2 synthesized under different conditions, these differences are surprisingly small in the 2-11 µm range where the insulating phase of VO 2 also has relatively low optical loss. It is anticipated that the refractive-index datasets from this article will be broadly useful for modeling and design of VO 2 -based optical and optoelectronic components, especially in the mid-wave and long-wave infrared.
“…This reversible phase transition is the result of an interplay between Mott‐ and Peierls‐type mechanisms, and can result in carrier‐density changes of up to four orders of magnitude across the IMT . The dramatic change in optical properties of VO 2 that accompanies the carrier‐density change has advanced a variety of applications, including optical limiting, nonlinear isolation, switching, and thermal‐emission engineering . Depending on the VO 2 film quality, grain size, the concentration and type of potential impurities, and the degree of strain, the phase transition can vary from abrupt to gradual .…”
The insulator-to-metal transition (IMT) in vanadium dioxide (VO 2 ) can enable a variety of optics applications, including switching and modulation, optical limiting, and tuning of optical resonators. Despite the widespread interest in VO 2 for optics, the wavelength-dependent optical properties across its IMT are scattered throughout the literature, are sometimes contradictory, and are not available at all in some wavelength regions. Here, the complex refractive index of VO 2 thin films across the IMT is characterized for free-space wavelengths from 300 nm to 30 µm, using broadband spectroscopic ellipsometry, reflection spectroscopy, and the application of effective-medium theory. VO 2 films of different thicknesses are studied, on two different substrates (silicon and sapphire), and grown using different synthesis methods (sputtering and sol-gel). While there are differences in the optical properties of VO 2 synthesized under different conditions, these differences are surprisingly small in the 2-11 µm range where the insulating phase of VO 2 also has relatively low optical loss. It is anticipated that the refractive-index datasets from this article will be broadly useful for modeling and design of VO 2 -based optical and optoelectronic components, especially in the mid-wave and long-wave infrared.
“…Consequently, if installed on spacecraft and satellites, the proposed optical solar reflector is expected to maintain system temperature in an optimal range through self‐regulated thermal emission according to the device temperature in space. Ito et al reported the emission control of a VO 2 resonator–insulator–tungsten metasurface, while Ligmajer et al recently demonstrated temperature tunable optical responses from an epitaxially grown VO 2 nanobeam based metasurface . It should be noted that, in addition to the top‐down methods, solution‐processable colloidal synthesis of VO 2 nanocrystals and assembly techniques that have been reported recently provide an alternative approach to achieving active photonic metasurfaces that exhibit reversible IMT …”
Evolving from metamaterials, optical metasurfaces not only inherit their unprecedented flexibility in tailoring light–matter interaction but also the generally fixed response determined by the metaatoms' geometries—an undesirable property that hinders the development of practical metadevices. Consequently, novel optical metasurfaces that can be tuned in an active manner are intensively studied in recent years. Due to their optically thin structures, optical metasurfaces present unique characteristics in the realization of dynamic tuning. In this review, a detailed summary of the recent advances in active optical metasurfaces is provided including discussions reviewing a variety of active materials and approaches that enable faster, stronger, and more accurate tuning. Beyond facilitating practical metadevices, studies of active metasurfaces have, and will continue to deepen the understandings of the interaction between light and nanostructured photonic architectures and to promote the development of nanofabrication techniques involving high‐complexity.
“…Thanks to its switchable polarization dependence (from weak to strong), the nanomaterial can thus be used as a switchable near‐infrared polarizer. The authors also demonstrated how to use it as a switchable near‐infrared phase shifter in the optical telecom window …”
Section: Plasmon Resonancesmentioning
confidence: 99%
“…b) Extinction spectra by far‐field transmittance measurements at normal incidence with the incident light polarized along the long axis of the nanoparticle and perpendicular to it at room temperature (below the dielectric–metal transition, plasmons off) and at 80° (above the transition, plasmons on). Reproduced with permission . Copyright 2018, American Chemical Society.…”
Section: Plasmon Resonancesmentioning
confidence: 99%
“…The authors also demonstrated how to use it as a switchable near-infrared phase shifter in the optical telecom window. [28] 2.5. "Interband" Plasmonic Materials An interesting outcome of the search for alternative plasmonic materials is that the condition ε 1 < 0 (moderately negative ε 1 ) to achieve LSPRs in nanoparticles with aspect ratio close to 1 can be fulfilled in the visible even without free charge carrier excitation, in contrast with the plasmonic materials mentioned earlier.…”
Section: Plasmons In Densely Packed or Random Polydisperse Assembliesmentioning
Functional materials with a tailored optical response are needed for applications including coloring, optical instrumentation, data encryption, thermal radiation shaping, energy harvesting, and environmental or biological sensing. A given application requires the material to absorb, transmit, reflect, or scatter light in a suitable way, in a specific, narrow, or broad spectral range selected in the visible or infrared. To achieve the optical response and functionality needed, it is appealing to design nanomaterials with a suitable composition and a properly engineered structure, and this should ideally be realized with high‐throughput and cost‐effective fabrication methods. Herein, the aim is to provide a view on some very recently reported functional nanomaterials showing an optical response tailored for applications by lithography‐free methods. It is illustrated how assembling tailor‐made nanostructures based on the expanding library of optical materials (and their specific wavelength‐dependent dielectric functions) enables harnessing a variety of optical resonances (plasmonic, epsilon near zero, high refractive index Mie, film interference) to tailor the nanomaterials' optical response. It is also shown that building the nanostructures from novel optical materials brings special functionalities, such as a switchable response. Examples of applications are put forward.
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