Recent Advances in Tunable and Reconfigurable Metamaterials

Abstract

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.

Keywords: color filter; graphene; indium tin oxide; metasurface; perfect absorber; phase change material; plasmonics; wavefront engineering.

Figure 1

(a) A VO₂ perfect absorber: The design of the material structure and experimental set up inside an infrared microscope. VO₂ is deposited on a sapphire substrate with height = 180 nm. Temperature is controlled by a stage plate; reflectivity is measured using a mid-infrared microscope, a Fourier transform infrared (FTIR) spectrometer and mercury-cadmium-telluride detector with normal incidence. (b) Reflectivity spectrum at temperatures from 297 K to 360 K. Reflectivity is 0.0025 at λ = 11.6 μm; i.e., absorption = 99.75%. Reproduced with permission from © 2012, AIP Publishing [44]. (c) Schematic of engineering selective defects in VO₂ by using ion beam irradiation with a mask. Defects in the checkerboard or striped patterns can be achieved by changing the masks. Energetic ions strike the VO₂ layer and trigger structural defects by moving the vanadium and oxygen atoms. As irradiated VO₂ has a lower transition temperature, the two states of VO₂ co-exist at a lower temperature than in the undamaged VO₂. (d) Stripe defected switchable polarizer: When the thickness of the VO₂ layer is much less than the wavelength, the stripe defected VO₂ can be used as a switchable polarizer. The polarization direction affects the absorption by the meta-device; this property provides a variable degree of optical anisotropy. Reproduced with permission from © 2016, American Chemical Society [53]. (e) A schematic of a bow-tie shaped unit cell that has a small amount of the VO₂. The sizes of the structure are length = 264 nm, width = 300 nm, wire width = 100 nm, and gap width = 34 nm. A spacer dielectric layer made of Al₂O₃ is located between the bow-tie structure and a thick gold backplane to give a gap. Reduction in the amount of VO₂ increases the switching speed and precision of controllability. (f) The temperature affects the relationship between wavelength and absorption. The tuning range is up to 360 nm (NIR). As the temperature increases, the curve shifts the peak of the absorption into the short-wavelength direction. Reproduced with permission from © 2017, American Chemical Society [52].

Figure 2

(a) The metal-dielectric-metal structure (MIM) structure with GST between an Au disk array and a SiO₂ insulating layer for the tunable perfect absorber. The Au disks are 300 nm in diameter and 20 nm thick; the GST layer is also 20 nm thick. The absorption of this structure changes depending on the crystallization level. Reproduced with permission from © 2015, Chinese Laser Press [56]. (b) To induce change to c-GST, a 100-mW pulsed laser beam with a 45-ns pulse is used; to transform to a-GST, a 310-mW pulsed laser beam with a 15-ns pulse is used. (bottom) The schematic of the process of switching from a-GST to c-GST in V-shape with a suitable laser. (right) Scattering-type scanning near-field optical microscope (s-SNOM) image of the SPhPs excited in this structure. In this structure, the SPhPs form; they reflect from the boundary of a-, c-GST, and the process can be visualized using s-SNOM. Reproduced with permission from © 2016, Springer Nature [57]. (c) Dynamically optically reconfigurable zone-plate device. (top-left) Two Fresnel zone patterns of c-GST obtained using 0.39-nJ write pulses that focus a plane wave onto two foci. (top-middle) A single l.25-nJ erase pulse changes one of the Fresnel zone patterns to a-GST. (top-right), then by using 0.39-nJ pulses write pulses, the a-GST returns to the c-GST state. (middle line) Optical images of the Fresnel zone pattern on the GST film. (bottom line) At λ = 730 nm, red dot: transmission focal spot. The write–erase–write reconfiguration cycle is achievable by changing the GST phase (Scale bar: 10 μm). Reproduced with permission from © 2015, Springer Nature [59]. (d) A schematic of silicon metasurface embedded in a liquid crystal (LC). The LC is nematic at temperature T > 58 °C, and isotropic at T < 58 °C. This state change is reversible, and according to the state, the metasurface resonances wavelength shifts and the tuning range of emission changes. Reproduced with permission from © 2018, American Chemical Society [67]. (e) Schematic of changing the volume of the poly(N-isopropyl acrylamide) (pNIPAM) film. The pNIPAM film is placed between Au mirror and Au nanoparticles (NPs); the film volume changes at 30 °C and thereby alters the distance between the mirror and the NPs. (Right: magnification of pNIPAM to show the change). The transition temperature is ≈30 °C. Insets: Scanning electron microscope (SEM) images of this structure from different angles. Reproduced with permission from © 2016, John Wiley and Sons [68].

Figure 3

(a) The schematic structure of the meta-molecule with a top-view. The meta-molecule consists of Au nanocuboids and a nanocomposite that provides a large third-order nonlinear susceptibility. The nanocomposite is composed of polycrystalline indium-tin oxide doped with Au nanoparticles. The bottom graph is the calculated transmission spectra of this MM according to pump intensity. With a weak pump laser (21 kW/cm²), the tuning range is 120 nm. Reproduced with permission from © 2014 AIP Publishing LLC [74]. (b) A schematic of how to operate the structure. The control beam is green light (532 nm) which interacts with a photoisomerizable azo ethyl red to cause a switch in the polarization effects of the metasurface. (Top) The single beam combined with control beam by a dichroic mirror passes through the nanostructure and a long pass filter that allows only a single beam to pass. (Bottom) Structures of trans and cis state of the ethyl red molecule. Green irradiation causes a change from the trans to the cis state; darkness allows a return to the trans state. Reproduced with permission from © 2017, Springer Nature [79]. (c) A graph of polarization parameters. Polarization parameters (ϕ, χ) measured from the transmitted beam; dots: measured data; lines: simulation results. When the changing ethyl-red molecule changes from trans to cis, both polarization parameters shift toward blue. Depending on the control light power, the measured parameters show nonlinear changes (Δϕ and Δχ), and each parameter is compared with the results without the control beam. Irradiation with 4 mW green light (λ = 820 nm) induced a nonlinear change Δϕ ~23.2° in the transmitted polarization azimuth. (d) A schematic of a switchable reflective polarizer. It consists of an indium-doped cadmium oxide (CdO:In) that has a high carrier mobility, an Au capping layer and a magnesium oxide (MgO) substrate. An unpolarized incident beam is reflected by the polarizer and the output beam can have different polarization states depending on whether the polarizer is on or off. Bottom graph: concept of how the polarizer works. When the polarizer is turned on or off, the reflection spectrum of the p-polarization (p-pol) beam changes. When it is switched on, the polarizer acts as a perfect absorber for the p-pol at wavelength λ₁; when the polarizer is switched off, it acts as a mirror for p-pol at λ₁. Right: set-up of the pump-probe measurement. The reflectance of p-pol is changed due to the photoexcitation of the CdO film, and this change is dependent on the energy density of the pump beam. Due to the changing reflectance, the degree of p-pol state of the reflected light varies with wavelength. Reproduced with permission from © 2017, Springer Nature [80]. (e) A schematic of the spiropyran (SPI)/PMMA layer is integrated into nanopatch antennas and changing the chemical structure of SPI. The SPI/PMMA layer is integrated into nanopatch antennas comprised of silver nanocubes and silver film (left). The chemical structure of an isomer of SPI changes reversibly between Spiropyran and Merocyanine (right). Depending on the types of light, the C-O ring opens or closes and this affects the optical properties of the isomer. Reproduced with permission from © 2017 American Chemical Society and https://pubs.acs.org/doi/pdf/10.1021/acs.nanolett.7b04109. Note that further permissions related to the material excerpted [81] should be directed to the ACS [81]. (f) The simulated electric field increase of the fundamental plasmonic mode (SPI: 565 nm and MC: 610 nm) and experimental data about the scattering intensity. Initially, the reset resonance wavelength is 577 nm and after ultra-violet (UV) exposure, it red-shifts by 39 nm to 616 nm and the full width at half maximum decreases to 0.75 times the initial value.

Figure 4

(a) The schematic of tunable metalens fabricated on an elastic substrate. The four corners of the elastic substrate are held by the arms, and when the force is applied in four directions, the elastic substrate is stretched and the spacing of the gold patterns changes. (b) Photograph of the fabricated metalens before and after stretching. Reproduced with permission from © 2016, American Chemical Society [92]. (c) A schematic of un-activated and activated MEMs tunable MM. The voltage applied to each arm in the four directions can release the arm from the surface. (d) A SEM image of the pattern. Reproduced with permission from © 2014, AIP Publishing LLC [93]. The cantilever on the right and bottom side have been released. A schematic of MM (e) before and (f) after transformation. Reproduced with permission from © 2017, Springer Nature [94]. When the magnetic fields are applied to the patterns on the top and bottom side, a force that pulls each side is generated. As a result, the elastic interlayer substrate shrinks, so the properties of the MMs change. (g) A schematic of total material, (h) Schematic of the tuning mechanism; depending on the applied voltage, the structure is bent up or down by piezoelectricity, so the volume of the voids changes and the property of the entire structure is tuned. Reproduced with permission from © 2016, Optical Society of America [95].

Figure 5

(a) The schematic of MIM with tunable ITO MM, with a basic MIM structure of aluminum square patterns in the top layer and an ITO layer between the top metal patterns and the insulator (Al₂O₃). (b) A cartoon of the mechanism of tuning MM by an ITO-induced structure. Reproduced with permission from © The Author(s) 2017 [113]. The voltage between ITO and metal layer increases the carrier accumulation; as a result, the reflectance change reaches 5.16 at wavelength = 2.56 μm. (c) A schematic of a graphene-added tunable MIM MM structure and (d) a SEM image. A MIM structure with patterning on the top layer makes the reflectance close to zero at a certain wavelength band. By putting a graphene layer in the middle of this structure and applying a voltage to that layer, the target wavelength band shifted. (e) Simulation of the reflectance change when the voltage was changed from 0 to 80 V, and the consequent change in the target frequencies. Reproduced with permission from © 2014, American Chemical Society [115].

Figure 6

(a) The schematics of the cross-section and total structure of 2D tunable MMs. Reproduced with permission from © 2017, Springer Nature [125]. The array of a 2D layered line structure can transmit light. The line is composed of a metal-oxide-semiconductor integrated with ITO, which is conventionally used in tunable MMs. (b,c) tunable color-generation and color-filtering MM using the LC model. The alignment structure of the LC is changed by the applied voltage and changes the color produced by the MMs. MM in (b) tuning color of reflected light; (c) transmitted light. Reproduced with permission from © 2017, Springer Nature [127], Reproduced with permission from © 2017 American Chemical Society [128].