Introduction
Fig.1 (a) A metamaterial composed of a periodic array of metal split-ring resonators, where the incident magnetic field perpendicular to the metal structure can excite a resonant magnetic response, resulting in (b) a negative magnetic permeability over a narrow frequency range above the resonance. Adapted from Ref. [2] |
Resonant response in planar terahertz metamaterials
Fig.2 (a) A simple split-ring resonator unit cell repeating in x and y directions to form a planar THz metamaterial. P = 50 mm, A = 36 mm, w = 4 mm, g = 2 mm, and the substrate is intrinsic GaAs; (b) an equivalent circuit of the split-ring resonator when the incident THz waves are polarized along the gap-bearing arm; (c) resonant transmission spectrum of the planar metamaterial normalized by the plain GaAs substrate. Adapted from Ref. [33] |
Fig.3 (a) A unit cell of a cross resonator array at the interface between two dielectric media; (b) schematic of the reflection and transmission at the metamaterial interface; (c) amplitude and (d) phase spectra of the complex S-parameters of the metamaterial interface corresponding to the reflection and transmission coefficients under normal incidence, where and . Adapted from Ref. [45] |
Functionalities in few-layer terahertz metamaterials
Fig.5 Experimentally measured reflectance and transmittance under nearly normal incidence to a metamaterial coated GaAs surface. The gray horizontal lines indicate the reflectance (32%) and transmittance (68%) at a plain GaAs surface. Inset: unit cell of the metamaterial antireflection coating. Adapted from Ref. [59] |
Optically tunable terahertz metamaterials
Fig.10 Scanning electron microscopy images of (a) an individual unit cell and (b) a square array of electric split-ring resonators where silicon strips were incorporated at the split gap; (c) experimentally measured THz transmission spectra at various photoexcitation power levels. Adapted from Ref.[87] |
Fig.11 (a) Scanning electron microscopy image of the dynamically switchable chiral meta-molecule; (b) simulated transmission spectra of left (solid curves) and right (dashed curves) handed circular polarizations before (black curves) and after (red curves) near-infrared photoexcitation; (c) circular dichroism before (black curve) and after (red curve) photoexcitation. Adapted from Ref. [90] |
Electrically switchable terahertz metamaterials and terahertz modulation
Fig.12 (a) Design schematic of the electrically switchable THz metamaterial, which is an array of interconnected electric split-ring resonators fabricated on top of a thin layer of n-doped GaAs substrate; (b) THz transmission spectra (intensity) as a function of the applied reverse voltage bias. Adapted from Ref. [94] |
Fig.13 Correlated transmission amplitude (a) and phase (b) spectra under various reverse voltage biases to an electrically switchable THz metamaterial; (c) THz modulation signal normalized to the incident THz spectrum under a square electrical signal alternating between 0 and -16 V. Adapted from Ref. [95] |
Fig.14 (a) Schematic of the single unit cell of the HEMT based electronically controllable THz metamaterial modulator, where the HEMT is identified and lies under each split gap of the metamaterial; (b) frequency dependent transmitted THz electric field for the HEMT/metamaterial device as a function of voltage bias. Adapted from Ref. [97] |
Fig.15 A unit cell illustrating an electric split-ring resonator array fabricated on top of the n-doped GaAs epilayer in the reverse biased state; (b) schematic showing a portion of the first four grating strips formed by interconnected electric split-ring resonators; (c) illustration of the entire metamaterial grating. The color profile illustrates that alternate columns are biased forming a diffraction grating, with each column being independently controlled by the voltage bias between its Schottky pads and the ohmic contacts. Adapted from Ref. [103] |