Terahertz Metamaterials for Free Space and Outdoors

Liquid Crystal-Metamaterial Composite Devices. Nematic-phase liquid crystals are widely used for phase modulation in the visible range and are valuable for terahertz applications. However, to achieve a sufficiently large phase shift, the thickness of the liquid crystal must be similar to the wavelength, which increases the modulation recovery time, which is a challenge for terahertz applications. Combining liquid crystal materials with metamaterials can significantly reduce the thickness of the liquid crystal, reduce the drive voltage, increase the switching speed of the device, and exploit the hypersurface to generate a variety of functional applications.

Phase Change Materials – Metamaterial Devices. The use of the process of phase change materials (GST, VO2, etc.) transforming from a crystalline form to an amorphous form under the influence of external driving modulation causes changes in the refractive index and absorption coefficient of materials, which can lead to rich application scenarios for active modulation of composite metamaterial devices to meet various application requirements (Fig. 1b). Currently, phase change materials are driven by various means, which have their own advantages and disadvantages.

Graphene-metamaterial devices. The conductivity of graphene in the terahertz band is positively correlated with the DC conductivity, so efficient and broadband modulation of terahertz waves can be realized by modulating the Fermi energy level. Typical modulation applications include diode-like optical devices, wideband phase modulation devices, optical memory devices, and nonlinear devices. The modulation speed of electrically driven graphene-metamaterial devices is still limited by the RC time; furthermore, their own limited thickness limits the strength of interaction with light and they must rely on the extremely strong local field enhancement of the metamaterial to improve the modulation depth.

MEMS-Metamaterials Devices. Microelectromechanical systems (MEMS) are the core part of the most advanced integrated systems available on the market. Based on the mature processing and wide range of MEMS applications, its technology can be well extended to terahertz band applications, and one of the typical application scenarios is the use of electrostatic control to realize dynamic modulation of the polarization state of electromagnetic waves (Fig. 1d). Combining MEMS with hypersurfaces can also realize programmable modulators, which are to integrate complex functions such as dynamic modulation of the polarization state, wavefront polarization, holographic display, etc. into one device. However, reliability is a problem faced by MEMS-hypersurface systems, such as non-uniform strain distribution, defects and failures of cantilever beams to release.

Silicon-metamaterial devices. The main semiconductor materials used in the terahertz band are silicon and gallium arsenide, and the dynamic modulation is achieved by two modes: all-optical modulation and electrical drive. All-optical modulation is usually the use of laser pulses to excite the transition carriers of the semiconductor material, the carrier concentration of the material changes in the modulation of the transmittance/reflectivity of the terahertz wave, through the combination of supersurfaces can be achieved by amplitude modulation, polarization beam splitting, radiation angle switching and other applications. The biggest advantage is that ultrafast modulation speed and remote modulation can be achieved. Electrically controlled modulation changes the carrier concentration of the material by electrically injecting or depleting the carriers, which tends to be more attractive from the point of view of device usability and integration, but the modulation speed is limited by the RC time of the control circuit.

Topological photonics. Terahertz waveguides based on topological photonics have the advantages of high transmission efficiency, good stability and insensitivity to large-angle deviation, which will help the development of terahertz communication applications. Most of the topological photonics applications are based on photonic crystals. This paper briefly describes the basic principles of topological photonics (Fig. 2a), several classifications of topological photonic crystals (Fig. 2b, topological photonic crystals with time reversal symmetry and broken symmetry), and the calculation of topological invariants, with special attention paid to photonic crystals endowed with time reversal symmetry, the Hall effect of quantum spin (Fig. 2c) and the Hall effect of quantum valley (Fig. 2d). Finally, the paper summarizes the current reports on terahertz communication applications based on topological photonic crystals (Fig. 2e).

Active metamaterials for free-space terahertz waves include driving methods including electrical, optical, thermal, and force, etc., which have their own advantages and disadvantages in applications, for example, the typical electric drive method has good integration, but the modulation speed is often limited by the RC time; the modulation time of optical drive can be up to femtoseconds and can be remotely controlled, but the integration is limited, and the modulation frequency is limited by the pulse repetition rate of the laser. For on-chip terahertz applications, the combination of topological photonics and active modulation is expected to practically accelerate the development of terahertz communication applications. It is also expected that the use of higher-order topological photonic crystals will further enhance the light-matter interaction and provide solutions for high-power terahertz radiation sources.

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