Terahertz metamaterials for free-space and on-chip applications: From active metadevices to topological photonic crystals

Liquid crystal-metamaterial composite devices. Nematic-phase liquid crystals have been widely used for phase modulation in the visible wavelength band, and are valuable for applications in the terahertz wavelength band. However, to obtain a sufficiently large phase shift, the thickness of the liquid crystal needs to be similar to the wavelength, thus increasing the modulation-recovery time, which poses a challenge for terahertz applications. Combining liquid crystal materials with metamaterials can greatly reduce the liquid crystal thickness, decrease the drive voltage, increase the device switching speed, and utilize the hypersurface to generate a range of functional applications.

Phase Change Materials-Metamaterial Devices. Utilizing the process of phase change materials (GST, VO2, etc.) converting from crystalline to amorphous form under external driving modulation brings about changes in the refractive index and absorption coefficient of the materials, which can bring about rich application scenarios for the active modulation of composite metamaterials devices to satisfy different application requirements (Fig. 1b). At present, phase change materials are driven by diverse means, possessing 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, broadband phase-modulation devices, optical memory devices, and nonlinear devices. The modulation speed of electrically-driven graphene-metamaterial-based devices is still limited by the RC time; in addition, their own limited thickness limits the strength of the interaction with light, and they need to rely on the metamaterial's extremely strong local field enhancement to improve the modulation depth.

MEMS-Metamaterials Devices. Microelectromechanical systems (MEMS) are a fundamental part of the most advanced integrated systems available. Based on the mature processing and wide range of applications of MEMS, its technology can be well extended for applications in the terahertz band, and one of the typical application scenarios is the use of electrostatic actuation to realize dynamic modulation of the polarization state of electromagnetic waves (Fig. 1d). Combining MEMS with hypersurfaces can also realize programmable modulators, which are expected to integrate complex functions such as dynamic polarization state modulation, wavefront bias, holographic display, etc. in one device. However, reliability is an issue faced by MEMS-hypersurface arrays, such as uneven deformation distribution, defects, and failure of cantilever beams to release.

Silicon-Metamaterial Devices. The main semiconductor materials used in the terahertz band are silicon and gallium arsenide, and dynamic modulation is achieved through two modes: all-optical modulation and electrical drive. All-optical modulation is usually the use of laser pulses to excite the transient carriers of the semiconductor material, the material carrier concentration changes on the terahertz wave transmittance/reflectance modulation, through the combination of super-surfaces can be achieved by amplitude modulation, polarization beam splitting, radiation angle switching and other applications. The biggest advantage is that ultra-fast modulation speed can be realized and remote modulation can be achieved. Electrically driven modulation changes the carrier concentration of the material by electrically injecting or depleting carriers, which tends to be more attractive from the point of view of device utility and integration, but the modulation speed is limited by the RC time of the driver circuit.

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

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