Static and Tunable Devices for Terahertz Focusing and Beam Steering

The electromagnetic radiation with wavelengths between 0.1 and 1 mm, and frequencies between 0.3 and 3 THz, is usually called "terahertz radiation". In this band, photons have energies in the milli-electronvolt range and interact with systems that have picosecond lifetimes and energetic transitions between 1.24 and 12.4 meV (Jepsen PU, Cooke DG, Koch M Laser Photonics Rev. https://doi. org/10.1002/lpor. 201000011). Currently, numerous practical applications for optoelectronic THz systems are studied, but their progress has closely been linked to the more general development of a well-established technology. In this context, two different types of devices for THz-wave control will be presented: polymeric zone plates (ZPs) as THz lenses, and leaky-wave antennas (LWAs) based on nematic liquid crystals (LCs) for THz beam steering. ZPs are circular diffraction gratings with radially increasing line density. The basic properties of ZPs were already described by J. L. Soret in 1875 (Soret JL, Ann Phys Chem 156, 1875), and they are commonly employed for extreme UV and hard X-ray focusing. ZPs are able to focus THz waves with a theoretical resolution comparable to the radiation wavelength and a theoretical diffraction efficiency of about 40%, but both these goals are difficult to reach in practice. Some considered implementations are: (i) to employ a multilevel ZP; (ii) to employ two stacked binary ZPs instead of a conventional single-layer binary ZP lens (double-sided ZP) (Tofani S, Zografopoulos DC, Missori M, Beccherelli R.