The terahertz (0.1–10 THz) range represents a fast‐evolving research and industrial field. The great interest for this portion of the electromagnetic spectrum, which lies between the photonics and the electronics ranges, stems from the unique and disruptive sectors where this radiation finds applications in, such as spectroscopy, quantum electronics, sensing and wireless communications beyond 5G. Engineering the propagation of terahertz light has always proved to be an intrinsically difficult task and for a long time it has been the bottleneck hindering the full exploitation of the terahertz spectrum. Amongst the different approaches that have been proposed so far for terahertz signal manipulation, the implementation of metamaterials has proved to be the most successful one, owing to the relative ease of realisation, high efficiency and spectral versatility. In this review, we present the latest developments in terahertz modulators based on metamaterials, while highlighting a few selected key applications in sensing, wireless communications and quantum electronics, which have particularly benefitted from these developments.

A complementary split ring resonator (CSRR)-based metallic layer is proposed as a route to mimic surface plasmon polaritons. A numerical analysis of the textured surface is carried out and compared to previous prominent topologies such as metal mesh, slit array, hole array, and Sievenpiper mushroom surfaces, which are studied as well from a transmission line perspective. These well-documented geometries suffer from a narrowband response, alongside, in most cases, metal thickness constraint (usually of the order of λ/4) and non-subwavelength modal size as a result of the large dimensions of the unit cell (one dimensions is at least of the order of λ/2). All of these limitations are overcome by the proposed CSRR-based surface. Besides, a planar waveguide is proposed as a proof of the potential of this CSRR-based metallic layer for spoof surface plasmon polariton guiding. Fundamental aspects aside, the structure under study is easy to manufacture by simple PCB techniques and it is expected to provide good performance within the frequency band from GHz to THz.

Weyl points are discrete locations in the three-dimensional momentum space where two bands cross linearly with each other. They serve as the monopoles of Berry curvature in the momentum space, and their existence requires breaking of either time-reversal or inversion symmetry^{1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16}. Although various non-centrosymmetric Weyl systems have been reported¹⁵, demonstration of Weyl degeneracies due to breaking of the time-reversal symmetry remains scarce and is limited to electronic systems^17,18. Here, we report the experimental observation of photonic Weyl degeneracies in a magnetized semiconductor—InSb, which behaves as a magnetized plasma¹⁹ for electromagnetic waves at the terahertz band. By varying the magnetic field strength, Weyl points and the corresponding photonic Fermi arcs have been demonstrated. Our observation establishes magnetized semiconductors as a reconfigurable²⁰ terahertz Weyl system, which may prompt research on novel magnetic topological phenomena such as chiral Majorana-type edge states and zero modes in classic systems^21,22.