Metasurfaces for Structured Terahertz Waves: Generation, Control and Applications

Terahertz (THz) structured fields, such as vortex beams carrying orbital angular momentum, Bessel beams, and vector beams, support mode multiplexing, extended depth of focus, and improved scattering tolerance in communication, sensing, and imaging. Metasurfaces, consisting of ultrathin subwavelength arrays, employ metallic or all-dielectric building blocks to enable wavefront manipulation through geometric-phase or resonance mechanisms. Building on these foundations, subsequent research has sought to further enhance functionality and efficiency by developing multilayer configurations that integrate multiple operations and support more complex modulation. In parallel, tunable materials – such as phase-change compounds, liquid crystals, and graphene, whose properties respond to external physical stimuli – have been incorporated to dynamically control optical responses. These approaches enable planar, programmable manipulation of phase, amplitude, and polarization in the THz regime while reducing losses and device footprint, thereby providing reconfigurability beyond that of conventional bulk optics. The chapter summarizes recent advances in THz metasurfaces for structured beams along four themes: high-efficiency beam-shaping mechanisms; control strategies using tunable materials and reconfigurable architectures; multifunctional integration incorporating frequency, polarization, and mode multiplexing; and applications in communication, imaging, and high-field emission. We discuss the future directions for scalable, tunable, and efficient platforms to facilitate chip-to-free-space interfaces, metrology, and manufacturable implementation. Two noteworthy areas of interest are ferroelectric nematic liquid-crystal platforms and spatiotemporally structured THz waves, which are also discussed.