Photoconductive antennas (PCAs) are widely used as terahertz (THz) detectors for spectroscopy and imaging. However, their relatively low efficiency and sensitivity often limits the signal-to-noise and measurement capabilities of experimental systems. By replacing the photoconductive region with an all-dielectric, fully absorbing metasurface the efficiency and sensitivity of PCAs is substantially improved. This thesis describes the design, modelling and experimental testing of highly absorbing metasurfaces made for the purpose of improving PCAs. Perfect absorption is achieved through the degenerate critical coupling of Mie modes. By simple modifications of the metasurface geometry, perfect absorption is obtained across the wavelength range of near-infrared ultrafast lasers commonly used for PCA excitation. When used as PCA detectors, high signal-to-noise is achieved at unprecendently low excitation powers, and extremely low dark resistance enables high sensitivity detection. Furthermore, when integrated with near-field aperture probes, the ultra-thin design of such metasurface PCAs could significantly enhance the spatial resolution and spectral sensitivity of THz near-field systems. In addition to PCA detectors, this thesis investigates whether GaAs metasurfaces could be used for THz emission via ultrafast charge carrier dynamics. Perfect absorption is demonstrated when the metasurface is excited at oblique angles, as necessary for THz emission and out-coupling. This work suggests the possibility of efficient, adaptable and integrable THz sources which do not require external bias for operation. Finally, this thesis explores whether metasurfaces can enhance absorption in low-temperature materials at sub-bandgap energies. It is shown that substantial improvements in absorption are possible using degenerate critical coupling, even for materials with very low absorption coefficients. This finding invites the future development of efficient PCA detectors that use convenient, turn-key operated fiber lasers for excitation - enabling cheaper, more functional THz spectroscopy and imaging systems
