Efficient probes for terahertz near-field microscopy and spectroscopy

This thesis focuses on improving the sensitivity and spatial resolution of two near-field microscopy techniques at terahertz (THz) frequencies: direct detection in the near-field using a collection-mode aperture probe, and using the apex of a metallic tip to scatter a near-field interaction into the far-field. The first technique is limited in spatial resolution primarily due to strong attenuation of THz fields transmitted through the subwavelength aperture. By integrating a terahertz detector with an optical metasurface it is possible to make nanoscale terahertz detectors, which can efficiently detect non-propagating THz fields close to the rear of the aperture, increasing probe sensitivity and spatial resolution. The scattering technique suffers from high background signals and weak scattering from the tip apex. By designing a scattering probe to act as a resonant dipole antenna, the efficiency of the scattering process can be improved, and by efficiently coupling THz radiation to the probe using a radially polarized THz source, interference from background signals can be reduced. These improvements can enable imaging of a variety of fascinating systems, including polaritons in monolayers and heterostructures of 2D materials, biological systems and topological insulators.