  Subwavelength metallic resonators provide a route to achieving strong
light-matter coupling by means of tight confinement of resonant electromagnetic
fields. Investigation of such resonators however often presents experimental
difficulties, particularly at terahertz (THz) frequencies. A single
subwavelength resonator weakly interacts with THz beams, making it difficult to
probe it using far-field methods; whereas arrays of resonators exhibit
inter-resonator coupling, which affects the resonator spectral signature and
field confinement. Here, traditional far-field THz spectroscopy is
systematically compared with aperture-type THz near-field microscopy for
investigating complementary THz resonators. Whilst the far-field method proves
impractical for measuring single resonators, the near-field technique gives
high signal-to-noise spectral information, only achievable in the far-field
with resonator arrays. At the same time, the near-field technique allows us to
analyze single resonators - free from inter-resonator coupling present in
arrays - without significant interaction with the near-filed probe.
Furthermore, the near-field technique allows highly confined fields and surface
waves to be mapped in space and time. This information gives invaluable insight
into resonator spectral response in arrays. This near-field microscopy and
spectroscopy method enables investigations of strong light-matter coupling at
THz frequencies in the single-resonator regime.
