  Terahertz (THz) and sub-THz frequency emitter and detector technologies are
receiving increasing attention, underpinned by emerging applications in
ultra-fast THz physics, frequency-combs technology and pulsed laser development
in this relatively unexplored region of the electromagnetic spectrum. In
particular, semiconductor-based ultrafast THz receivers are required for
compact, ultrafast spectroscopy and communication systems, and to date, quantum
well infrared photodetectors (QWIPs) have proved to be an excellent technology
to address this given their intrinsic ps-range response However, with research
focused on diffraction-limited QWIP structures (lambda/2), RC constants cannot
be reduced indefinitely, and detection speeds are bound to eventually meet un
upper limit. The key to an ultra-fast response with no intrinsic upper limit
even at tens of GHz is an aggressive reduction in device size, below the
diffraction limit. Here we demonstrate sub-wavelength (lambda/10) THz QWIP
detectors based on a 3D split-ring geometry, yielding ultra-fast operation at a
wavelength of around 100 {\mu}m. Each sensing meta-atom pixel features a
suspended loop antenna that feeds THz radiation in the ~20 m3 active volume.
Arrays of detectors as well as single-pixel detectors have been implemented
with this new architecture, with the latter exhibiting ultra-low dark currents
below the nA level. This extremely small resonator architecture leads to
measured optical response speeds - on arrays of 300 devices - of up to ~3 GHz
and an expected device operation of up to tens of GHz, based on the measured
S-parameters on single devices and arrays.
