Ultrafast terahertz detectors based on three-dimensional meta-atoms

Terahertz (THz) and sub-THz frequency emitter and detector technologies are receiving increasing attention, in part, 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), measured detection speeds are limited to the MHz/sub-MHz range. The key to a true ultra-fast response at the GHz level is an aggressive reduction in device size, below the diffraction limit, and here we demonstrate sub-wavelength (λ/10) THz QWIP detectors based on a 3D split-ring geometry, yielding ultra-fast operation at a wavelength of around 100 um. Each sensing meta-atom pixel features a suspended loop antenna that feeds THz radiation in the ~20 µm3 active volume. This extremely small resonator architecture leads to measured optical response speeds of up to 3 GHz, and an expected device operation of up to tens of GHz, based on our measured S-parameters. 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.