Ultrafast terahertz detectors based on three-dimensional meta-atoms

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 picosecond-range response. However, with research focused on diffraction-limited QWIP structures ( /2), RC constants cannot be reduced indefinitely, and detection speeds are bound to eventually meet an upper limit. The key to an ultra-fast response with no intrinsic upper limit even at tens of gigahertz (GHz) is an aggressive reduction in device size, below the diffraction limit. Here we demonstrate sub-wavelength ( /10) THz QWIP detectors based on a 3D split-ring geometry, yielding ultra-fast operation at a wavelength of around 100 μm. Each sensing meta-atom pixel features a suspended loop antenna that feeds THz radiation in the ∼20  μm³ active volume ( eff~3×10‾⁴( /2)³). 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 parameters on single devices and arrays.