  Although the detection of light at terahertz (THz) frequencies is important
for a large range of applications, current detectors typically have several
disadvantages in terms of sensitivity, speed, operating temperature, and
spectral range. Here, we use graphene as a photoactive material to overcome all
of these limitations in one device. We introduce a novel detector for terahertz
radiation that exploits the photothermoelectric (PTE) effect, based on a design
that employs a dual-gated, dipolar antenna with a gap of 100 nm. This
narrow-gap antenna simultaneously creates a pn junction in a graphene channel
located above the antenna and strongly concentrates the incoming radiation at
this pn junction, where the photoresponse is created. We demonstrate that this
novel detector has an excellent sensitivity, with a noise-equivalent power of
80 pW-per-square-root-Hz at room temperature, a response time below 30 ns
(setup-limited), a high dynamic range (linear power dependence over more than 3
orders of magnitude) and broadband operation (measured range 1.8-4.2 THz,
antenna-limited), which fulfills a combination that is currently missing in the
state-of-the-art detectors. Importantly, on the basis of the agreement we
obtained between experiment, analytical model, and numerical simulations, we
have reached a solid understanding of how the PTE effect gives rise to a
THz-induced photoresponse, which is very valuable for further detector
optimization.
