  Terahertz (THz) radiation has uses from security to medicine; however,
sensitive room-temperature detection of THz is notoriously difficult. The
hot-electron photothermoelectric effect in graphene is a promising detection
mechanism: photoexcited carriers rapidly thermalize due to strong
electron-electron interactions, but lose energy to the lattice more slowly. The
electron temperature gradient drives electron diffusion, and asymmetry due to
local gating or dissimilar contact metals produces a net current via the
thermoelectric effect. Here we demonstrate a graphene thermoelectric THz
photodetector with sensitivity exceeding 10 V/W (700 V/W) at room temperature
and noise equivalent power less than 1100 pW/Hz^1/2 (20 pW/Hz^1/2), referenced
to the incident (absorbed) power. This implies a performance which is
competitive with the best room-temperature THz detectors for an optimally
coupled device, while time-resolved measurements indicate that our graphene
detector is eight to nine orders of magnitude faster than those. A simple model
of the response, including contact asymmetries (resistance, work function and
Fermi-energy pinning) reproduces the qualitative features of the data, and
indicates that orders-of-magnitude sensitivity improvements are possible.
