  We present a micro-mechanical terahertz (THz) detector fabricated on a
silicon on insulator (SOI) substrate and operating at room-temperature. The
device is based on a U-shaped cantilever of micrometric size, on top of which
two aluminum half-wave dipole antennas are deposited. This produces an
absorption extending over the $\sim 2-3.5$THz frequency range. Due to the
different thermal expansion coefficients of silicon and aluminum, the absorbed
radiation induces a deformation of the cantilever, which is read out optically
using a $1.5\mu$m laser diode. By illuminating the detector with an amplitude
modulated, 2.5 THz quantum cascade laser, we obtain, at room-temperature and
atmospheric pressure, a responsivity of $\sim 1.5 \times 10^{8}$pm/W for the
fundamental mechanical bending mode of the cantilever. This yields an
noise-equivalent-power of 20 nW/Hz$^{1/2}$ at 2.5THz. Finally, the low
mechanical quality factor of the mode grants a broad frequency response of
approximately 150kHz bandwidth, with a response time of $\sim 2.5\mu$s.
