  The detection of terahertz (THz) radiation promises intriguing applications
in biology, telecommunication, and astronomy but remains a challenging task so
far. For example, semiconductor infrared detectors (e.g., HgCdTe) utilize
photo-excited electrons across the bandgap and hardly reach the far-infrared
terahertz regime because they are vulnerable to thermally excited carriers. In
this work, we propose the THz sensing by the bulk photovoltaic effect (BPVE) in
the twisted bilayer graphene (TBG). The BPVE converts light into a coherent DC
at zero bias or an open-circuit voltage. As a quantum response from the wave
function's geometry (different from the $p$-$n$ junction), BPVE is more robust
against temperature excitation and disorders. We predict that the TBG (bandgap
of several meV) exhibits a sizeable BPVE response in a range of 0.2 -- 1 THz.
Beyond the ordinary shift current scenario, BPVE in TBG comes from a
momentum-space shift of flat bands. Our work provides a pathway to design
twisted photonics for resonant terahertz detection.
