  An unusual photo-galvanic effect is predicted on the topological insulator
surface when its semi-metallic electronic spectrum is modified by an adjacent
ferromagnet. The effect is correlated with light absorption in a wide frequency
range (from a few to hundreds of meV) and produces a pronounced response that
is not only resonant to the photon energy but also tunable by an external
electrical bias. The exceptionally strong peak photocurrent of the order of
$\mu$A/cm may be achieved at elevated temperatures with the illumination power
of 1 W/cm$^2$ in the THz range on Bi$_2$Se$_3$. These advantages could enable
room-temperature detection of far-infrared radiation.
