  The gapless surface states of topological insulators (TI) can potentially be
used to detect and harvest low-frequency infrared light. Nonetheless, it was
shown that significant surface photocurrents due to light with frequency below
the bulk gap are rather hard to produce. Here we demonstrate that a periodic
magnetic pattern added to the surface dramatically enhances surface
photocurrents in TI's. Moreover, the sensitivity of this set-up to the
wavelength of the incident light can be optimized by tuning the geometry of the
magnetic pattern. The ability to produce substantial photocurrents on TI
surfaces from mid-range and far-infrared light could be used in photovoltaic
applications, as well as for detection of micrometer wavelength radiation. For
light of wavelength greater than 15$\mu$m we estimate that at room temperature,
a detector based on the effect we describe can have a specific detectivity as
high as 10$^7$ cm$\sqrt{\text{Hz}}$/W (i.e. 10$^9$ Jones). The device can
therefore operate at much larger wavelengths than existing infrared detectors,
while maintaining a comparable figure of merit.er wavelength radiation.
