Lighting up topological insulators: large surface photocurrents from magnetic superlattices

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.