  We have fabricated black phosphorus photodetectors and characterized their
full spectral responsivity. These devices, which are effectively in the bulk
thin film limit, show broadband responsivity ranging from <400 nm to the ~3.8
$\mu$m bandgap. In the visible, an intrinsic responsivity >7 A/W can be
obtained due to internal gain mechanisms. By examining the full spectral
response, we identify a sharp contrast between the visible and infrared
behavior. In particular, the visible responsivity shows a large photoconductive
gain and gate-voltge dependence, while the infrared responsivity is nearly
independent of gate voltage and incident light intensity under most conditions.
This is attributed to a contribution from the surface oxide. In addition, we
find that the polarization anisotropy in responsivity along armchair and zigzag
directions can be as large as 103 and extends from the band edge to 500 nm. The
devices were fabricated in an inert atmosphere and encapsulated by Al$_2$O$_3$
providing stable operation for more than 6 months.
