  Black phosphorus has attracted interest as a material for use in
optoelectronic devices due to many favorable properties such as a high carrier
mobility, field-effect, and a direct bandgap that can range from 0.3 eV in its
bulk crystalline form to 2 eV for a single atomic layer. The low bandgap energy
for bulk black phosphorus allows for direct transition photoabsorption that
enables detection of light out to mid-infrared frequencies. In this work we
characterize the room temperature optical response of a black phosphorus
photoconductive detector at wavelengths ranging from 1.56 $\mu$m to 3.75
$\mu$m. Pulsed autocorrelation measurements in the near-infrared regime reveal
a strong, sub-linear photocurrent nonlinearity with a response time of 1 ns,
indicating that gigahertz electrical bandwidth is feasible. Time resolved
photoconduction measurements covering near- and mid-infrared frequencies show a
fast 65 ps rise time, followed by a carrier relaxation with a time scale that
matches the intrinsic limit determined by autocorrelation. The sublinear
photoresponse is shown to be caused by a reduction in the carrier relaxation
time as more energy is absorbed in the black phosphorus flake and is well
described by a carrier recombination model that is nonlinear with excess
carrier density. The device exhibits a measured noise-equivalent power of 530
pW/$\sqrt{\text{Hz}}$ which is the expected value for Johnson noise limited
performance. The fast and sensitive room temperature photoresponse demonstrates
that black phosphorus is a promising new material for mid-infrared
optoelectronics.
