  We report the fabrication, characterization, and modeling of photoconductive
antennas using 40 nm thin-film flakes of black phosphorus (BP) as the
photoconductor and hexagonal boron nitride (hBN) as a capping layer to prevent
oxidation of BP. Dipole antennas were fabricated on oxidized high-resistivity
Si substrates, and BP and hBN flakes were picked up and transferred onto the
antenna inside a nitrogen glovebox. The transfer matrix technique was used to
optimize the thickness of BP and hBN for maximum absorption. BP flakes were
aligned with the armchair axis along the anode-cathode gap of the antenna, with
crystal orientation measured using reflection anisotropy. Photocurrent imaging
under illumination with 100 fs pulses at 780 and 1560 nm showed a
bias-dependent maximum photocurrent localized to the antenna gap with a peak
photoconductivity of 1 (2) S/cm in the linear regime of bias for excitation at
780 (1560) nm. Photocurrent saturation in bias (pump fluence) occurred at
approximately 1 V (0.25 mJ/cm$^2$). Device performance was modeled numerically
by solving Maxwell's equations and the drift-diffusion equation to obtain the
photocurrent density in response to pulsed laser excitation, which was largely
in qualitative agreement with the experimental observations. THz output
computed from surface current density suggests that BP THz PCA performance is
at least comparable to more traditional devices based on low-temperature-grown
GaAs. These devices represent a step toward high-performance THz
photoconductive antennas using BP.
