THz Photoconductive Antennas Using Thin Black Phosphorus

We report the fabrication and characterization of THz photoconductive antennas using thin (~50 nm) black phosphorus (BP) as the photoconductor and hexagonal boron nitride (hBN) as a capping layer to prevent oxidation of BP and enhance absorption. Dipole antennas were fabricated on oxidized high-resistivity silicon substrates, and BP and hBN flakes were picked up and transferred onto the antennas inside a nitrogen glovebox. BP flakes were aligned with the armchair axis along the anode-cathode gap of the antenna, with crystal orientation measured using reflection anisotropy. The transfer matrix technique was used to optimize the thickness of BP and hBN for maximum absorption. Photocurrent imaging under illumination with ~100 fs pulses at 1550 nm showed a bias-dependent maximum photocurrent near the anode side of the antenna gap with a responsivity at 50 kHz of about 0.2 mA/W at a source-drain bias of -100 mV. We will also report ongoing measurements to compare THz output power of these antennas with reference devices in which silicon is used as the photoconductor.