Terahertz photoresponse of black phosphorus.

Two-dimensional black phosphorus is a new material that has gained widespread interest as an active material for optoelectronic applications. It features high carrier mobility that allows for efficient free-carrier absorption of terahertz radiation, even though the photon energy is far below the bandgap energy. Here we present an efficient and ultrafast terahertz detector, based on exfoliated multilayer flakes of black phosphorus. The device responsivity is about 1 mV/W for a 2.5 THz beam with a diameter of 200 µm, and is primarily limited by the small active area of the device in comparison to the incident beam area. The intrinsic responsivity is determined by Joule heating experiments to be about 44 V/W, which is in agreement with predictions from the Drude conductivity model. Time resolved measurements at a frequency of 0.5 THz reveal an ultrafast response time of 20 ps, making black phosphorus a candidate for high performance THz detection at room temperature.

Optical Gating of Black Phosphorus for Terahertz Detection.

Photoconductive antennas are widely used for time-resolved detection of terahertz (THz) pulses. In contrast to photothermoelectric or bolometric THz detection, the coherent detection allows direct measurement of the electric field transient of a THz pulse, which contains both spectral and phase information. In this Letter, we demonstrate for the first time photoconductive detection of free-space propagating THz radiation with thin flakes of a van der Waals material. Mechanically exfoliated flakes of black phosphorus are combined with an antenna that concentrates the THz fields to the small flake (∼10 µm). Similar performance is reached at gating wavelengths of 800 and 1550 nm, which suggests that the narrow bandgap of black phosphorus could allow operation at wavelengths as long as 4 µm. The detected spectrum peaks at 60 GHz, where the signal-to-noise ratio is of the order of 40 dB, and the detectable signal extends to 0.2 THz. The measured signal strongly depends on the polarization of the THz field and the gating pulse, which is explained by the role of the antenna and the anisotropy of the black phosphorus flake, respectively. We analyze the limitations of the device and show potential improvements that could significantly increase the efficiency and bandwidth.