Abstract
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.
Highlights
Black phosphorus (BP) is the most stable phosphorus allotrope and has been known for several decades [1] before it again gained interest in recent years as a material that can be produced in atomically thin layers
Its band gap depends on the thickness of the flake, so while a single layer of BP is characterized by a direct band gap of about 2 eV, the band gap decreases to its bulk value of about 0.3 eV within about 10 atomic layers [7]
Even though the band gap of 0.3 eV does not allow for direct electron-hole pair excitation in the THz regime, its high mobility allows for THz detection in nano-transistors [12,13]
Summary
Black phosphorus (BP) is the most stable phosphorus allotrope and has been known for several decades [1] before it again gained interest in recent years as a material that can be produced in atomically thin layers. While its properties are not promising in bulk crystals, it can be exfoliated to two-dimensional flakes [2] that can be electrostatically gated to control carrier density [3]. In contrast to graphene and many other 2D materials, its properties like thermal conductivity [4], carrier mobility [5], and optical absorption [6], are highly anisotropic due to the corrugated structure of each layer. A very recent study by Wang et al [14] demonstrates photoconductive detection in the microwave frequency range with a bandwidth of several kHz. Here we study the THz photoresponse of thin flakes of BP at room temperature. Time resolved measurements with a pulsed THz source show a fast response time of about 20 ps, indicating that BP is a promising material for THz detection
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