  The mid-infrared (MIR) spectral range, pertaining to important applications
such as molecular 'fingerprint' imaging, remote sensing, free space
telecommunication and optical radar, is of particular scientific interest and
technological importance. However, state-of-the-art materials for MIR detection
are limited by intrinsic noise and inconvenient fabrication processes,
resulting in high cost photodetectors requiring cryogenic operation. We report
black arsenic-phosphorus-based long wavelength infrared photodetectors with
room temperature operation up to 8.2 um, entering the second MIR atmospheric
transmission window. Combined with a van der Waals heterojunction, room
temperature specific detectivity higher than 4.9*10^9 Jones was obtained in the
3-5 um range. The photodetector works in a zero-bias photovoltaic mode,
enabling fast photoresponse and low dark noise. Our van der Waals
heterojunction photodector not only exemplify black arsenic-phosphorus as a
promising candidate for MIR opto-electronic applications, but also pave the way
for a general strategy to suppress 1/f noise in photonic devices.
