  Black phosphorus (BP), a relative new plasmonic two-dimensional (2D)
material, offers unique photonic and electronic properties. In this work, we
propose a new tunable and broadband ultrathin coherent perfect absorber (CPA)
device operating in the terahertz (THz) frequency range. It is based on a
bifacial metasurface made of BP patch periodic arrays separated by a thin
dielectric layer. Broadband CPA bandwidth is realized due to the ultrathin
thickness of the proposed device and the extraordinary properties of BP. In
addition, a substantial modulation between CPA and complete transparency is
achieved by adjusting the phase difference between the two counter-propagating
incident waves. The CPA performance can be tuned by dynamically changing the
electron doping level of BP. The CPA response under normal and oblique
transverse magnetic (TM) and electric (TE) polarized incident waves is
investigated. It is derived that CPA can be achieved under both incident
polarizations and across a broad range of incident angles. The presented CPA
device can be used in the design of tunable planar THz modulators, all-optical
switches, detectors, and signal processors.
