  Black phosphorus (BP), an emerging two-dimensional (2D) material with
intriguing optical properties, forms a promising building block in optics and
photonics devices. In this work, we propose a simple structure composed of BP
sandwiched by polymer and dielectric materials with low index contrast, and
numerically demonstrate the perfect absorption mechanism via the critical
coupling of guided resonances in the mid-infrared. Due to the inherent in-plane
anisotropic feature of BP, the proposed structure exhibits highly
polarization-dependent absorption characteristics, i.e., the optical absorption
of the structure reaches 99.9$\%$ for TM polarization and only 3.2$\%$ for TE
polarization at the same wavelength. Furthermore, the absorption peak and
resonance wavelength can be flexibly tuned by adjusting the electron doping of
BP, the geometrical parameters of the structure and the incident angles of
light. With high efficiency absorption, the remarkable anisotropy, flexible
tunability and easy-to-fabricate advantages, the proposed structure shows
promising prospects in the design of polarization-selective and tunable
high-performance devices in the mid-infrared, such as polarizers, modulators
and photodetectors.
