  Black phosphorus thin films might offer attractive alternatives to narrow gap
compound semiconductors for optoelectronics across mid- to near-infrared
frequencies. In this work, we calculate the optical conductivity tensor of
multilayer black phosphorus thin films using the Kubo formula within an
effective low-energy Hamiltonian. The optical absorption spectra of multilayer
black phosphorus are shown to vary sensitively with thickness, doping, and
light polarization. In conjunction with experimental spectra obtained from
infrared absorption spectroscopy, we also discuss the role of interband
coupling and disorder on the observed anisotropic absorption spectra.
