  Black phosphorus is an infrared layered material. Its bandgap complements
other widely studied two-dimensional materials: zero-gap graphene and
visible/near-infrared gap transition metal dichalcogenides. Though highly
desirable, a comprehensive infrared characterization is still lacking. Here we
report a systematic infrared study of mechanically exfoliated few-layer black
phosphorus, with thickness ranging from 2 to 15 layers and photon energy
spanning from 0.25 to 1.36 eV. Each few-layer black phosphorus exhibits a
thickness-dependent unique infrared spectrum with a series of absorption
resonances, which reveals the underlying electronic structure evolution and
serves as its IR fingerprints. Surprisingly, unexpected absorption features,
which are associated with the forbidden optical transitions, have been
observed. Furthermore, we unambiguously demonstrate that controllable uniaxial
strain can be used as a convenient and effective approach to tune the
electronic structure of few-layer black phosphorus. Our study paves the way for
black phosphorus applications in infrared photonics and optoelectronics.
