  The strength of light-matter interaction is of central importance in
photonics and optoelectronics. For many widely studied two-dimensional
semiconductors, such as MoS2, the optical absorption due to exciton resonances
increases with thickness. However, here we will show, few-layer black
phosphorus exhibits an opposite trend. We determine the optical conductivity of
few-layer black phosphorus with thickness down to bilayer by infrared
spectroscopy. On the contrary to our expectations, the frequency-integrated
exciton absorption is found to be enhanced in thinner samples. Moreover, the
continuum absorption near the band edge is almost a constant, independent of
the thickness. We will show such scenario is related to the quanta of the
universal optical conductivity of graphene, with a prefactor originating from
the band anisotropy.
