  Thanks to its tunable infrared band-gap and to its anisotropic conduction
properties, black phosphorus represents a very unique 2D material, whose
potential in the engineering of new devices still needs to be fully explored.
We investigate here the nonlinear terahertz (THz) electrodynamics of black
phosphorus along the more conducting armchair direction. Similarly to the case
of other 2D systems like graphene and topological insulators, the THz saturable
absorption properties of black phosphorus can be understood within a
thermodynamic model by assuming a fast thermalization of the electron bath.
While black phosphorus does not display the presence of massless fermions at
ambient pressure and temperature, our analysis shows that its anomalous THz
nonlinear properties can be accounted for by a relativistic massive Dirac
dispersion, provided the Fermi temperature is low enough. An optimal tuning of
the Fermi level therefore represents a strategy to engineer strong THz
nonlinear response in other massive Dirac materials as in transition metal
dichalchogenides or high-temperature superconductors.
