  Black phosphorus (BP) has emerged as a promising material candidate for next
generation electronic and optoelectronic devices due to its high mobility,
tunable band gap and highly anisotropic properties. In this work, polarization
resolved ultrafast mid-infrared transient reflection spectroscopy measurements
are performed to study the dynamical anisotropic optical properties of BP under
magnetic fields up to 9 T. The relaxation dynamics of photoexcited carrier is
found to be insensitive to the applied magnetic field due to the broadening of
the Landau levels and large effective mass of carriers. While the anisotropic
optical response of BP decreases with increasing magnetic field, its
enhancement due to the excitation of hot carriers is similar to that without
magnetic field. These experimental results can be well interpreted by the
magneto-optical conductivity of the Landau levels of BP thin film, based on an
effective k*p Hamiltonian and linear response theory. These findings suggest
attractive possibilities of multi-dimensional controls of anisotropic response
(AR) of BP with light, electric and magnetic field, which further introduces BP
to the fantastic magnetic field sensitive applications.
