Transient Anisotropic Photocurrent Induced Terahertz Emission from the Surface of Black Phosphorus

Abstract

Anisotropic second‐order nonlinear optical effects have not been experimentally observed in bulk black phosphorus (BP) because of the presence of inversion symmetry. Herein, it is experimentally shown that the anisotropic second‐order nonlinear optical effect can be observed at the surface of BP with inversion symmetry breaking by terahertz emission spectroscopy. The dependence of the terahertz emission on the azimuthal angle, pump power, and polarization states is investigated in a reflection configuration. The results can be well fitted by photogalvanic theory, which proves that the terahertz emission process is dominated by a second‐order nonlinear optical effect. In addition, it is demonstrated that the oxidization process at the surface can protect the terahertz surface emission from the bulk BP. These results prove the BP crystal can be used as an alternative terahertz emitter and the radiation mechanism shows that the second‐order nonlinear optical effect can occur at the surface of the bulk BP crystal.