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Although abundant research on the anisotropy of van der Waals (vdW) materials has been published, we undertake an in-depth study of their optical properties as they have an important guiding role for light control in two-dimensional (2D) nanospace. As an example, we study the reflectance of few-layered black phosphorus (BP) in the total internal reflection (TIR) mode in detail. We demonstrate that its optical anisotropy can be changed on a large scale by varying the incident angles, polarization states, and the in-plane rotation angles of the BP samples. Theoretical analysis indicates that the phenomena observed are common to all the atom-thick biaxial crystals, so these conclusions can be widely applied to other anisotropic 2D materials. This research furthers the current understanding of the properties of BP more comprehensively, and provides guidance for developing new optoelectronic applications, especially when BP and other atom-thick biaxial crystals are integrated with TIR devices. Keywords: Black phosphorus, Optical anisotropy, Total internal reflection, Biaxial crystal

Femtosecond degenerate and non degenerate pump-probe spectroscopy in graphene reveals the evolution of photoexcited carrier distribution in the energy band during relaxation: the initial occupation of photoexcited carriers centered at the excitation state is anisotropic in momentum space; this anisotropic distribution rapidly relaxes through an intermediate state argued to be fully isotropic in the energy band due to phonon-involved cascade scattering. In addition, the experiment suggests that graphene optical absorbances for in-plane and out-of-plane optical fields are identical. [ABSTRACT FROM AUTHOR]