Optical Anisotropy of Few-Layer Black Phosphorus Visualized by Scanning Polarization Modulation Microscopy

In-plane anisotropy, as one of the most intriguing properties of newly rediscovered 2D black phosphorus (BP), supplies another degree of freedom to design novel optical and optoelectronic devices. Here, the optical in-plane anisotropy of few-layer BP was directly visualized and studied by scanning polarization modulation microscopy (SPMM). The polarization analysis and experiment showed that the SPMM signal was dependent on the polarization angle of the laser beam in a sinusoidal way, by which the crystallographic orientation of BP was determined. The differential reflectance (Delta R = R-zz - R-ac) showed the excitation wavelength and thickness dependence, which was well explained by considering the anisotropic multireflection effect. The intrinsic anisotropic complex refractive indices (n + ik) were also derived on the basis of the Fresnel equation.