Visualizing Photothermal Anisotropy in Black Phosphorus by Total Internal Reflection Pump-Probe Technique

Black phosphorus (BP) attracts increasing attention due to its moderate bandgap, high carrier mobility, and striking in-plane anisotropy. In particular, its high-extinction coefficient combined with elevated photothermal conversion efficiency and nontoxicity in biological tissues would make BP promising in photothermal therapy. In this study, a new pump-probe technique for photothermal measurement is developed based on polarization-dependent absorption of BP under total internal reflection. The photothermal anisotropy of BP is experimentally observed for the first time. The values of photothermal anisotropy reach 2.9, which is stronger than thermal and electrical conductivity anisotropic ratios. The difference in crystal orientations of BP can accurately be distinguished by analyzing the relationship between the polarization of pump light and photothermal signal. Also, it is found that the BP not only exists as heat source yielding changes in refractive index of the photothermal media but also can detect minor photothermal signals as sensing layer. Overall, these findings that BP surface and the media significantly impact the photothermal signal provide not only an accurate way for measuring crystal orientation but also can expand the practical applications of biosensors.