Ultrafast dynamics in atomically thin black phosphorus

Black Phosphorous (BP) is a new emerging material in the family of two-dimensional van der Waals semiconductors. Unlike transition metal dichalcogenides, BP has direct band gap ranging from bulk to monolayer, significantly higher carrier mobility, and an unusual in-plane anisotropy in its optical response. Despite its importance for potential optoelectronics applications, efforts in understanding carrier dynamics have only been limited to bulk BP due to its rapid photo-oxidation in ambient conditions. Here we report on the ultrafast and nonlinear optical response of mechanically exfoliated bilayer BP. By covering the BP with hBN, or eliminating exposure to air during preparation and measurement, we observe substantial inhibition of its degradation over several days in multiple samples. We study the nonlinear optical response of the bilayer BP for different pump intensities, probe wavelengths, and substrates.