Near-Field Absorption Spectroscopy of Quantum Well in Black Phosphorus

2D van der Waals (vdW) semiconductors are prime candidates for the next generation of optoelectronics due to their large electron mobilities and high on/off ratio which can be modulated via a gateable field effect. Moreover, their bandgap energies vary with material type, layer number, and applied field effect. Further, due to the quantum confinement in the dimension of thickness, they are host to quantum wells states, providing a bespoke material platformfor next-generation photodetectors in the infrared. Black phosphorus is an exciting example of a naturally layer-tuned quantum well owing to its narrow bandgap and ease of mechanical exfoliation to specified thickness. However, due to its moisture reactivity it is challenging to explore and exploit with nano-scale optics. Its characteristic NIR energy scale poses challenges for nano-spectroscopic analysis needed to characterize small and thin flakes. We present a new method of efficiently measuring near-field absorption in the near infrared as well as methodically cataloging the effects of encapsulation for preserving the quantum well photoresponse of nano-thick black phosphorus crystals