The strength of light-matter interaction is of central importance in photonics and optoelectronics. For many widely studied two-dimensional semiconductors, such as MoS2, the optical absorption due to exciton resonances increases with thickness. However, here we will show, few-layer black phosphorus exhibits an opposite trend. We determine the optical conductivity of few-layer black phosphorus with thickness down to bilayer by infrared spectroscopy. On the contrary to our expectations, the frequency-integrated exciton absorption is found to be enhanced in thinner samples. Moreover, the continuum absorption near the band edge is almost a constant, independent of the thickness. We will show such scenario is related to the quanta of the universal optical conductivity of graphene, with a prefactor originating from the band anisotropy.
@article{arxiv.2004.07114,
title = {The optical conductivity of few-layer black phosphorus by infrared spectroscopy},
author = {Guowei Zhang and Shenyang Huang and Fanjie Wang and Qiaoxia Xing and Chaoyu Song and Chong Wang and Yuchen Lei and Mingyuan Huang and Hugen Yan},
journal= {arXiv preprint arXiv:2004.07114},
year = {2020}
}