The quest for two-dimensional materials with a tunable band gap and high carrier mobility brought several semiconductors with interesting optoelectronic properties into the spotlight. Photodetectors based on this material display a high carrier mobility, broadband photoresponse, and intrinsic anisotropy. In order to gain more insights we investigate the electronic states in reciprocal space. To our big surprise, the photoexcited state is less homogeneous than the equilibrium one. As shown in the Figure, a Stark broadening smears the electronic states at an early delay time. This effect is ascribed to a spatially varying potential that builds up along the surface upon the sudden change of electronic screening. We argue that carriers with a high excess energy ionize charged defects and induce the inhomogeneous screening of a local potential. As a consequence, the pinning of the valence band is perturbed by development of fields parallel to the surface plane.
 Band Gap Renormalization, Carrier Multiplication, and Stark Broadening in Photoexcited Black Phosphorus, Zhesheng Chen, Jingwei Dong, Evangelos Papalazarou, Marino Marsi, Christine Giorgetti,Zailan Zhang, Bingbing Tian, Jean-Pascal Rueff, Amina Taleb-Ibrahimi, and Luca Perfetti, Nano Letters 19, 488 (2019).