  The evolution of excitons from 2D to 3D is of great importance in
photo-physics, yet the layer-dependent exciton polarizability has not been
investigated in 2D semiconductors. Here, we determine the exciton
polarizabilities for 3- to 11-layer black phosphorus-a direct bandgap
semiconductor regardless of the thickness-through frequency-resolved
photocurrent measurements on dual-gate devices and unveil the carrier screening
effect in relatively thicker samples. By taking advantage of the broadband
photocurrent spectra, we are also able to reveal the exciton response for
higher-index subbands under the gate electrical field. Surprisingly, dark
excitons are brightened with intensity even stronger than the allowed
transitions above certain electrical field. Our study not only sheds light on
the exciton evolution with sample thickness, but also paves a way for
optoelectronic applications of few-layer BP in modulators, tunable
photodetectors, emitters and lasers.
