Layer-dependent exciton polarizability and the brightening of dark excitons in few-layer black phosphorus

TL;DR

This study investigates how exciton polarizability varies with layer thickness in black phosphorus, revealing layer-dependent effects and the brightening of dark excitons, with implications for optoelectronic device applications.

Contribution

It provides the first measurement of layer-dependent exciton polarizability in black phosphorus and uncovers the brightening of dark excitons under electrical fields.

Findings

Exciton polarizability varies with the number of layers in black phosphorus.

Dark excitons become brighter under electrical fields, surpassing allowed transitions.

Carrier screening effects are observed in thicker samples.

Abstract

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.