Signatures of sub-band excitons in few-layer black phosphorus

TL;DR

This paper presents a theoretical model explaining the observed excitonic peaks in few-layer black phosphorus, highlighting sub-band excitons and their tunability via external fields, advancing understanding of its optical properties.

Contribution

It introduces a continuum and tight-binding combined model for sub-band excitons in few-layer BP, explaining experimental absorption features and their controllability.

Findings

Identification of high and low oscillator strength excitonic states.

Explanation of bright and dark exciton peaks in absorption spectra.

Potential for tuning excitonic energies with electric and magnetic fields.

Abstract

Recent experimental measurements of light absorption in few-layer black phosphorus (BP) reveal a series of high and sharp peaks, interspersed by pairs of lower and broader features. Here, we propose a theoretical model for these excitonic states in few-layer black phosphorus (BP) within a continuum approach for the in-plane degrees of freedom and a tight-binding approximation that accounts for inter-layer couplings. This yields excitonic transitions between different combinations of the sub-bands created by the coupled BP layers, which leads to a series of high and low oscillator strength excitonic states, consistent with the experimentally observed bright and dark exciton peaks, respectively. The main characteristics of such sub-band exciton states, as well as the possibility to control their energies and oscillator strengths via applied electric and magnetic fields, are discussed, towards a full understanding of the excitonic spectrum of few-layer BP and its tunability.