Biexciton oscillator strength

TL;DR

This paper analyzes the physical coupling between photons and biexcitons, deriving their oscillator strength and contrasting their absorption behavior with trions, highlighting the quantum nature and density dependence of biexciton formation.

Contribution

It introduces a new coordinate framework and a Fourier transform approach to describe biexcitons, providing a detailed physical understanding of their oscillator strength and absorption characteristics.

Findings

Biexciton oscillator strength is extremely small, scaled by the ratio of biexciton volume to sample volume.

Biexciton absorption peak rises linearly with exciton density, unlike fermionic trions.

Biexciton and trion absorption lines behave differently due to their bosonic and fermionic quantum statistics.

Abstract

Our goal is to provide a physical understanding of the elementary coupling between photon and biexciton and to derive the physical characteristics of the biexciton oscillator strength, following the procedure we used for trion. Instead of the more standard two-photon absorption, this work concentrates on molecular biexciton created by photon absorption in an exciton gas. We first determine the appropriate set of coordinates in real and momentum spaces to describe one biexciton as two interacting excitons. We then turn to second quantization and introduce the "Fourier transform in the exciton sense" of the biexciton wave function which is the relevant quantity for oscillator strength. We find that, like for trion, the oscillator strength for the formation of one biexciton out of one photon plus a \emph{single} exciton is extremely small: it is one biexciton volume divided by one sample volume smaller than the exciton oscillator strength. However, due to their quantum nature, trion and biexciton have absorption lines which behave quite differently. Electrons and trions are fermionic particles impossible to pile up all at the same energy. This would make the weak trion line spread with electron density, the peak structure only coming from singular many-body effects. By contrast, the bosonic nature of exciton and biexciton makes the biexciton peak mainly rise with exciton density, this rise being simply linear if we forget many-body effects between the photocreated exciton and the excitons present in the sample.