Hot biexcitons driven by extreme optical confinement

TL;DR

This paper reports the experimental observation of four-body hot biexcitons in bilayer WS2 using extreme optical confinement via a photonic crystal cavity, revealing room-temperature many-body interactions with potential for advanced quantum phenomena.

Contribution

It introduces a novel method of creating quasi-3D optical confinement to observe high-order many-body excitons at room temperature in 2D materials.

Findings

Observation of four-body hot biexcitons in bilayer WS2.

Room-temperature biexcitons with valley polarization and coherence.

Potential for exploring higher-order many-body phenomena.

Abstract

A powerful means to understanding condensed matter that possesses a multi-constituent, non-isolated, and complex nature, with a preeminent example being two-dimensional (2D) materials, is studying many-body interactions. However, experimentally observing high-order many-body interactions is a daunting task due to its heavy reliance on the abundance of low-order complexes. Here, we report the observation of four-body hot biexcitons in an energetically unfavorable bilayer of tungsten disulfide (WS2) through creating extreme optical confinement. Specifically, we integrate a non-radiative bound state in the continuum (BIC) into a photonic crystal (PhC) defect cavity, forming a quasi-three-dimensional (q-3D) but open confinement for photons at the driving frequency. The extremely confined photons in both reciprocal and physical spaces then excite inherently unproductive two-body hot excitons situated slightly above the indirect bandgap so efficiently that they form overwhelmed higher-order four-body hot biexcitons. Distinctively, these hot biexcitons exhibit substantial valley polarization and coherence at room temperature, which we attribute to the topological nature of BICs and the associated q-3D confinement with an orbital angular momentum. Besides achieving room-temperature biexcitons, the q-3D confinement could be valuable for higher-order interactions, such as triexcitons, and many other many-body phenomena, including Bose-Einstein condensation.