# Collective two-boson decay of excitons in Bose-Einstein condensate and generation of coherent photon-phonon radiation

## TL;DR

This paper theoretically investigates the collective decay of excitons in a Bose-Einstein condensate, revealing how spontaneous emission of bosonic pairs can be amplified, leading to potential applications like phonon lasers.

## Contribution

It introduces a new theoretical model for collective two-boson decay in exciton BECs, highlighting the amplification mechanism and its nonlinear density dependence, with implications for phonon laser development.

## Key findings

- Spontaneous bosonic pair emission is amplified in exciton BECs.
- Decay rate depends nonlinearly on excitonic density.
- Potential for realizing phonon laser applications.

## Abstract

The collective decay of excitons from initial Bose-Einstein condensate state is investigated theoretically. As practically more interesting case we consider excitons of the yellow series in the semiconductor cuprous oxide where we have collective photon and phonon assisted decay of excitons. It is shown that because of intrinsic instability of recoilless two-boson decay of Bose-Einstein condensate, the spontaneously emitted bosonic pairs are amplified leading to an exponential buildup of a macroscopic population into the certain modes. The collective decay rate has a nonlinear dependence on the excitonic density being comparable or larger than Auger recombination loss rate up to the high densities, which makes obtainable its observation. The considering phenomenon can also be applied for the realization of phonon laser.