Self-formation of coherent emission in a cavity-free system

TL;DR

This paper demonstrates that in a cavity-free system, active atoms can induce mode pulling that leads to self-formation of coherent emission once a certain pump threshold is exceeded.

Contribution

It reveals a novel mechanism where intermodal coupling via active atoms causes mode pulling and coherence in cavity-free lasing systems.

Findings

Mode pulling aligns free-space modes with atomic transition frequency.

Constructive interference realizations emerge at low pump rates.

Coherent radiation appears when pump rate surpasses a threshold.

Abstract

It is commonly accepted that a collection of pumped atoms without a resonator, which provides feedback, cannot lase. We show that intermodal coupling via active atoms pulls the frequencies of the free-space modes towards the transition frequency of the atoms. Although at a low pump rate mode phases randomly fluctuate, phase realizations at which interference of pulled modes is constructive emerge. This results in an increase of stimulated emission into such realizations and makes their lifetime longer. Thus, mode pulling provides positive feedback. When the pump rate exceeds a certain threshold, the lifetime of one of the realizations diverges, and radiation becomes coherent.