Chiral modes and directional lasing at exceptional points

TLDR

Exceptional points arise when loss or gain in a system causes its eigenvalues and eigenstates to coalesce. Operating near two exceptional points in a whispering‑gallery‑mode resonator enabled dynamic control of light rotation, allowing emission to be tuned from bidirectional to unidirectional and reversed, thereby providing new functionalities for lasing, sensing, optomechanics, cavity quantum electrodynamics, and on‑chip light‑flow control.

Abstract

Significance Exceptional points (EPs) emerge when a system with loss or gain is tuned such that its eigenvalues and the corresponding eigenstates coalesce. By operating close to two EPs, we achieved a dynamic control of the rotation direction of light in a whispering-gallery-mode (WGM) resonator. This enabled us to tune the emission direction of a WGM microlaser from bidirectional, when the system is away from the EPs, to unidirectional emission at EPs and to reverse the emission direction by moving from one EP to the other. This feature provides WGM resonators new functionalities useful for lasing, sensing, optomechanics, and cavity quantum electrodynamics. Our findings will help to develop novel technologies for controlling the light flow on-chip and could affect scientific fields beyond optics.