We clarify theoretically that the topological ring-cavity (TRC) modes propagating along the interface between two honeycomb-type photonic crystals distinct in topology can be exploited for achieving stable single-mode lasing, with the maximal intensity larger than a whispering-gallery-mode counterpart by order of magnitude. Especially, we show that the TRC modes located at the bulk band-gap center benefit maximally from the gain profile since they are most concentrated and uniform along the interface, and that, inheriting from the Dirac-like dispersion of topological interface states, they are separated in frequency from each other and from other photonic modes, both favoring intrinsically single-mode lasing. A TRC mode running in a specific direction with desired orbital angular momentum can be stimulated selectively by injecting a circularly polarized beam. With the scheme proposed in the present work one can fabricate a topological laser of tens of micrometers in linear size by means of advanced semiconductor nanotechnologies, which generates chiral laser beams ideal for novel photonic functions.

• Received 11 March 2020
• Revised 21 May 2021
• Accepted 24 May 2021

DOI:https://doi.org/10.1103/PhysRevB.103.245305