Complex Edge-State Phase Transitions in 1D Topological Laser Arrays

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We report the first observation of lasing in topological edge states in a 1D Su-Schrieffer-Heeger active array of resonators.

• We show that in the presence of chiral-time ($\mathcal{CT}$) symmetry, this non-Hermitian topological structure can experience complex phase transitions that alter the emission spectra as well as the ensued mode competition between edge and bulk states.
• The onset of these phase transitions is found to occur at the boundaries associated with the complex geometric phase- a generalized version of the Berry phase in Hermitian settings.
• Our experiments and theoretical analysis demonstrate that the topology of the system plays a key role in determining its operation when it lases: topologically controlled lasing.