Optical loss in general is assumed to be detrimental, however, its judicious combined action with gain in a parity-time (PT) symmetric system has given rise to a plethora of intriguing phenomena. Of interest is the interplay between optical gain (loss) and magnetization, as two different time-reversal symmetry breaking mechanisms. It can provide a curious playground to go beyond conventional PT-symmetric platforms that solely rely on the manipulation of gain/loss through the scalar refractive index of the constituent material.

Towards this goal, researchers from City College of the City University of New York, have leveraged the vector nature of electromagnetic waves and chirality of the optical near field in a PT-symmetric gain/loss double ring resonator scheme by integrating one of the resonators with a magneto-optical (MO) material whose response is dominated by circular dichroism. Such a nonreciprocal non-Hermitian system manifests different PT-symmetry conditions for two counter-propagating modes, thus introducing a route to optical tuning of nonreciprocal devices, for example, in isolators and circulators.

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