We introduce a new family of spectral singularities with highly directional response in parity-time (PT) symmetric cavities. These spectral singularities support modes with infinite reflection from one side and zero reflection from the other side of the cavity, results in simultaneous unidirectional laser and unidirectional reflectionless parity-time symmetric cavity. Such unidirectional spectral singularities emerge from resonance trapping induced by the interplay between parity-time symmetry and Fano resonances.
We introduce a class of unidirectional lasing modes associated with the frozen mode regime of non-reciprocal slow-wave
structures.1 Such asymmetric modes can only exist in cavities with broken time-reversal and space inversion
symmetries. The lasing frequency coincides with a spectral stationary inflection point of the underlying passive
structure and it is virtually independent of the size of the cavity. These unidirectional lasers can be indispensable
components of photonic integrated circuitry.
We show that nonlinear optical structures involving a balanced gain-loss profile can act as optical diodes. This is made
possible by exploiting the interplay between the fundamental symmetries of parity (P) and time (T), with optical
nonlinear Kerr effects. This unidirectional propagation is demonstrated for the case of a PT -symmetric nonlinear coupler
and a PT-symmetric Bragg grating.