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.