New classes of non-parity-time-symmetric optical potentials with all-real spectra and exceptional-point-free phase transition

Abstract: Paraxial linear propagation of light in an optical waveguide with material gain and loss is governed by a Schr\"odinger equation with a complex potential. Properties of parity-time-symmetric complex potentials have been heavily studied before. In this article, new classes of non-parity-time-symmetric complex potentials featuring conjugate-pair eigenvalue symmetry in its spectrum are constructed by operator symmetry methods. Due to this eigenvalue symmetry, it is shown that the spectrum of these complex potentials is often all-real. Under parameter tuning in these potentials, phase transition can also occur, where pairs of complex eigenvalues appear in the spectrum. A peculiar feature of the phase transition here is that, the complex eigenvalues may bifurcate out from an interior continuous eigenvalue inside the continuous spectrum, in which case a phase transition takes place without going through an exceptional point. In one spatial dimension, this class of non-parity-time-symmetric complex potentials is of the form $V(x)=h'(x)-h^2(x)$, where $h(x)$ is an arbitrary parity-time-symmetric complex function. These potentials in two spatial dimensions are also derived. Diffraction patterns in these complex potentials are further examined, and unidirectional propagation behaviors are demonstrated.