A universal form of complex potentials with spectral singularities

We establish necessary and sufficient conditions for complex potentials in the Schrödinger equation to enable spectral singularities (SSs) and show that such potentials have the universal form $U(x) = -w^2(x) - iw_x(x) + k_0^2$, where $w(x)$ is a differentiable function, such that $\lim_{x\to \pm \infty}w(x)=\mp k_0$, and $k_0$ is a nonzero real. We also find that when $k_0$ is a complex number, then the eigenvalue of the corresponding Shrödinger operator has an exact solution which, depending on $k_0$, represents a coherent perfect absorber (CPA), laser, a localized bound state, a quasi bound state in the continuum (a quasi-BIC), or an exceptional point (the latter requiring additional conditions). Thus, $k_0$ is a bifurcation parameter that describes transformations among all those solutions. Additionally, in a more specific case of a real-valued function $w(x)$ the resulting potential, although not being $PT$ symmetric, can feature a self-dual spectral singularity associated with the CPA-laser operation. In the space of the system parameters, the transition through each self-dual spectral singularity corresponds to a bifurcation of a pair of complex-conjugate propagation constants from the continuum. The bifurcation of a first complex-conjugate pair corresponds to the phase transition from purely real to complex spectrum.

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