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Bound states in the continuum (BICs) enable unique features in tailoringlight-matter interaction on nanoscale. These radiationless localized statesdrive theoretically infinite quality factors and lifetimes for modernnanophotonics, making room for a variety of emerging applications. Here we usethe peculiar properties possessed by the so-called $\mathcal{PT}$-symmetricoptical structures to propose the novel mechanism for the quasi-BICmanifestation governed by the $\mathcal{PT}$-symmetry breaking. In particular,we study regularities of the spontaneous $\mathcal{PT}$-symmetry breaking intrilayer structures with the outer loss and gain layers consisting of materialswith permittivity close to zero. We reveal singular points on the curvesseparating $\mathcal{PT}$-symmetric and broken-$\mathcal{PT}$-symmetry statesin the parametric space of the light frequency and the angle of incidence.These singularities remarkably coincide with the BIC positions at the frequencyof volume plasmon excitation, where the dielectric permittivity vanishes. Theloss and gain value acts as an asymmetry parameter that disturbs conditions ofthe ideal BIC inducing the quasi-BIC. Fascinating properties of thesequasi-BICs having ultrahigh quality factors and almost perfect transmission canbe utilized in sensing, nonlinear optics, and other applications.

Quasi-bound states in the continuum induced by $\mathcal{PT}$-symmetry breaking