We discuss phase transitions in PT-symmetric optical systems. We show that, due to frequency dispersion of the dielectric permittivity, an optical system can have PT symmetry at isolated frequency points only. An assumption of the existence of a PT-symmetric system in a continuous frequency interval violates the causality principle. Therefore, the ideal symmetry-breaking transition cannot be observed by simply varying the frequency.
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Zyablovsky et al. (2014) studied this question.
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