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The prospect of judiciously utilizing both optical gain and loss has been recently suggested as a means to control the flow of light. This proposition makes use of some newly developed concepts based on non-Hermiticity and parity-time (PT) symmetry-ideas first conceived within quantum field theories. By harnessing such notions, recent works indicate that novel synthetic structures and devices with counter-intuitive properties can be realized, potentially enabling new possibilities in the field of optics and integrated photonics. Non-Hermitian degeneracies, also known as exceptional points (EPs), have also emerged as a new paradigm for engineering the response of optical systems. As opposed to standard degeneracies, at an EP, not only do the eigenvalues coalesce but so do the corresponding eigenstates. At such bifurcations, the relevant eigenvectors collapse on each other and as a result, the dimensionality of the system is abruptly reduced. In this case, when a perturbation of strength ε acts on an Nth-order EP (when N eigenvalues and eigenvectors merge), the resulting eigenvalue (frequency) splitting is now proportional to ε^(1/N). This indicates that the sensitivity of a set-up can be enhanced by several orders of magnitude by exploiting the physics of EPs. Among many different non-conservative photonic configurations, parity-time (PT) symmetric arrangements are of particular interest since they provide an excellent platform to explore the physics of EPs for enhanced sensing applications. In this talk, we will provide an overview of recent developments in this field. The use of other type symmetries in photonics will be also discussed.
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