Exceptional points make for exceptional sensors

A ring-shaped optical cavity has degenerate resonant modes, because clockwise and anticlockwise waves resonate at the same frequencies. The degeneracy can be lifted, and the frequencies split, by a perturbation such as a physical rotation or the presence of a molecule or nanoparticle. Typically, the frequency splitting is proportional to the perturbation’s magnitude, as illustrated in the top panel of the figure for a hypothetical complex-valued perturbation ε (that is, one that can affect both the light’s frequency and its phase). Because the plot’s shape resembles a yo-yo-like toy called a diabolo, the degeneracy has been dubbed a diabolic point. The mode splitting around a diabolic point is the basis for optical gyroscopes, and it’s been explored for other sensing applications.

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