Non-Hermitian effects due to asymmetric backscattering in optical microresonators

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Optical microresonators play a fundamental role in many fields of basic and applied research in physics. Due to optical losses, such as absorption and radiation, these resonators are open systems. An implication of the openness is the recently discovered asymmetry in the coherent backscattering of counter-propagating waves in whispering-gallery microresonators which do not possess a mirror-reflection symmetry. This asymmetric backscattering has surprising consequences, such as the appearance of pairs of strongly nonorthogonal optical modes, which mainly co-propagate. These interesting effects are strongly enhanced near so-called non-Hermitian degeneracies at exceptional points in parameter space. In this project several important aspects of asymmetric backscattering will be studied in detail in different configurations. To this belongs the numerical and analytical investigation of coupled microresonators without mirror-reflection symmetry in the context of non-Hermitian degeneracies of higher order and exceptional points in complex band structures. Moreover, a perturbation theory will be developed which allows to describe asymmetric backscattering and its consequent effects analytically. Further, we plan to analyze the asymmetric backscattering in a microresonator coupled to two waveguides and to directly verify it experimentally in a cooperation with Prof. Hui Cao (Yale University).