Gate-defined quantum dots (GQDs) are promising solid-state quantum structures for realizing spin-photon quantum interfaces that convert the arbitrary polarization state of a photon to the corresponding spin state of an electron. For an efficient conversion, the optical absorption of GQDs must be enhanced without polarization dependence. In this study, we design an optical cavity based on the bull's-eye structure. The symmetry of the structure allows the cavity to support doubly degenerate modes. We numerically demonstrate that for an incident light of any polarization state, the optical absorption of a GQD embedded in a bull's-eye cavity is equally enhanced. A 450× absorption enhancement is obtained in an experimentally feasible structure. We elucidate the physical origins of this significant enhancement by analyzing the light absorption of GQDs using coupled mode theory. Our results indicate the potential of the bull's-eye cavity structure as a building block for efficient quantum media conversion using GQDs.
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