When spinning particles, such as electrons and photons, undergo spin–orbit coupling, they can acquire an extra phase in addition to the well-known dynamical phase. This extra phase is called the geometric phase (also known as the Berry phase), which plays an important role in a startling variety of physical contexts such as in photonics, condensed matter, high-energy and space physics. The geometric phase was originally discussed for a cyclically evolving physical system with an Abelian evolution, and was later generalized to non-cyclic and non-Abelian cases, which are the most interesting fundamental subjects in this area and indicate promising applications in various fields. Here, we enable optical spin–orbit coupling in asymmetric microcavities and experimentally observe a non-cyclic optical geometric phase acquired in a non-Abelian evolution. Our work is relevant to fundamental studies and implies promising applications by manipulating photons in on-chip quantum devices.

Optoplasmonic whispering‐gallery‐mode (WGM) microcavities, consisting of plasmonic nanostructures and optical microcavities, provide excellent platforms for exploring fundamental mechanisms as well as facilitating novel optoplasmonic applications. These integrated systems support hybrid modes with both subwavelength mode confinement and high‐quality factor which do not exist in either pure optical WGM microcavities or plasmonic resonators. In this progress report, geometric designs and fabrication strategies of optoplasmonic microcavities, which efficiently bridge the interaction between resonant light and plasmonic resonances, are reviewed in detail. Three types of hybrid modes in the optoplasmonic microcavities, that is, surface‐plasmon‐polariton whispering‐gallery modes, hybrid photon–plasmon whispering‐gallery modes, and heterostructured metal–dielectric whispering‐gallery modes, are considered. These modes are characterized by a largely enhanced evanescent field that is referred to as a plasmon‐type field in hybrid whispering‐gallery modes. Moreover, the coupling effect between localized surface plasmon resonances and whispering‐gallery modes is summarized. The underlying coupling mechanisms and their influence on mode shifts, Q factor, mode splitting, and line shapes of the whispering‐gallery modes are discussed. Applications based on optoplasmonic WGM microcavities including enhanced sensing, nanolasing, and free‐space coupling are highlighted, followed by an outlook of the opportunities and challenges in developing large‐scale on‐chip integrated optoplasmonic systems.