Strong Coupling in a Photonic Molecule Formed by Trapping a Microsphere in a Microtube Cavity

Abstract

role for the efficient intercavity coupling. However, the evanescent field is relatively weak in the reported photonic mole cules because most of the optical field is strongly confined within the coupled cavities (e.g., microdisks, [4,8,11,18] micro toroids, [19] microspheres, [3] microrods, [5,20] and microfibers [7] ). As such, one needs a deliberate control on both the cavity geometries and the intercavity coupling gap to ensure a good spectral match and efficient evanescent coupling between the coupled cavities. [18] Moreover, dynamic tuning of the intercavity coupling strength has been investigated in recent years, which were carried out by advanced and sophisticated techniques such as strain tuning, [21,22] acoustooptic control, [23] and precise micromanipulation techniques. [3] To extend and promote the research in the field of photonic molecules, it is of high interest to design novel photonic molecules extending from adjacent solid microcavities to thinwalled hollow cavities which possess intense evanescent field facilitating intercavity coupling and provide novel strategy for tuning of the coupling strength.Microtube cavities, which are formed by selfrolling of pre strained nanomembranes, feature unique properties such as hollowcore structures and ultrathin cavity walls (≈100-300 nm) for the study of lightmatter interactions and the integration of "labinatube" systems. [24][25][26][27] These key merits enable extensive applications ranging from optofluidic sensing, [28][29][30][31] single cell analysis, [32] dynamic molecular process detection, [33] photon plasmon coupling, [34] to optical spin-orbit coupling.[35] To combine with other media/objects, luminescent quantum dots, [36,37] quantum wells, [38] and organic molecules [39] have been enwrapped into the microtube wall by the rolling up process, which couple photoluminescence (PL) light to the microtube cavities to support whisperinggallery mode (WGM) resonances. [37,40] In this context, a design and demonstration of photonic molecule based on microtube cavity is of fundamental interest for the study of strong optical coupling and the promo tion of its potential applications.Herein, we report a novel design of photonic molecule by trapping a microsphere cavity into the hollow core of a rolled up microtube cavity. We focus on studying the WGM coupling between the trapped microsphere and microtube cavity with a significant difference of cavity sizes, which is in contrast to pre vious reports where the two externally adjacent microcavities possess highly similar sizes [11,12,41] or slightly mismatched sizes (less than two times). [4][5][6][7]18,[42][43][44] Periodic modulations on A photonic molecule formed by trapping a microsphere cavity into a hollow microtube cavity is demonstrated, which provides a novel design over conventional photonic molecules comprised of solid-core whispering gallery mode microcavities with externally tangent configuration. Periodic spectral modulations of mode intensity, resonant mode shift, and quality factor are observed...

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