Ultrathin spiral microtube cavities for enhanced free-space coupling

Optical whispering-gallery-mode (WGM) microcavities have gained tremendous attention as key elements in photonic applications. Among the coupling techniques for introducing light into the microcavity to pump WGM resonances, free-space coupling presents a straightforward and convenient way for transferring light into microcavities, which eliminates the requirement for additional assistant coupling components. Nevertheless, achieving simplified designs without compromising robustness and high efficiency in free-space coupling systems remains a significant challenge. Here, we demonstrate free-space coupling in ultrathin spiral microtube cavities, achieving a remarkable enhancement in coupling efficiency of up to 32.02%. This significant improvement is enabled by the combination of the ultrathin cavity wall (i.e., the nanomembrane frame of the tube) and a nanonotch. In particular, our design exhibits angle dependence, which provides phase-difference-steered flexible excitation of mode profiles, ranging from peaks, Fano-type curves, to dips. Our straightforward strategy of free-space coupling avoids the complex designs of luminescent materials, precise alignments, and mechanical instability of coupling components, greatly facilitating fundamental explorations on light--matter interactions and related applications.