Demonstration of a high- Q subwavelength dielectric nanocylinder

The development of subwavelength dielectric cavities is essential for reducing the size of photonic devices and enabling dense optoelectronic integration. However, previously demonstrated subwavelength optical cavities exhibit measured Q -factors <400, limiting their applications. Here, we demonstrate a high- Q subwavelength nanocylinder by leveraging bound states in the continuum (BICs). We track BIC modes of different longitudinal orders while maintaining an ultrasmall footprint. We find that the Q -factor initially increases but then saturates at higher orders. By linking quasi-normal-mode perturbation theory with coupled-mode analysis, we clarify the origin of this saturation, estimate the attainable Q -factor limit, and identify an optimized geometry that balances performance with fabrication feasibility. By suspending this design in free space using nanobridges and optimizing the nanofabrication process, we experimentally realize an InP subwavelength nanocylinder with a measured Q -factor exceeding 1000. Compared with a lower- Q substrate-supported counterpart, the suspended high- Q BIC nanocylinder exhibits stronger scattering and photoluminescence signals. Our work provides a route to high- Q optical devices with ultrasmall footprints.