All-dielectric silicon microring resonators with deeply sub-diffractive mode volumes

We report an approach to achieve deeply sub-diffractive mode volumes in all-dielectric ring resonators. Specifically, we leverage simultaneous usage of nanostructured subwavelength metamaterial waveguides and excitation of standing wave rather than traveling wave whispering gallery mode (WGM) resonances. This enables the mode volume to be reduced by factors greater than 10 to 100 compared to a diffraction-limited silicon microring resonator. We also highlight how a combination of diffractive mode volume scaling and sub-diffractive near-field intensity scaling indicates a lower limit for the sub-diffractive all-dielectric mode volume, $V_{\text{min}}'$, which scales in proportion to the mode order times the refractive index raised to the seventh power, i.e., $V_{\text{min}}' \sim m n^{-7}$. Through both numerical modeling and experimental characterization, we systematically investigate the modal properties of metawaveguide ring resonators comprised of Si and SiO$_2$. The sub-diffractive devices are experimentally shown to support standing wave resonances with high intrinsic quality factors, on the order of $10^4$ to $10^5$. These metamaterial ring resonators present a promising WGM platform for interfacing wavelength-scale optics with sub-wavelength matter.