Metamaterial engineered silicon photonic coupler for whispering gallery mode microsphere and disk resonators

Optical microresonators with high quality factors are key in photonic circuits requiring fine spectral filtering or resonant storage of optical power. Silicon (Si) photonics provides high-performance optoelectronic circuits but yields planar Si microresonators with rather low quality factors ( $\mathrm{Q}<{10}^5$ ). On the other hand, bulk resonators achieve exceptionally high quality factors, $\mathrm{Q}>{10}^7$ . Si photonic waveguides and bulk resonators have very different sizes and refractive indices that preclude efficient coupling. Here, we show an efficient method to couple bulk resonators and Si waveguides based on subwavelength metamaterial engineering. We demonstrate up to 99% light coupling efficiency for microspheres and microdisks made of silica, lithium niobate, and calcium fluoride, with $0.3-3.6\mathrm{m}\mathrm{m}$ diameter. This achievement could enable the heterogeneous integration of bulk resonators and silicon photonic circuits, with potential applications in sensing, communications, and quantum information.