  Optical microresonators are of paramount importance in photonic circuits
requiring fine spectral filtering or resonant light recirculation. Key
performance metrics improve with increasing resonance quality factor (Q) across
all applications. The performance of silicon photonic circuits is often
hampered by the low-quality factor of planar silicon microresonators, typically
of Q~10^4-10^5. On the other hand, bulk whispering gallery mode resonators
provide a wide range of materials with intriguing optical properties and
exceptionally high resonant quality factors Q>10^7. However, the efficient
coupling between bulk resonators and planar Si photonic waveguides is
considered challenging, if not impossible, due to remarkably large mismatch in
size and refractive index. Here, we show an efficient method to couple bulk
resonators and Si waveguides based on subwavelength metamaterial engineering of
silicon. Based on this approach, we experimentally demonstrate coupling between
220-nm-thick Si waveguides and bulk microresonators made of silica, lithium
niobate and calcium fluoride with diameters in the 0.3-3.5 mm range, achieving
high coupling efficiency of 75-99% and exceptional Q of 10^6-10^7. These
results open a new route for the heterogeneous integration of bulk resonators
and silicon photonic circuits, with great potential for applications in
sensing, microwave-photonics, and quantum photonics, to name a few.
