Universal coupler for bulk whispering gallery mode resonators based on silicon subwavelength grating waveguides

TL;DR

This paper introduces a silicon subwavelength grating waveguide approach enabling highly efficient coupling between bulk whispering gallery mode resonators and silicon photonic circuits, achieving high quality factors and broad material compatibility.

Contribution

It presents a novel coupling method using subwavelength metamaterials to connect bulk resonators with silicon waveguides, overcoming size and refractive index mismatches.

Findings

Achieved coupling efficiencies of 75-99%.

Demonstrated high Q factors of 10^6-10^7.

Enabled integration of diverse bulk resonator materials.

Abstract

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.