Alumina coating for dispersion management in ultra-high Q microresonators

TL;DR

This paper demonstrates that conformal alumina coatings on silica microspheres can precisely engineer group-velocity dispersion while maintaining ultra-high quality factors, enabling advanced nonlinear optical applications like Kerr frequency combs.

Contribution

The study introduces a nanometric alumina coating technique via atomic layer deposition to finely tune dispersion in silica microresonators without degrading their high Q factors.

Findings

Alumina coating preserves ultra-high Q (~10^7) in silica microspheres.

Coating allows precise control of group-velocity dispersion.

Kerr frequency combs are successfully generated using coated microspheres.

Abstract

Silica optical microspheres often exhibit ultra-high quality factors, yet, their group velocity dispersion, which is crucial for nonlinear optics applications, can only be coarsely tuned. We experimentally demonstrate that group-velocity dispersion of a silica microsphere can be engineered by coating it with conformal nanometric layers of alumina, yet preserving its ultra-high optical quality factors (\num{\sim e7}) at telecom wavelengths. Using the atomic layer deposition technique for the dielectric coating, which ensures nm-level thickness control, we not only achieve a fine dispersion tailoring but also maintain a low surface roughness and material absorption to ensure a low optical loss. Numerical simulations supporting our experimental results show that the alumina layer thickness is a promising technique for precise tuning of group-velocity dispersion. As an application we demonstrate the generation of Kerr optical frequency combs, showing that the alumina coatings can also sustain the high optical intensities necessary for nonlinear optical phenomena.