Alumina coating for dispersion management in ultra-high Q microresonators

Marvyn Inga,Laís Fujii,João Henrique Quintino Palhares,Francisco C Marques,Thiago P Mayer Alegre,Andre Santarosa Ferlauto,Gustavo Wiederhecker,José Maria C Da Silva Filho

doi:10.1063/5.0028839

Marvyn Inga, Laís Fujii + Show 6 more

Open Access

https://doi.org/10.1063/5.0028839

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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 (∼107) 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.

Highlights

Despite the consistent progress in mitigating optical losses within integrated photonics microcavities,1,2 fused-silica microspheres still rank among the highest optical quality factor microresonators ever fabricated
Group velocity dispersion in microresonators translates into a free-spectral range (FSR) that varies with frequency
Using alumina scitation.org/journal/app coatings with roughly 100 nm thickness, we could achieve a large degree of group-velocity dispersion (GVD) control, significantly reducing the degree of anomalous GVD yet not drastically impacting their high Q-factors

Summary

INTRODUCTION

Despite the consistent progress in mitigating optical losses within integrated photonics microcavities, fused-silica microspheres still rank among the highest optical quality factor microresonators ever fabricated. These ultra-high Q-factors, reported above 109 at visible and near-infrared and around 108 at telecom wavelengths, result from silica’s low material absorption and atomiclevel surface roughness achieved by the standard thermal-fusion fabrication process. Despite the consistent progress in mitigating optical losses within integrated photonics microcavities, fused-silica microspheres still rank among the highest optical quality factor microresonators ever fabricated.3 These ultra-high Q-factors, reported above 109 at visible and near-infrared and around 108 at telecom wavelengths, result from silica’s low material absorption and atomiclevel surface roughness achieved by the standard thermal-fusion fabrication process. Those properties, combined with a relatively small modal volume, allowed microspheres to host pioneer lowpower nonlinear optical phenomena, such as Raman lasing, thirdharmonic generation, Kerr optical frequency combs, and optomechanical effects.. Given the conformal characteristics of the ALD technique, the demonstrated approach could be readily extended to other high-Q resonator geometries and material platforms

GVD CONTROL WITH ALUMINA COATING

HIGH-Q ALUMINA-COATED MICROSPHERES

Findings

CONCLUSIONS