Tailoring Broadband Kerr Soliton Microcombs via Post-Fabrication Tuning of the Geometric Dispersion

TL;DR

This paper demonstrates a method for post-fabrication tuning of geometric dispersion in silicon nitride microresonators using controlled dry etching, enabling precise control of broadband Kerr soliton microcombs without degrading device quality.

Contribution

The authors introduce a dry-etching technique to fine-tune silicon nitride thickness after fabrication, improving dispersion control for microcombs in integrated photonics.

Findings

Dry etching allows precise thickness adjustment without surface roughness increase.

Post-fabrication tuning enables spectral shape control of Kerr solitons.

Device quality remains high after etching, preserving optical performance.

Abstract

Geometric dispersion in integrated microresonators plays a major role in nonlinear optics applications, especially at short wavelengths, to compensate the natural material normal dispersion. Tailoring of geometric confinement allows for anomalous dispersion, which in particular enables the formation of microcombs which can be tuned into the dissipative Kerr soliton (DKS) regime. Due to processes like soliton-induced dispersive wave generation, broadband DKS combs are particularly sensitive to higher-order dispersion, which in turn is sensitive to the ring dimensions at the nanometer-level. For microrings exhibiting a rectangular cross section, the ring width and thickness are the two main control parameters to achieve the targeted dispersion. The former can be easily varied through parameter variation within the lithography mask, yet the latter is defined by the film thickness during growth of the starting material stack, and can show a significant variation (few percent of the total thickness) over a single wafer. In this letter, we demonstrate that controlled dry-etching allows for fine tuning of the device layer (silicon nitride) thickness at the wafer level, allowing multi-project wafers targeting different wavelength bands, and post-fabrication trimming in air-clad ring devices. We demonstrate that such dry etching does not significantly affect either the silicon nitride surface roughness or the optical quality of the devices, thereby enabling fine tuning of the dispersion and the spectral shape of the resulting DKS states.