Fabricating silicon nitride microrings for optical frequency comb generation

Author:

Gabriel Colación, Doctoral Student, Optical Science and Engineering, University of New Mexico

Lala Rukh, University of New Mexico

Tara Drake, University of New Mexico

Abstract:

Since their invention over two decades ago, optical frequency combs have enabled dramatic advances in the way we measure light and time. The utility of these devices follows from their characteristic output of broadband optical frequencies along with their well-timed pulsed output. Optical frequency combs are now used in a multitude of applications including microwave and optical frequency generation, synchronization between state-of-the-art atom-based clocks, as well as atomic and molecular spectroscopy. However, the complexity and scale of optical frequency combs generally limits them to specialized optics laboratories. In response, there has been a significant effort to leverage advances in nano- and micro-fabrication to produce compact chip-based optical frequency comb devices.

Here, we describe our current work in the fabrication and testing of micro-scale ring (microring) geometries for optical frequency comb generation. In microrings, the geometric requirements for frequency comb generation call for devices several hundreds of nanometers thick. This poses a challenge for “subtractive” fabrication in materials such as silicon nitride (SiN) where the device is chemically cut or “etched” from a blank layer of material utilizing a protective mask that transfers the pattern. The required thickness and etch resistivity of SiN necessitates masks of either sufficient thickness or hardness to last the entirety of the etch process. We implement a novel method for subtractive processing of thick SiN microrings implementing a chromium metallic mask as an etch template. By leveraging the etch resistivity of the metallic layer, we fabricate high quality SiN microrings that exhibit exceptional uniformity, near vertical sidewall angles and moderate sidewall roughness. This work serves to highlight both the benefits and drawbacks to this technique as a robust approach to achieving high quality microrings for optical frequency comb generation.

Biography:

Gabriel Colación is currently a doctoral student in the Optical Science and Engineering program at the University of New Mexico. He received his B.A. in physics from Reed College in 2017. Upon graduating, he joined the Fiber Sources and Applications group at the National Institute of Standards and Technology (NIST) in Boulder, Colorado as a research assistant. While at NIST, he conducted research on the design, construction and implementation of fiber optic-based frequency comb devices for spectroscopic applications. In 2020, He joined the research group of Dr. Tara Drake at UNM focusing his research on nonlinear photonic microresonators for frequency comb generation. His current work involves the engineering and fabrication of these devices, investigating novel techniques for thermal noise mitigation, and low-noise microwave synthesis. 

Dr. Tara Drake's Research Group: