III-Nitride nanophotonics for beyond-octave soliton generation and self-referencing

TL;DR

This paper demonstrates a nanophotonic aluminum nitride chip that generates stabilized octave-spanning microcombs with integrated offset frequency detection, advancing portable optical frequency devices.

Contribution

It introduces a fully integrated aluminum nitride nanophotonic platform for octave soliton microcombs with on-chip offset frequency stabilization capabilities.

Findings

Achieved 1.5-octave comb span with dual dispersive waves

Enabled on-chip heterodyne detection of offset frequency

Generated microcombs with sub-terahertz repetition rates down to 220 GHz

Abstract

Frequency microcombs, successors to mode-locked laser and fiber combs, enable miniature rulers of light for applications including precision metrology, molecular fingerprinting, and exoplanet discoveries. To enable the frequency ruling function, microcombs must be stabilized by locking their carrier-envelop offset frequency. So far, the microcomb stabilization remains compounded by the elaborate optics external to the chip, thus evading its scaling benefit. To address this challenge, here we demonstrate a nanophotonic chip solution based on aluminum nitride thin films, which simultaneously offer optical Kerr nonlinearity for generating octave soliton combs and Pockels nonlinearity for enabling heterodyne detection of the offset frequency. The agile dispersion control of crystalline III-Nitride photonics permits high-fidelity generation of solitons with features including 1.5-octave comb span, dual dispersive waves, and sub-terahertz repetition rates down to 220 gigahertz. These attractive characteristics, aided by on-chip phase-matched aluminum nitride waveguides, allow the full determination of the offset frequency. Our proof-of-principle demonstration represents an important milestone towards fully-integrated self-locked microcombs for portable optical atomic clocks and frequency synthesizers.