Third-harmonic-assisted four-wave mixing in a chip-based microresonator frequency comb generation.

Area of Science:

• Photonics
• Nonlinear Optics
• Integrated Optics

Background:

• Microcombs offer broadband, coherent optical frequency combs with high repetition rates.
• Coherent microcombs in normal dispersion microresonators typically exhibit a platicon (flat-top) temporal profile.
• Generating platicons in pure $\chi^{(3)}$ microresonators, especially in the visible spectrum, remains challenging.

Purpose of the Study:

• To propose and demonstrate a novel scheme for generating platicons in silicon nitride microresonators.
• To leverage third-harmonic generation (THG) to achieve phase matching for four-wave mixing in normal dispersion.
• To enable coherent and visible microcomb generation in a $\chi^{(3)}$ microresonator.

Main Methods:

• Utilizing a silicon nitride microresonator with engineered third-harmonic generation.
• Exploiting nonlinear coupling between fundamental and third-harmonic waves via third-order sum/difference frequency generation.
• Tuning the THG nonlinear coupling strength and phase matching conditions.

Main Results:

• Successfully generated platicon states in a silicon nitride microresonator.
• Demonstrated the ability to obtain single or multiple platicons by adjusting experimental parameters.
• Showcased a new mechanism for phase matching four-wave mixing in normal dispersion regimes.

Conclusions:

• The proposed THG-assisted scheme is a promising route for generating coherent and visible microcombs.
• This method facilitates platicon generation in pure $\chi^{(3)}$ microresonators.
• The findings have potential applications in self-referencing combs and optical clock stabilization.