Geometry optimization for dark soliton combs in thin multimode silicon nitride microresonators.

Area of Science:

• Integrated photonics
• Materials science
• Nonlinear optics

Background:

• Silicon nitride (Si3N4) is crucial for ultralow-loss integrated photonics and frequency comb generation.
• Thick Si3N4 films (>600 nm) are needed for bright soliton generation but suffer from stress and cracking.
• Thin Si3N4 films (<400 nm) offer advantages like one-step deposition but require optimized designs for nonlinearity.

Purpose of the Study:

• To engineer an integrated Si3N4 structure for optimal effective nonlinearity and compact footprint.
• To investigate the trade-offs between waveguide thickness, nonlinearity, and device stability.
• To demonstrate the feasibility of using thin Si3N4 films for advanced photonic applications.

Main Methods:

• Comparative analysis of Si3N4 resonators with varying waveguide thicknesses.
• Experimental demonstration of dark pulse generation in a multimode Si3N4 resonator.
• Characterization of resonator footprint, quality factor, and generated dark comb properties.

Main Results:

• A 400-nm thin Si3N4 film provides a balance between effective nonlinearity and device stability.
• Low-noise coherent dark pulses with a 25 GHz repetition rate were generated.
• A compact spiral resonator (0.28 mm²) with a high quality factor (4×10⁶) was fabricated.

Conclusions:

• Thin (400 nm) silicon nitride films are a viable alternative for integrated photonics, avoiding stress issues.
• The demonstrated dark combs have significant potential for applications in microwave photonics, spectroscopy, and telecommunications.
• Optimized thin-film Si3N4 resonators offer a promising platform for generating high-performance frequency combs.