Fig. 1. Chalcogenide microresonators-based optical frequency combs generation. Three types of optical frequency combs can be generated: Bright soliton microcomb; Dark pulse microcomb; Raman-Kerr comb

Fig. 2. Fabrication process of high-QGeSbS microresonators and linear transmission characterization^[34] . (a) Fabrication procedure of GeSbS microresonators based on RIE-ICP etching; (b), (c) Top view and cross section view of scanning electron microscopy (SEM) of a GeSbS microresonator with a radius of 100 μm; (d), (g) Experimentally measured transmission spectra of TE₀₀ and TM₀₀ mode in fabricated devices, showing linewidths to be 144 MHz and 204 MHz from the Lorentz fitting curves; (e), (h) Corresponding mode distributions of TE₀₀and TM₀₀ mode; (f), (i) Measured intrinsic linewidth distributions of TE₀₀and TM₀₀ mode

Fig. 3. Bright soliton comb generation in a GeSbS microresonator^[34]. (a) The calculated and measured resonator dispersion for TM₀₀ mode with a radius of 100 μm, and the cross-section is 2.4 μm×0.8 μm (width × height), respectively; (b) Transmission spectrum of the resonance when a laser is swept with a higher pump power, the "soliton step" can be observed; (c) The measured optical spectra as the pump laser is red-detuned into the cavity resonance (top to bottom), the pump power is fixed at ~20 mW

Fig. 4. Dark-pulse comb generation in a GeSbS microresonator^[34]. (a) The measured dispersion curve of the same GeSbS microresonator in TE₀₀ mode. Dashed and solid lines show the simulated integrated dispersion of the TE₀₀ mode and high-order mode. Blue circles represent the measured integrated dispersion of the TE₀₀ mode; (b) Normalized comb power when the laser was scanned from the blue side to the red side with high power; (c) The measured output optical spectra of dark-pulse combs at different stages as indicated in (b), black line denotes the simulated spectrum in stage III, the pump power is fixed at ~25 mW

Fig. 5. Raman-Kerr comb generation in a GeSbS microresonator^[37]. (a) Measured Raman spectrum of GeSbS film with a thickness of 0.8 μm; (b) Schematic of the degenerate FWM between the pump wave, the first and the second Stokes waves; (c) Calculated dispersion curve for TE₀₀ mode of the microresonators, with different waveguide widths (1.7 and 2.4 μm); (d) Calculated phase mismatch of the degenerate FWM process for (c); (e) The measured Raman-Kerr comb when increasing the pump power to ~30 mW in the 2.4-μm microresonator

Table 1. Comparison of the on-chip chalcogenide microresonators