Fig. 1. Dispersion engineering and characterization of the microdisk resonators. (a) Curves of the group velocity dispersion as a function of wavelength for various thicknesses of the microdisk resonator, whereas the wedge angle (37 deg) and diameter (86μm) are fixed. The left inset shows the Dint curve centered at 780 nm with a thickness of 1μm. The right inset shows the schematic diagram of a microdisk resonator. (b) Scanning electron micrograph image of the microdisk resonator. (c) Transmission trace (black curve) of TM10 mode at 775.9 nm with a resonance doublet fitting (red curve). The blue sinusoidal curve is a frequency calibration from a Mach–Zehnder interferometer. The total resonance linewidth is 5.8 MHz with a resonance splitting of 6.5 MHz, corresponding to an intrinsic Q-factor (Q0) of 6.7×107. (d) Optical microscope image of the microresonator with visible single-soliton microcomb generation (with 780 nm light filtered).

Fig. 2. Generation of visible soliton microcomb with a DW at the short wavelength. (a) Optical spectrum of the single-soliton observed at the pump power of 1.1 mW. The gray curve indicates the simulated soliton microcomb spectrum based on the Lugiato–Lefever equation. (b) Transmission of comb power with the soliton steps indicating the existence of the soliton states. (c) Radio frequency noise of the chaotic comb and the single-soliton comb, together with a noise floor of the photodiode. (d) Simulated integrated dispersion of the TM10 mode family of the microdisks with various wedge angles ranging from 34.4 to 43.1 deg. The thickness (1μm) and diameter (82.3μm) are fixed. The measured dispersion of the microdisk is shown by hollow dots. Inset: zoom-in view of measured and simulated dispersion in the center. (e) Optical spectra of the single-soliton state generated in three different samples with wedge angles of 34.4 deg (top panel), 39.2 deg (middle panel), and 43.1 deg (bottom panel). The corresponding pump powers are 2.5, 8.2, and 32.6 mW, respectively. The intrinsic (loaded) Q-factors in the experiment are 6.7×107 (2.4×107) in the top panel, 3.1×107 (1.0×107) in the middle panel, and 1.7×107 (0.4×107) in the bottom panel, respectively. The insets show the SEM images of the microdisk edges with different angles.

Fig. 3. Visible single-soliton microcomb with dual DWs. Soliton microcomb spectra of the silica microdisk resonators with a wedge angle of 39.2 deg, a thickness of 1μm, and various diameters of 86.5μm in panel (a), 89.0μm in panel (b), and 91.3μm in panel (c). The corresponding pump powers are 123.6, 131.1, and 180.4 mW, respectively. The short-wavelength DWs are induced by higher order dispersion, and the long-wavelength DWs are induced by the spatial mode interactions between TM10 and TM20 mode families. The intrinsic (loaded) Q-factors in the experiment are 3.0×107 (0.3×107) in panel (a), 3.4×107 (0.3×107) in panel (b), and 2.8×107 (0.2×107) in panel (c), respectively. (d)–(f) The relative mode frequency (relative to pump mode) of TM10 (green curve) and TM20 (purple curve) mode families correspond to panels (a)–(c), respectively.

Fig. 4. Precise control of DW. (a) Zoom-in spectra of short-wavelength DW blue-shifted as the pump red-detuning increased. (b) Spectra of single-soliton microcombs with increasing pump detuning using microdisk resonators with a wedge angle of 37 deg, a thickness of 1μm, and a diameter of 85.9μm. The soliton self-frequency shift is measured by fitting the center of the optical spectrum.