Fig. 1. Fundamental of soliton dynamics in microcavities：(a) Input and output of a coherently pumped microcavity^[46]; (b) Double balance between gain and loss as well as dispersion and nonlinearity is needed to support dissipative solitons in microcavities; (c) Dispersion curve of an anomalous dispersion cavity; (d) Simulated intracavity power change when scanning the pump frequency detuning. The power change features several steps indicating soliton states; (e) Experimentally measured transmitted power when scanning the laser from blue-detuning to red-detuning^{[10, 12]}

Fig. 2. Soliton interaction in microcavities：(a) Illustration of co-propagating (CoP) solitons trapped by dispersive wave emission^[79]; (b) Perfect soliton crystals (PSC) comprising X solitons in a microcavity^[80]; (c) Counter-propagating (CP) solitons interaction via Rayleigh backscattering^[82]; (d) Synchronization of solitons in two different microcavities via injecting one of the solitons into the other one^[84]

Fig. 3. Dual-microcomb applications：(a) Dual-comb spectroscopy using microcombs^[87]; (b) A microcavity based vernier spectrometer^[89]; (c) Dual-comb imaging using microcombs^[90]; (d) Ultrafast distance measurement with dual-microcombs^[92]

Fig. 4. Application of optical frequency combs for spectroscopy：(a) Scheme of mid-infrared dual-comb spectroscopy using silicon microcavities^[96]; (b) Dual-comb spectroscopy (DCS) analysis of methane using iDFG mid-IR combs^[102]; (c) Intrapulse difference-frequency-generation technology^[100]; (d) Interleaved difference-frequency-generation (iDFG) technology^[101]