| Microresonator-based optical frequency combs（i.e.microcombs）are versatile coherent light sources due to their advantages of broad bandwidth,compactness and low power consumption.Mid-infrared（MIR）region covers specific absorption frequencies of various molecules as well as two atmospheric windows,and corresponds to the transition frequencies of narrow-band-gap materials,together with low Rayleigh scattering losses.Therefore,MIR microcombs can find promising applications in extensive areas including molecular spectroscopy,photoexcitation dynamics and optical imaging.Here,microring resonators are designed with optimized structures based on different material platforms,and MIR microcomb generation along with influences of several effects on the dynamic processes and microcomb properties is investigated.Firstly,the Lugiato-Leferver model describing microcomb generation is derived from the nonlinear Schr?dinger equation,and the analytical expression of microcavity dissipative solitons is obtained via Lagrange variation method.Factors affecting the microcomb stability and mechanism are investigated via analyzing the homogeneous steady state solution,suggesting the frequency detuning region that supports steady dissipative soliton generation.Then,a microring resonator consisting of Li Nb O3-Si slot waveguides is proposed.With optimized structure design,it possesses a flattened near-zero anomalous dispersion in a wide wavelength range.MIR microcombs ranging from 2810 nm to4630 nm,which exceeds four-fifths of an octave spanning,are generated with pump power as low as 50 m W.Influences of the high-order dispersion and self-steepening effect on the temporal and spectral properties of microcombs are further investigated,manifesting that the soliton drift has a maximum velocity with the Cherenkov radiation and phase-mismatching caused by the third-order dispersion,while the fourth-order dispersion can reduce the Cherenkov radiation intensity and drift velocity.Based on this,a method to improve the temporal stability of solitons is proposed,that is,the soliton drift caused by the third-order dispersion can be suppressed by the self-steepening effect.Next,the theoretical model describing the electro-optic modulation and second harmonic（SH）generation is established based on the dual-pump structure.A Li Nb O3microcavity is designed with optimized parameters to enable it having a strong electrical tuning ability.MIR microcavity solitons and mode-locked SH are obtained through resonance wavelength scanning with proper control of applied voltage.By altering the frequency interval of the two pumps,the repetition rate of the microcomb can be tuned and reach 2 THz.Finally,the theoretical model and analytical expression for microcavity solitons containing multi-photon absorption and free-carrier effects are derived to investigate properties and dynamic process of MIR soliton microcomb generation on Si and Ge platforms.The results suggest that the free-carrier absorption is equivalent to the linear propagation loss while the free-carrier dispersion is similar to the linear detuning.Therefore,the nonlinear detuning introduced by the free-carrier dispersion can help to establish the dynamic equilibrium of the intracavity field and enable the microcomb converting from the modulation instability state into the steady single-soliton state.Stable soliton steps are obtained in both Si and Ge microcavities with high nonlinearities by further studying the influences of pump power and free-carrier lifetime as well as appropriately controlling parameters.Besides,it takes different time to establish dynamic balance with different parameters. |
