Multi-photon absorption (MPA) and free-carrier (FC) effects, usually considered to be detrimental to microcomb generation by introducing strong nonlinear loss, also offer opportunities to overwhelm the thermal-optic effect by modifying the refractive index. Here, we derive the theoretical expression of solitons expanded with MPA and FC effects, accompanied by numerical simulations to reveal the dynamics and mechanism for capturing steady mid-infrared solitons in both Si and Ge microcavities. It is found that with increased FC lifetime, the intensity-dependent MPA underlies non-monotonous variation of nonlinear detuning which enables soliton generation. More interestingly, proper control on the FC lifetime admits bidirectional switching of soliton states (i.e., both decreasing and increasing the number of solitons) as well as self-starting solitons, yielding different technique routes toward microcavity solitons. This research could contribute to a better understanding of nonlinear behaviors influenced by FC effects and find practical applications by releasing the demand on precise control of laser sources, which is especially meaningful for the mid-infrared region.

中文翻译：

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自由载流子辅助的中红外微腔孤子生成

多光子吸收（MPA）和自由载流子（FC）效应通常被认为会通过引入强烈的非线性损耗而损害微梳子的产生，它们还提供了通过改变折射率来抵消热光效应的机会。在这里，我们推导了通过MPA和FC效应扩展的孤子的理论表达式，并通过数值模拟揭示了在Si和Ge微腔中捕获稳定的中红外孤子的动力学和机理。发现随着FC寿命的增加，强度相关的MPA成为非线性失谐的非单调变化的基础，这使得孤子产生成为可能。更有趣的是，对FC寿命的适当控制允许孤子状态的双向切换（即，孤子数量的减少和增加）以及自启动孤子，针对微腔孤子产生了不同的技术路线。通过释放对激光源的精确控制的需求，该研究可以有助于更好地理解受FC效应影响的非线性行为，并找到实际应用，这对于中红外区域尤其有意义。