Route to pure-quartic solitons in the mid-infrared

Pure-quartic solitons (PQSs), as a new branch of soliton families, have attracted significant attention recently due to their potential in achieving higher energy than classical solitons for short pulse durations, providing innovations in nonlinear optics. Here, we report the excitation of PQSs at telecom and mid-infrared (MIR) wavelengths in silicon microcavities, enabled by slot waveguide structures for dominant fourth-order dispersion (FOD). We demonstrate that PQSs can be generated in the three-photon absorption (3 PA) and four-photon absorption (4 PA) regimes due to appropriate free-carrier (FC) lifetime or weak absorption coefficient, while PQSs are suppressed in the C-band ascribed to strong two-photon absorption (2 PA). Importantly, different PQS states are accessible by tuning the FC lifetime. Analytically, the approximate PQS solutions involving FOD and high-order nonlinearities are deduced with the Lagrangian perturbation method. These results provide a promising route to dispersion engineering for PQS formation in emerging platforms and shed new insights into understanding the dynamics of PQSs combined with multiphoton absorption (MPA) and FC effects.