  Solitary waves have consistently captured the imagination of scientists,
ranging from fundamental breakthroughs in spectroscopy and metrology enabled by
supercontinuum light, to gap solitons for dispersionless slow-light, and
discrete spatial solitons in lattices, amongst others. Recent progress in
strong-field atomic physics include impressive demonstrations of attosecond
pulses and high-harmonic generation via photoionization of free-electrons in
gases at extreme intensities of 1014 Wcm2. Here we report the first
phase-resolved observations of femtosecond optical solitons in a semiconductor
microchip, with multiphoton ionization at picojoule energies and 1010 Wcm2
intensities. The dramatic nonlinearity leads to picojoule observations of
free-electron-induced blue-shift at 1016 cm3 carrier densities and self-chirped
femtosecond soliton acceleration. Furthermore, we evidence the time-gated
dynamics of soliton splitting on-chip, and the suppression of soliton
recurrence due to fast free-electron dynamics. These observations in the highly
dispersive slow-light media reveal a rich set of physics governing
ultralow-power nonlinear photon-plasma dynamics.
