  Nonlinear wave mixing in mesoscopic silicon structures is a fundamental
nonlinear process with broad impact and applications. Silicon nanowire
waveguides, in particular, have large third-order Kerr nonlinearity, enabling
salient and abundant four-wave-mixing dynamics and functionalities. Besides the
Kerr effect, in silicon waveguides two-photon absorption generates high
free-carrier densities, with corresponding fifth-order nonlinearity in the
forms of free-carrier dispersion and free-carrier absorption. However, whether
these fifth-order free-carrier nonlinear effects can lead to six-wave-mixing
dynamics still remains an open question until now. Here we report the
demonstration of free-carrier-induced six-wave mixing in silicon nanowires.
Unique features, including inverse detuning dependence of six-wave-mixing
efficiency and its higher sensitivity to pump power, are originally observed
and verfied by analytical prediction and numerical modeling. Additionally,
asymmetric sideband generation is observed for different laser detunings,
resulting from the phase-sensitive interactions between free-carrier
six-wave-mixing and Kerr four-wave-mixing dynamics. These discoveries provide a
new path for nonlinear multi-wave interactions in nanoscale platforms.
