  Optically generated electron-hole plasma in high-index dielectric
nanostructures was demonstrated as a means of tuning of their optical
properties. However, until now an ultrafast operation regime of such plasma
driven nanostructures has not been attained. Here, we perform pump-probe
experiments with resonant silicon nanoparticles and report on dense optical
plasma generation near the magnetic dipole resonance with ultrafast (about 2.5
ps) relaxation rate. Basing on experimental results, we develop an analytical
model describing transient response of a nanocrystalline silicon nanoparticle
to an intense laser pulse and show theoretically that plasma induced optical
nonlinearity leads to ultrafast reconfiguration of the scattering power
pattern. We demonstrate 100 fs switching to unidirectional scattering regime
upon irradiation of the nanoparticle by an intense femtosecond pulse. Our work
lays the foundation for developing ultracompact and ultrafast all-optical
signal processing devices.
