  Femtosecond-scale polarization state conversion is experimentally found in
optical response of a plasmonic nanograting by means of time-resolved
polarimetry. Simultaneous measurements of the Stokes parameters as a function
of time with an averaging time-gate of 130 fs reveal a remarkable alteration of
polarization state inside a single fs-pulse reflected from a plasmonic crystal.
Time-dependent depolarization is experimentally found and described within an
analytical model which predicts the four-fold enhancement of the polarization
conversion effect with the use of the narrower gate. The effect is attributed
to excitation of time-delayed polarization-sensitive surface plasmons with a
highly birefringent Fano-type spectral profile.
