  We present the generation of transient polarization gratings at the
nanoscale, achieved using a tailored accelerator configuration of the FERMI
free electron laser. We demonstrate the capabilities of such a transient
polarization grating by comparing its induced dynamics with the ones triggered
by a more conventional intensity grating on a thin film ferrimagnetic alloy.
While the signal of the intensity grating is dominated by the thermoelastic
response of the system, such a contribution is suppressed in the case of the
polarization grating. This exposes helicity-dependent magnetization dynamics
that have so-far remained hidden under the large thermally driven response. We
anticipate nanoscale transient polarization gratings to become useful for the
study of any physical, chemical and biological systems possessing chiral
symmetry.
