  Ultrafast nanophotonics is an emerging research field aimed at the
development of nanodevices capable of light modulation with unprecedented
speed. A promising approach exploits the optical nonlinearity of nanostructured
materials (either metallic or dielectric) to modulate their effective
permittivity via interaction with intense ultrashort laser pulses. While the
ultrafast temporal dynamics of such nanostructures following photoexcitation
has been studied in depth, sub-ps transient spatial inhomogeneities taking
place at the nanoscale have been so far almost ignored. Here we theoretically
predict and experimentally demonstrate that the inhomogeneous space-time
distribution of photogenerated hot carriers induces a transient symmetry
breaking in a plasmonic metasurface made of highly symmetric metaatoms. The
process is fully reversible, and results in a broadband transient dichroic
optical response with a recovery of the initial isotropic state in less than 1
picosecond, overcoming the speed bottleneck caused by slower relaxation
processes, such as electron-phonon and phonon-phonon scattering. Our results
pave the way to the development of ultrafast dichroic devices, capable of Tera
bit/s modulation of light polarization.
