Photoinduced transient symmetry breaking in plasmonic structures for ultrafast nanophotonics

We study the spatio-temporal evolution of hot electrons generated in plasmonic nanostructures under resonant excitation with fs-laser pulses. A spatially inhomogeneous version of the Three-Temperature Model for hot-electrons dynamics, coupled to semiclassical calculations of third-order optical nonlinearity in gold, enabled us to engineer a transient symmetry breaking of the optical properties at the nanoscale. This effect is exploited to achieve all-optical control of light with unprecedented speed. For instance, a photoinduced broadband dichroism, fully reversible and transiently vanishing in less than 1 picoseconds (overcoming the speed bottleneck caused by slower, electron-phonon and phonon-phonon relaxation processes), has been experimentally demonstrated in plasmonic metasurfaces with nanocross metaatoms. Also, we designed a nonlinear plasmonic metagrating (based on cross-polarized gold nanostrip dimer metaatoms), where the nanoscale symmetry breaking enables ultrafast reconfiguration of diffraction orders via control laser pulses. The photoinduced power imbalance between symmetrical diffraction orders is calculated to exceed 20% under moderate (similar to 2 mJ/cm(2)) laser fluence, and returns to the balanced diffraction in about 2 ps. Our design has been developed for gold nanomaterials, but the concept of ultrafast all-optical symmetry breaking can be exploited beyond plasmonics (e.g. in semiconductor nanostructures), with potential impact on a broad range of applications in nanophotonics.