Spectral synthesis of temporal response of nonlinearity through tuneable electron and phonon dynamics in a metamaterial.

Manipulating intensity, phase and polarisation of the electromagnetic fields on ultrafast timescales is essential for all-optical switching, optical information processing and development of novel time-variant media. Noble metal based plasmonics has provided numerous platforms for optical switching and control, enabled by strong local field enhancement, artificially engineered dispersion and strong Kerr-type free-electron nonlinearities. However, achieving precise control over switching times and spectral response remains challenging, often limited by hot-electron gas relaxation on picosecond timescales and by the intrinsic band structure of the materials. Here, we experimentally demonstrate a strong and tunable nonlinearity in a metamaterial-on-a-mirror geometry, controlled by the wavelength of excitation, which imprints a specific, non-uniform hot-electron population distribution, driving targeted electron and lattice dynamics. The synergistic exchange of electromagnetic, electronic and mechanical energies enables reflection changes on sub-300 fs timescales in selected spectral ranges, surpassing the limitations imposed by the inherent material response of metamaterial constituents. The observed effect-present in reflection due to leaky guided modes of the metamaterial, but absent in transmission-is highly spectrally selective and sensitive to polarisation of light, opening a pathway to tailoring switching rates through the choice of operating wavelength and nanostructure design. The ability to manipulate temporal, spectral, and mechanical aspects of light-matter interactions underscores new opportunities for nonlinear optical applications where polarisation diversity, spectral selectivity, and ultrafast modulation are important.