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

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.