Ultrafast All-Optical Polarization Control in a Plasmonic Metasurface by Selective Hot-Electron Heating of Orthogonal Gold Nanorods

Abstract

Metamaterials tailor light's degrees of freedom through static designs, but sub-picosecond, all-optical control without sacrificing signal throughput remains challenging. Here, a metasurface is presented whose unit cell contains two independent, orthogonal and spectrally detuned Au nanorod resonators. A femtosecond pump polarized along either nanorod selectively heats its electrons, modulating one polarization channel while the other serves as an essentially unperturbed phase reference, turning a static wave retarder into an ultrafast, all-optically reconfigurable one. The transient response is computed by coupling a Three-Temperature Model, benchmarked against published experiments, with Finite-Difference Time-Domain simulations. Hot electrons damp and red-shift the pumped resonance, modulating the amplitude ratio and phase retardation of the two channels. In transmission at normal incidence and sub-mJ cm${}^{-2}$ fluence, a 45${}^{\circ }$ probe undergoes changes of $\approx$10${}^{\circ }$ in ellipticity and $\approx$15${}^{\circ }$ in optical rotation within $\approx$100 fs (relaxation time $\approx$4 ps), while its transmittance ($\approx$0.4) changes by at most $\approx$0.05. A second, delayed pump along the other nanorod raises these changes to $\approx$15${}^{\circ }$ and $\approx$22${}^{\circ }$ and, for a suitably polarized probe, cancels the intensity change of the first within $\approx$60 fs while the polarization stays changed. Selective all-optical control of independent resonators offers a practical route to transmissive ultrafast polarization control.