ИСТИНА

Abstract: Modern, nanotechnologies allow to create nanostructured multi-component systems supporting different types of electromagnetic excitations such as plasmon-polaritons, surface lattice resonances, waveguide modes. Ultrafast process in such systems are of great interest. These process are studied by means of pump-probe technique. Action of an ultrashort laser “pump” pulse leads to perturbation of material properties and the second “probe” pulse detects the change. In metals, laser pulse causes heating of an electron gas and a consecutive modification of the dielectric permittivity. Therefore, transmittance is one of the quantities that evolves on a sub-picosecond scale. In this work, the sub- and picosecond optical response dynamics of the hybrid metasurface based on gold nanospheres placed in a layer of bismuth-substituted yttrium iron garnet has been studied using femtosecond pump-probe spectroscopy. Differential transmittance ∆T/T, which is a normalized change in the transmittance of a sample under the action of a laser pulse, were measured for various wavelengths λ, time delays τ and pump fluences J. The studied multimodal metasurface supports qualitatively disparate electromagnetic modes. The plasmon modes demonstrates typical bulk gold dynamics, while ΔT/T (λ,τ) and ΔT/T (λ,J) dependencies near quasi-waveguide mode shows anomalous behaviour: temporal evolution relaxes slowly and can’t be described by two-temperature model and differential transmittance grows sub-linearly with increasing fluence. Numerical simulations based on two-temperature model and band theory of gold were used to describe ultrafast dynamics at plasmon resonances, but were inconsistent in the vicinity of waveguide mode.