Anomalous picosecond optical transmittance dynamics in Au-Bi:YIG hybrid metasurface

The sub- and picosecond optical response dynamics of the metal–dielectric metasurface based on gold nanospheres placed in a layer of bismuth-substituted yttrium iron garnet has been studied via the pump–probe femtosecond spectroscopy with time resolution. It is shown that the plasmon modes of the metasurface demonstrate bulk gold dynamics, while the observed relaxation of the polariton mode is slowed down by several picoseconds. The difference arise because of the detection of different processes at the wavelengths of the plasmon and polariton modes by the probe beam.