Tailoring the Ultra‐Fast Infrared Optical Response of Al:ZnO Through Nanostructuration

Aluminum-doped zinc oxide (AZO) is one of the most promising transparent conductive oxides, valued for its low cost, high transparency and low electrical resistivity. Its tunable plasmonic response in the near-infrared region and the low optical losses make it a promising material for photonic applications. Here, we investigate the role of nanostructuration in the ultrafast optical response of engineered AZO metasurfaces. We design periodic arrays of nanocylinders, nanorods, and L-shaped nanostructures as a function of their relevant structural parameters. Selected metasurfaces are fabricated after an electron beam lithography and focused ion beam process. They are characterized from a dynamic point of view via ultra-fast mid-infrared pump probe technique, demonstrating marked differences of the optical behavior in the nanostructures. Simulations predict broad resonances in the 1500–3000 nm range, with peak absorption from 20% to 40%. In the AZO film and larger nanostructures, hot carriers decay rapidly (∼100 fs) in a thermalized population and in ∼300 fs to the ground state, while for nanorods only one population decaying rapidly is evident. Our results establish that it is possible to modify and control the ultrafast response of AZO metasurfaces through nanostructuration making them promising building blocks for infrared plasmonics and nanophotonic applications.