Tunable Optical Response Based on Au@GST Core–Shell\nHetero-nanostructures

Abstract

Active control over the optical response\nhas been investigated\nextensively, which is highly required in next-generation photonic\ndevices. Here, we report a core–shell nanostructure combined\nwith the plasmonic nanostructure with an active phase change material,\ntermed as Au@GST, where the individual Au nanoparticle is wrapped\nby isolated active Ge₂Sb₂Te₅ (GST)\ntuning medium as the shell. We demonstrate the active control of optical\nresponse by the simultaneous control of size and crystallization degree\nof the phase change shell within each isolated core–shell nanostructure.\nThe hetero-nanostructures are synthesized by direct sputtering of\nstoichiometric GST on Au nanoparticles with different sizes based\non a seeding effect of the metal nanostructures. The tunable optical\nresponse is enabled by inducing the GST shell phase states between\namorphous and crystalline phases through thermal annealing, accompanied\nby high optical contrast. We demonstrate the tunable optical response\n(from 570 to 630 nm) of the hetero-nanostructures by tailoring the\nGST shell’s optical property. The experimental results show\nthat the optical response of the Au@GST core–shell hetero-nanostructures\nexhibits high sensitivity to the size of the nanoparticles (GST thickness\nand Au diameter). The simulation results are in good agreement with\nthe experiments. This confirms the dominant role of the phase states\non the optical modulation. Furthermore, this study provides a single-step\nand lithography-free 3D printing process to arrange the hybrid Au@GST\ncore–shell nanostructures in a large area deliberately, giving\nadditional flexibility to control over the architecture and geometry.\nSuch Au@GST core–shell hetero-nanostructures have the potential\nfor active control of nanophotonic devices, such as super-resolution\nimaging and programmable metasurfaces.