Unexpected ultraviolet plasmonic properties of Ge2Sb2Te5: Exploration through double Tauc–Lorentz model

Plasmonic materials have driven nanophotonic advancements, with ultraviolet (UV) functionality and dynamic reconfigurability emerging as critical needs. This work addresses these needs by investigating the UV optical properties of Ge2Sb2Te5 (GST-225), a prototypical phase-change material. Spectroscopic ellipsometric analysis reveals that precise descriptions of UV dielectric functions of GST-225 require a double Tauc–Lorentz (DTL) model, uncovering an additional resonance at 6–8 eV. Density functional theory (DFT) calculations trace the dual oscillators in the DTL model to two distinct interband transitions. The two oscillators associated with resonant bonding give rise to broadband plasmonic properties spanning visible to extreme-ultraviolet wavelengths for GST-225, which is an unusual trait among UV plasmonic materials. The plasmonic performances are further substantiated by finite element simulations and electron energy loss spectroscopy (EELS) measurements. This study establishes GST-225 as a promising candidate for dynamically tunable UV photonic devices while broadening the scope of phase-change material applications to UV adaptive optics, sensing, and nanoscale light manipulation.