Design Parameters for Phase-Change Materials for Nanostructure Resonance Tuning

Phase-change materials (PCMs) are a promising platform for programmable metamaterials in the infrared (IR) spectral range. This is based on the large refractive index change between the amorphous and crystalline phases of PCMs accompanied by their low dielectric losses in the IR. These PCMs can exist in various stoichiometries, leading to distinct changes in dielectric properties which can be exploited for resonance tuning. To investigate these differences, nanoantennas with thin films of three (GeTe)(x)-(Sb2Te3)(1-x) compounds such as Ge2Sb2Te5 (GST-225), Ge3Sb2Te6 (GST-326), and Ge8Sb2Te11 (GST-8211) are combined. Different PCM stoichiometries regarding their suitability for resonance tuning are systematically compared for the first time and the PCM GST-8211 is introduced into the field of photonics. An increase of the antenna resonance frequency shift based on an increasing GeTe content in the compound is experimentally demonstrated. This leads to the largest figure of merit for antenna resonance tuning in the IR using PCMs reported so far. Based on the insights obtained, the changes in the real part of the permittivity epsilon(1), the band gap E-G, and the charge carrier concentration N are identified as design parameters for resonance tuning with PCMs.