Overcoming optical performance and diffusion issues in thermally tunable phase-change metasurfaces

Thermally tunable optical metasurfaces based on chalcogenide phase-change materials (PCMs) - whose refractive indices between their amorphous and crystalline phases can be abruptly controlled via optical, electrical or thermal heat stimuli - are currently one of the most promising approaches towards the creation of novel, compact, and fast reconfigurable nanophotonic devices. Yet, such a technology currently faces various non-trivial technological and engineering challenges that need to be overcome before implementing them in real-world applications [1] . One of the most critical aspects for PCM metasurfaces to be successful is the achievement of in-situ electrical switching without severely compromising the optical performance. For this purpose, good electrical conductors (metals) with high melting points (i.e. above the melting point of GeSbTe based alloys where T melt ~ 630 ᵒC) are required to avoid long-term degradation. In addition, such metallic elements suffer from thermally activated atomic diffusion into PCMs and/or semiconductors such as Ge 2 Sb 2 Te 5 (GST) or silicon, which results in the creation of parasitic interfacial gold/silver tellurides and gold/silver silicides that can catastrophically and irreversibly degrade device optical performance [2] , leading to unreliable device designs.