Ultra-Thin Aluminum-Doped Silver for Transmissive Thermally Reconfigurable Visible Photonics

Functional materials with high electrical conductivity and optical transmittance are vital for thermally tunable free-space photonic systems. Conventional transparent conductors such as graphene and indium tin oxide are limited by high contact resistance, poor mechanical stability, or complex fabrication. Ultra-thin metals, such as pure silver, have also been explored with limited success due to thermal instability and dewetting. Here, we propose an ultra-thin Al-doped Ag film to tackle these challenges. Aluminum promotes heterogeneous nucleation of silver, enabling the formation of continuous, smooth films that are thermally stable at reduced thicknesses while maintaining excellent electrical conductivity and transparency. We find that a 12 nm Al-doped Ag film exhibits an average transmittance of 80% across the visible range with a sheet resistance of 8.3$\pm$1.16 $\Omega$cm$^2$. Moreover, on-chip Al-doped Ag microheaters exhibit uniform, rapid thermal response, and stable electrical performance, maintaining functionality for over $10^7$ ON and OFF cycles at temperatures below 400$\deg$C. Furthermore, as a benchmark, we demonstrate reversible phase-change switching in Ge$_2$Sb$_2$Se$_4$Te (GSST) and VO$_2$. 30$\times$30 $\mu$m$^2$ GSST cells exhibited complete crystallization and amorphization under 2.2 V - 200 ms and 4.1V - 50$\mu$s pulses, respectively, resulting in a 40% transmission contrast at 780 nm and a tenfold improvement in power consumption compared to similar devices. Additionally, VO$_2$ films displayed reversible insulator-to-metal transitions near 65{\deg}C with reflectance and transmittance modulation in the visible and the near-infrared at frequencies up to 25 Hz with room for improvement. These results establish Al-doped Ag as a robust transparent metallic heater for integration in dynamic metasurfaces, optical coatings, and more.