Microheater Engineering for Phase-Change Material-Based Photonic Devices

Abstract

Reconfigurable photonic devices that dynamically modulate optical properties are enabling key functions in communications, sensing, and computing. Phase-change materials (PCMs) are particularly attractive because solid-state phase transitions provide a large, nonvolatile refractive-index contrast, supporting compact devices with zero static power. Realizing these advantages in practice, however, requires a precise, repeatable definition of amorphous and crystalline domains at micron scales while preserving optical performance and long-term reliability. This thesis addresses that challenge by establishing thermal-actuation and electrode design—especially focused on microheater geometry and material selection—as a unifying route to deterministic switching, improved stability, and scalable integration across both free-space and integrated photonic platforms.

This thesis demonstrates an ultrathin, optically transparent, and highly conductive metal heating film based on co-sputtering Al-doped Ag, which maintains continuous, smooth metallic layers at reduced thickness while retaining high electrical conductivity and optical transparency. The on-chip Al-doped Ag heater exhibits uniform, rapid thermal response, and stable electrical performance, maintaining functionality for over {\unboldmath $10^7$} ON and OFF cycles at temperatures below 400~$^\circ$C. These heaters enable reversible thermally driven transitions in representative tunable materials, including Ge$_2$Sb$_2$Se$_4$Te and VO$_2$, and demonstrate efficient optical modulation in transmissive free-space devices.

Beyond material engineering, the thesis demonstrates repeatable multilevel programming in waveguide-integrated PCM devices by engineering on-chip microheaters to shape the spatial and temporal thermal profile. Foundry-compatible doped-silicon heaters integrated on silicon-on-insulator waveguides with Sb$_2$Se$_3$ and Ge$_2$Sb$_2$Se$_4$Te claddings enable deterministic switching of targeted regions, achieving 27 cycles with 7 repeatable levels each. We used transient thermoreflectance imaging to quantify heater dynamics and validate thermal design strategies. Finally, the microstructure-engineering framework is extended to hybrid PCM–ferroelectric systems, enabling spatially controlled ferroelectric domain switching for nonvolatile, multilevel tuning of guided light.

Overall, this thesis focuses on multi-physics field control and provides a scalable, energy-efficient pathway to high-fidelity, multilevel reconfigurable photonics, improving switching repeatability and device longevity while remaining compatible with dense photonic integration.