Reversible Structural Phase Transition in MoO_x Thin Films for Tunable Optical Devices

Abstract

Optical phase-change materials (OPCMs) offer a powerful platform for electrically driven reconfigurable photonics, yet the development of low-loss optical phase change materials, in particular in the visible range, remains a significant challenge. Currently investigated materials such as Sb2Se3 and Sb2S3 remain quite absorptive in that wavelength range. In this work, we demonstrate the use of molybdenum oxide (MoOx, x ≈ 3) as a low-loss OPCM. Through ellipsometry, we observed large changes in the refractive index between its amorphous and crystalline phases. This transition is achieved by annealing the material above its crystallization temperature, with crystallization confirmed via x-ray diffraction. In both phases, the imaginary part of the refractive index for green light was below 1×10-4, demonstrating the material’s potential for reconfigurable photonic devices. With an on-chip heater, Joule heating pulses were used to induce a reversible amorphous-to-crystalline phase transition, confirmed with selected area electron diffraction (SAED). During optical characterization, a change in reflectivity was observed. However, signs of layer delamination were also identified, which are expected to contribute to the reflectivity variation. Nevertheless, the observed reversible crystallization of MoOx using electrical pulses marks an important step toward its use in reconfigurable flat optics. Thermal simulations guided the microheater design to enable localized switching, while optical simulations provide a baseline for expected modulation. This work represents the first experimental demonstration of electrically induced phase switching of MoOx thin films integrated into an optical device, highlighting both the promise and practical challenges of this emerging material system.