Xueqi Xing | Searcharxiv

arXiv2026

Multiscale simulations guided advances for all-optical phase-change waveguides

Photonic computing using chalcogenide phase-change materials (PCMs) is under active development for energy-efficient artificial intelligence (AI) applications. A key requirement is to enable as many optically programmable levels per device as possible, while maintaining relatively low optical loss. In this work, we carry out multiscale simulations using density functional theory and finite-difference time-domain methods, proposing a "the shorter the better" strategy to optimize the performance of Sb2Te photonic waveguide devices. Our subsequent experimental characterizations of Sb2Te thin films and optical device measurements fully verify our theoretical predictions. In particular, we reveal the unconventional optical properties of metastable crystalline Sb2Te, and utilize these features for device design, yielding a simultaneous improvement in both the programming window and the optical loss. Overall, an optical programming precision exceeding 7-bit is achieved using a single waveguide cell, setting a new record for all-optical phase-change memory devices. Our work serves as a compelling example of computational material design, which demonstrates the predictive power of multiscale simulations in guiding the design of phase-change photonic devices for enhanced performance.

cond-mat.mtrl-sci↗

arXiv2025

Atomistic understanding of two-dimensional monatomic phase-change material for non-volatile optical applications

Elemental antimony (Sb) is a promising material for phase-change memory, neuromorphic computing and nanophotonic applications, because its compositional simplicity can prevent phase segregation upon extensive programming. Scaling down the film thickness is a necessary step to prolong the lifetime of amorphous Sb, but the optical properties of Sb are also significantly altered as the thickness is reduced to a few nanometers, adding complexity to device optimization. In this work, we aim to provide atomistic understanding of the thickness-dependent optical responses in Sb thin films. As thickness decreases, both the extinction coefficient and optical contrast reduce in the near-infrared spectrum, consistent with previous optical measurements. Such thickness dependence gives rise to a bottom thickness limit of 2 nm in photonic applications, as predicted by coarse-grained device simulations. Further bonding analysis reveals a fundamentally different behavior for amorphous and crystalline Sb upon downscaling, resulting in the reduction of optical contrast. Thin film experiments are also carried out to validate our predictions. The thickness-dependent optical trend is fully demonstrated by our ellipsometric spectroscopy experiments, and the bottom thickness limit of 2 nm is confirmed by structural characterization experiments. Finally, we show that the greatly improved amorphous-phase stability of the 2 nm Sb thin film enables robust and reversible optical switching in a silicon-based waveguide device.

cond-mat.mtrl-sci↗