image: Figure 1 | Electro-optic performance enhancement enabled by a self-buffered layer. a, Schematic of the self-buffered design, in which interface-induced edge dislocations give rise to a periodically modulated in-plane stress distribution within the film. Inset: Phase-field simulation results showing the emergence of orthorhombic (O-phase) polar nanoregions at the domain boundaries. b, Cross-sectional scanning transmission electron microscopy (STEM) image of the BTO film, revealing a ~40 nm-thick self-buffered layer at the film–substrate interface. Inset: High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image acquired across a domain boundary and the corresponding polarization distribution. A high density of transitional phases dominated by the orthorhombic (O) phase is induced, giving rise to a multiphase-coexisting polymorphic phase boundary, which enables a pronounced enhancement of electro-optic performance. c, Strain maps of the in-plane (top) and out-of-plane (bottom) components reconstructed from Nanobeam Electron Diffraction (NED) results. Clear periodic strain gradient stripes are observed in the in-plane component. d, Temperature dependence of Second Harmonic Generation (SHG) intensity and out-of-plane lattice constant. Inset: Schematic of the temperature-dependent domain evolution.Labels a, c, and o correspond to T-phase a/c domains and O-phase domains. e, Variation of refractive index as a function of the AC electric field applied along the in-plane <110> and <100> directions. The extracted effective electro-optic coefficient is 253 pm V-1.
Credit: Changzheng Sun et al.
As data traffic and computing demands continue to grow, photonic chips that process information with light instead of only with electrons are seen as a promising way to boost speed and energy efficiency. A central building block of such chips is an electro-optic modulator, which converts electrical signals into changes of an optical beam. Ideally, these modulators should be fast, work at low driving voltages and occupy as little chip area as possible. Ferroelectric barium titanate (BTO) is very attractive in this context because it offers an exceptionally strong Pockels effect, thereby enabling compact and energy-efficient devices. In practice, however, it has been difficult to grow BTO thin films on low-refractive index oxide insulators that are prerequisite for guiding light on a chip. Lattice mismatch and strain tend to degrade the crystalline quality and weaken the electro-optic response, so earlier attempts to integrate BTO on such substrates have struggled to combine strong modulation performance, low optical loss and scalable film growth.
In a new article published in Light: Science & Applications, a team led by Assoc. Prof. Qian Li from the School of Materials Science and Engineering and Prof. Changzheng Sun from the Department of Electronic Engineering at Tsinghua University, together with collaborators from Zhejiang University and the Chinese Academy of Sciences, report a strategy that breaks through this long-standing limitation. They demonstrate that BTO can be epitaxially integrated on low-index oxide substrates such as LaAlO3-Sr2TaAlO6 (LSAT), a widely used perovskite oxide. In this form, the film retains a very strong electro-optic response and a ferroelectric state that stays stable well above typical chip operating temperatures. Using this film as the active layer, the team builds the first on-chip electro-optic modulator based on BTO/LSAT. It can switch light efficiently with only a small driving voltage, handle radio-frequency signals in the tens of gigahertz and fit into a footprint suitable for dense integration, offering a route to low-power, high-capacity photonic links on a practical oxide platform.
The key to this performance is a self-buffered film architecture that forms during growth. By tuning the growth conditions, the bottom part of the BTO film relaxes most of the lattice mismatch with the LSAT substrate, effectively acting as a buried self-buffer layer. Above this layer, the film adopts a complex polar state characterized by a mixture of domains and nanoscale polar regions. This built-in structure enables the film to accommodate strain while preserving strong ferroelectricity, thereby enhancing its electro-optic response. The researchers comment: “With the assistance of these polymorphic polar nanoregions, the effective electro-optic coefficient reaches approximately 253 pm V-1, exceeding 8 times of LiNbO3 thin films. Notably, the Curie temperature is elevated from the conventional 120 °C of bulk BTO to roughly 200 °C. We attribute this significant thermal enhancement to the structural robustness and thermal stability provided by the orthorhombic polar regions. This combination of strong modulation strength and a wide working temperature window makes the material well suited to photonic chips that need to operate reliably in real-world environments.”
The next step was to see how this performance translates into a real device. To demonstrate that the materials platform is ready for practical use, the team built an on-chip electro-optic modulator using BTO on LSAT. Light is guided in silicon nitride strip-loaded waveguides, while the BTO layer serves as the active section where the optical mode and the applied electric field overlap, so that the strong electro-optic response of the film can be used directly on a compact chip without complicated layer sequences. Tests at telecommunications wavelengths show that the modulator can reach full optical modulation with only a small driving voltage, and still works reliably when driven by radio-frequency signals in the tens of gigahertz, supporting data rates in line with modern optical links.
The researchers also emphasise the broader implications of their approach. “By solving the epitaxy problem of BTO on oxide insulators, we now have a low-voltage, high-speed modulation platform that can be made on wafer-scale LSAT substrates,” they comment. “Such a platform could help build dense optical links on chips and between chips, so that future data centres, communication networks and photonic processors can move large amounts of information using much less electrical power.”
Article Title
Self-buffered epitaxy of barium titanate on oxide insulators enables high-performance electro-optic modulators