Advances in novel electro-optic thin films and their applications in high-speed electro-optic modulators (<italic>invited</italic>)

SignificanceIn recent years, global data traffic has experienced sustained growth. The rising demand for data in communication networks, high-performance computing, and intelligent computing systems has led to the exponential expansion of new ultra-large-scale data centers, thereby increasing the bandwidth requirements for optical transceiver modules and accelerating the development of advanced on-chip integrated electro-optic modulators. Silicon photonic devices have encountered significant development opportunities owing to their advantages in cost-effectiveness, high performance, and reliability. Although high-performance electro-optic modulators based on silicon have been realized over the past decade, they are constrained by relatively weak carrier dispersion effects, making it challenging to meet the stringent demands of next-generation photonic integrated circuits. Thin-film lithium niobate has garnered significant attention due to its inherent electro-optic efficiency, decades of research and industrial validation, as well as improvements in durability and rapid manufacturing. In addition to lithium niobate, electro-optic modulators based on emerging thin-film materials such as lithium tantalate, barium titanate, and lead zirconate titanate have also been increasingly investigated. With advances in high-quality thin-film fabrication techniques, these materials have demonstrated breakthroughs and notable progress in achieving high-speed and efficient electro-optic modulation. This underscores the importance of exploring novel ferroelectric thin films for high-speed electro-optic modulators.Progress While high-performance silicon electro-optic modulators have gradually reached commercialization, leveraging their CMOS-compatibility as a first-mover advantage, their physical mechanisms impose limitations on further improvements in modulation performance. In contrast, ferroelectric materials are ideal candidates for electro-optic modulators due to their excellent properties, including low loss, wide bandgap, intrinsic electro-optic effect, and fast response times. Advances in equipment and processing methods have enabled the successful development of high-quality ferroelectric thin films such as lithium niobate, lithium tantalate, barium titanate, and lead zirconate titanate, providing outstanding platforms for on-chip high-efficiency and broadband electro-optic modulators. After decades of development, lithium niobate-based electro-optic modulators with large bandwidth and high data rates have matured and been commercialized, with various modulator configurations thoroughly explored. Lithium tantalate exhibits similar material properties, and over the past decade, benefiting from its established use in commercial surface acoustic wave filters and the application of smart-cut technology, it has achieved performance comparable to thin-film lithium niobate modulators while gaining a cost advantage. Compared to lithium niobate and lithium tantalate, barium titanate and lead zirconate titanate possess intrinsic electro-optic coefficients that provide a natural advantage in electro-optic modulation. However, although barium titanate has a relatively regular domain structure, its high-frequency domain dynamics and associated losses limit its high-speed modulation performance, resulting in constrained modulation bandwidth. Thus, a fundamental understanding of its domain structure is essential for optimizing and improving modulation performance. Lead zirconate titanate exhibits a more complex domain structure, yet its properties can be macroscopically tailored by adjusting the Zr/Ti ratio. Due to this complexity, its macroscopic polarization responds more rapidly to applied electric fields, yielding better high-speed performance compared to barium titanate. For both materials, achieving high-performance modulators first requires the deposition of high-quality buffer layers to ensure good film orientation. Although the development stages of electro-optic modulators based on these emerging thin-film materials vary, their excellent electro-optic coefficients indicate growing application potential. The current developmental status of high-speed electro-optic modulators employing different materials is summarized in Tab.2 through 5.Conclusions and ProspectsThe aforementioned four ferroelectric materials are currently being investigated for high-speed electro-optic modulators owing to their large Pockels coefficients. However, in functionally diverse integrated systems, different materials may be utilized for fabricating various functional devices based on their distinct electrical or nonlinear optical properties. Correspondingly, their electro-optic modulators may be integrated into different functional units depending on the application context. This study aims to provide reference and inspiration for the future design and application directions of high-speed electro-optic modulators based on different material systems.