Lithium tantalate for low-cost, high-efficiency PICs
Scientists have developed a platform for photonic circuits based on lithium tantalate, which they say has excellent electro-optic properties similar to those of lithium niobate, but can be more easily and cheaply produced at scale
Researchers have reported the development of a new platform for PICs based on lithium tantalate, which they say can transform the field by making high-quality PICs more economically viable. Their results have been published in Nature.
For decades, silicon-based PICs have dominated the field due to their cost-effectiveness and through their integration with existing semiconductor manufacturing technologies, despite their limitations with regard to their electro-optical modulation bandwidth. Nevertheless, silicon-on-insulator optical transceiver chips have been successfully commercialised, driving information traffic through millions of glass fibres in modern datacentres.
Recently, the lithium niobate-on-insulator wafer platform has emerged as a superior material for photonic integrated electro-optical modulators, due to its strong Pockels coefficient, which is essential for high-speed optical modulation. Nonetheless, high costs and complex production requirements have kept lithium niobate from being adopted more widely, limiting its commercial integration.
According to the authors of the new paper, lithium tantalate (LiTaO3), a close relative of lithium niobate, promises to overcome these barriers. It features similar excellent electro-optic qualities but has an advantage over lithium niobate in scalability and cost, as it is already being widely used in 5G radiofrequency filters by telecom industries.
Led by Tobias Kippenberg, a professor at École Polytechnique Fédérale de Lausanne (EPFL), and Xin Ou, a professor at the Shanghai Institute of Microsystem and Information Technology (SIMIT), the scientists have begun to pursue the material’s potential by creating a new PIC platform based on lithium tantalate.
They developed a wafer-bonding method for lithium tantalate, which is compatible with silicon-on-insulator production lines. They then masked the thin-film lithium tantalate wafer with diamond-like carbon and proceeded to etch optical waveguides, modulators, and ultra-high quality factor microresonators.
The etching was achieved by combining deep ultraviolet (DUV) photolithography and dry-etching techniques, developed initially for lithium niobate and then carefully adapted to etch the harder and more inert lithium tantalate. This adaptation involved optimising the etch parameters to minimise optical losses, a crucial factor in achieving high performance in photonic circuits.
With this approach, the researchers report that they were able to fabricate high-efficiency lithium tantalate PICs with an optical loss rate of just 5.6 dB/m at telecom wavelength. Another highlight is the electro-optic Mach-Zehnder modulator (MZM), a device widely used in today’s high-speed optical fibre communication. The team said that the lithium tantalate MZM offers a half-wave voltage-length product of 1.9 V cm and an electro-optical bandwidth reaching 40 GHz.
“While maintaining highly efficient electro-optical performance, we also generated soliton microcomb on this platform,” says Chengli Wang, the study’s first author. “These soliton microcombs feature a large number of coherent frequencies and, when combined with electro-optic modulation capabilities, are particularly suitable for applications such as parallel coherent LiDAR and photonic computing.”
According to the researchers, the lithium tantalate PIC has reduced birefringence (the dependence of refractive index on light polarisation and propagation direction), allowing dense circuit configurations and ensuring broad operational capabilities across all telecommunication bands. They say that the development paves the way for scalable, cost-effective manufacturing of advanced electro-optical PICs.
Image credit: Tobias Kippenberg (EPFL)
