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5.15.24
Nik Papageorgiou
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Lithium tantalate photonic integrated circuits. Credit: Nikolai Kuznetcov (EPFL)
Researchers at EPFL have developed scalable photonic integrated circuits, based on lithium tantalate, marking a significant advancement in optical technologies with potential to widespread commercial applications.
The rapid advancement in photonic integrated circuits (PICs), whichcombine multiple optical devices and functionalities on a single chip, has revolutionized optical communications and computing systems.
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 were successfully commercialized, driving information traffic through millions of glass fibers in modern datacenters.
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.
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.
Now, scientists led by Professor Tobias J. Kippenberg at EPFL and Professor Xin Ou at the Shanghai Institute of Microsystem and Information Technology (SIMIT) have created a new PIC platform based on lithium tantalate. The PIC leverages the material’s inherent advantages and can transform the field by making high-quality PICs more economically viable. The breakthrough is published in Nature.
Fig. 1: LTOI substrates and optical waveguides.
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a, Schematic of the LTOI wafer-bonding workflow showing hydrogen-ion implantation, bonding, splitting and chemical mechanical polishing (CMP). b, Photograph of the bonded wafer demonstrating uniform and defect-free bonding. c, Thickness map of the LiTaO3 thin film on the wafer. The x,y axes represent the distance from the wafer centre. d, Atomic force micrograph of the LiTaO3 thin film surface. Scale bar, 500 nm. e, High-resolution scanning transmission electron-microscopy image of the LiTaO3–SiO2 bonding interface. The arrow represents the x-cut crystal orientation. Scale bar, 2 nm. f, Schematic of the fabrication workflow for LTOI optical waveguides, including DLC hard-mask deposition by plasma-enhanced chemical vapour deposition (PECVD) from the methane precursor, DLC dry etching through oxygen plasma, and LiTaO3 etching by argon ion-beam etching (IBE), followed by redeposition and mask removal. The layers are DLC (black), LiTaO3 (light blue), SiO2 (purple) and Si (grey). Spheres show C (black), O (red) and Ar+ (green). g, Colourized scanning electron micrograph (SEM) of LTOI microring resonator (blue). Scale bar, 50 μm. h, Colourized SEM of etched LTOI microring and bus waveguide coupling section. Scale bar, 2 μm. i, Colourized SEM of etched LTOI waveguide and sidewall. Scale bar, 2 μm. j, Colourized SEM cross-section of etched LTOI waveguide (blue) on top of SiO2 bottom cladding (purple). Scale bar, 500 nm.
See the science paper for further instructive material with images.
The researchers 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 optimizing the etch parameters to minimize optical losses, a crucial factor in achieving high performance in photonic circuits.
With this approach, the team was 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 fiber communication. 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.”
The lithium tantalate PIC’s reduced birefringence (the dependence of refractive index on light polarization and propagation direction) allows dense circuit configurations and ensures broad operational capabilities across all telecommunication bands. The work paves the way for scalable, cost-effective manufacturing of advanced electro-optical PICs.
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The Swiss Federal Institute of Technology in Lausanne [EPFL-École Polytechnique Fédérale de Lausanne] (CH) is a research institute and university in Lausanne, Switzerland, that specializes in natural sciences and engineering. It is one of the two Swiss Federal Institutes of Technology, and it has three main missions: education, research and technology transfer.
The QS World University Rankings ranks EPFL(CH) very high, whereas Times Higher Education World University Rankings ranks EPFL(CH) as one of the world’s best schools for Engineering and Technology.
EPFL(CH) is located in the French-speaking part of Switzerland; the sister institution in the German-speaking part of Switzerland is The Swiss Federal Institute of Technology ETH Zürich [Eidgenössische Technische Hochschule Zürich] (CH). Associated with several specialized research institutes, the two universities form The Domain of the Swiss Federal Institutes of Technology (ETH Domain) [ETH-Bereich; Domaine des Écoles Polytechniques Fédérales] (CH) which is directly dependent on the Federal Department of Economic Affairs, Education and Research. In connection with research and teaching activities, EPFL(CH) operates a nuclear reactor CROCUS; a Tokamak Fusion reactor; a Blue Gene/Q Supercomputer; and P3 bio-hazard facilities.
ETH Zürich, EPFL (Swiss Federal Institute of Technology in Lausanne) [École Polytechnique Fédérale de Lausanne](CH), and four associated research institutes form The Domain of the Swiss Federal Institutes of Technology (ETH Domain) [ETH-Bereich; Domaine des Écoles polytechniques fédérales] (CH) with the aim of collaborating on scientific projects.
The roots of modern-day EPFL(CH) can be traced back to the foundation of a private school under the name École Spéciale de Lausanne in 1853 at the initiative of Lois Rivier, a graduate of the École Centrale Paris (FR) and John Gay the then professor and rector of the Académie de Lausanne. At its inception it had only 11 students and the offices were located at Rue du Valentin in Lausanne. In 1869, it became the technical department of the public Académie de Lausanne. When the Académie was reorganized and acquired the status of a university in 1890, the technical faculty changed its name to École d’Ingénieurs de l’Université de Lausanne. In 1946, it was renamed the École polytechnique de l’Université de Lausanne (EPUL). In 1969, the EPUL was separated from the rest of the University of Lausanne and became a federal institute under its current name. EPFL(CH), like ETH Zürich (CH), and it is thus directly controlled by the Swiss federal government. In contrast, all other universities in Switzerland are controlled by their respective cantonal governments. EPFL(CH) has started to develop into the field of life sciences. It absorbed the Swiss Institute for Experimental Cancer Research (ISREC) in 2008.
In 1946, there were 360 students. In 1969, EPFL(CH) had 1,400 students and 55 professors. In the past two decades the university has grown rapidly and over 14,000 people study or work on campus, about 10,000 of these being Bachelor, Master or PhD students. The environment at modern day EPFL(CH) is highly international with the school attracting students and researchers from all over the world. More than 125 countries are represented on the campus and the university has two official languages, French and English.
Organization
EPFL is organized into eight schools, themselves formed of institutes that group research units (laboratories or chairs) around common themes:
School of Basic Sciences
Institute of Mathematics
Institute of Chemical Sciences and Engineering
Institute of Physics
European Centre of Atomic and Molecular Computations
Bernoulli Center
Biomedical Imaging Research Center
Interdisciplinary Center for Electron Microscopy
MPG-EPFL Centre for Molecular Nanosciences and Technology
Swiss Plasma Center
Laboratory of Astrophysics
School of Engineering
Institute of Electrical Engineering
Institute of Mechanical Engineering
Institute of Materials
Institute of Microengineering
Institute of Bioengineering
School of Architecture, Civil and Environmental Engineering
Institute of Architecture
Civil Engineering Institute
Institute of Urban and Regional Sciences
Environmental Engineering Institute
School of Computer and Communication Sciences
Algorithms & Theoretical Computer Science
Artificial Intelligence & Machine Learning
Computational Biology
Computer Architecture & Integrated Systems
Data Management & Information Retrieval
Graphics & Vision
Human-Computer Interaction
Information & Communication Theory
Networking
Programming Languages & Formal Methods
Security & Cryptography
Signal & Image Processing
Systems
School of Life Sciences
Bachelor-Master Teaching Section in Life Sciences and Technologies
Brain Mind Institute
Institute of Bioengineering
Swiss Institute for Experimental Cancer Research
Global Health Institute
Ten Technology Platforms & Core Facilities (PTECH)
Center for Phenogenomics
NCCR Synaptic Bases of Mental Diseases
College of Management of Technology
Swiss Finance Institute at EPFL
Section of Management of Technology and Entrepreneurship
Institute of Technology and Public Policy
Institute of Management of Technology and Entrepreneurship
Section of Financial Engineering
College of Humanities
Human and social sciences teaching program
EPFL Middle East
Section of Energy Management and Sustainability
In addition to the eight schools there are seven closely related institutions
Swiss Cancer Centre
Center for Biomedical Imaging (CIBM)
Centre for Advanced Modelling Science (CADMOS)
École Cantonale d’art de Lausanne (ECAL)
Campus Biotech
Wyss Center for Bio- and Neuro-engineering
Swiss National Supercomputing Centre