Thin-film lithium niobate (TFLN) is rapidly emerging as a versatile photonics platform with outstanding electro-optics and nonlinear optics performance, leading to breakthroughs in electro-optic modulation, frequency conversion, Kerr frequency comb generation, transduction, quantum photonics, and more on. To unlock the full potential of the TFLN platform, propagation loss arises as a fundamental challenge. At the device level, enhancing light-matter interaction in various applications necessitates ultra-low propagation loss, as the ratio between coupling and loss determines the strength of the desired interaction. At the system level, the requirement of integrating numerous devices to form large-scale photonic circuits is ultimately limited by loss.

Extensive research has focused on minimizing propagation loss in TFLN waveguides -- effectively maximizing the intrinsic quality (Q) factor of TFLN resonators -- through advances in both resonator design and fabrication techniques. Despite these efforts, the highest Q factor achieved using dry etching remains at 12 million, significantly lower than the 442 million demonstrated on silicon nitride platforms or the theoretical TFLN limit of 163 million. Alternative approaches such as wet etching and chemical mechanical polishing (CMP) yield smoother surfaces, yet face limitations in etch anisotropy and feature resolution. As most high-performance TFLN devices still exhibit Q factors of only a few million, further advances in fabrication and design are needed to overcome this bottleneck.

To address this challenge, Xinrui Zhu, a PhD student, Dr. Yaowen Hu, a Postdoc scientist (current assistant professor in Peking University, China), and Dr. Marko Lončar, the research group leader from Laboratory for Nanoscale Optics at John A. Paulson School of Engineering and Applied Sciences (SEAS), Harvard University, studied the loss mechanism and demonstrated a monolithic TFLN racetrack resonator with a record-high intrinsic Q factor of 29 million, which corresponds to a propagation loss less than 1.3 dB per meter.

The relevant research results are published in Photonics Research, Volume. 12, Issue 8, 2024

(Xinrui Zhu, Yaowen Hu, Shengyuan Lu, Hana K. Warner, Xudong Li, Yunxiang Song, Letícia Magalhães, Amirhassan Shams-Ansari, Andrea Cordaro, Neil Sinclair, and Marko Lončar, Twenty-nine million intrinsic Q-factor monolithic microresonators on thin-film lithium niobate, Photonics Research, 2024, 12(8), pp. A63-A68).

The microresonators adopt a racetrack geometry with ultra-wide waveguides. As shown in Fig. 1(a), Lumerical simulations reveal the fundamental mode supported by a 3 μm-wide racetrack. The ultra-wide design reduces the mode overlap with the sidewalls, thereby minimizing scattering loss and lowering the overall propagation loss. Fig. 1(b) shows the microscope image of a typical racetrack resonator featuring a 3 μm-wide waveguide and a 500 μm-long straight section, and Fig. 1(c) is the SEM image of a coupling region with 0.5 μm gap size. The optimized fabrication process -- including electron-beam lithography, dry etching, chemical cleaning, and thermal annealing -- has demonstrated excellent stability and repeatability in consistently yielding ultra-high-Q devices.

Fig. 1 (a) Lumerical eigenmode simulation representing the fundamental TE mode at the cross-section of 3 μm width ring racetrack resonator. (b) Optical microscope image showing a racetrack resonator with 3 μm width and 500 μm length straight section. (c) SEM image (false-colored) offering a detailed view of the coupling region of a racetrack with 0.5 μm coupling gap and 3 μm width. (d) Selected resonator spectrum spanning from wavelengths 1573.78 nm to 1574.06 nm. The corresponding racetrack features a width of 4.5 μm, length of 10 mm, coupling gap of 0.6 μm, and bending radius of 200 μm.(e) The highest-Q resonance features an intrinsic Q factor of 29 million at the wavelength of 1574 nm. (f) Average intrinsic Q of racetracks with a width of 3 μm and lengths of 1 mm, 2.5 mm, 5 mm, 7.5 mm, and 10 mm. (g) Average intrinsic Q of racetracks with a length of 10 mm and widths of 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, and 5.0 μm.

The microresonators were characterized using a tunable laser and a low-noise photodetector. As shown in Fig. 1(d), the transmission spectrum reveals multiple resonances exhibiting ultra-high Q factors. As shown in Fig. 1(e), the record-high intrinsic Q factor reaches 29.32 million, corresponding to a propagation loss less than 1.3 dB/m. To understand the impact of device geometry on intrinsic Q, authors statistically compare devices with varying waveguide lengths and widths, as summarized in Fig. 1(f, g). For devices with a fixed waveguide width of 3 μm, increasing the racetrack length from 1 mm to 10 mm results in higher average intrinsic Q, attributed to the increased proportion of straight waveguide in each round-trip. Additionally, for devices with a constant racetrack length of 10 mm, increasing the waveguide width from 3 μm to 5 μm leads to higher intrinsic Q, due to the reduced mode overlap with sidewall roughness.

Moving forward, the enhanced TFLN high-Q resonators could substantially improve the device performances, facilitating applications in microwave photonics, quantum computing, and nonlinear optics. This work further pushes the state-of-the-art, showcasing the potential of the TFLN platform and paving the way for future innovative explorations in integrated photonics.

Author Biography:

Marko Lončar

Tiantsai Lin Professor of Electrical Engineering

Harvard University, United States

Marko Lončar is Tiantsai Lin Professor of Electrical Engineering at Harvard's John A Paulson School of Engineering and Applied Sciences (SEAS). Lončar received his Diploma from University of Belgrade (R. Serbia) in 1997, and PhD from Caltech in 2003 (with Axel Scherer), both in Electrical Engineering. After completing his postdoctoral studies at Harvard (with Federico Capasso), he joined Harvard faculty in 2006. Lončar is expert in nanophotonics and nanofabrication, and his group has done pioneering work in the field of quantum and nonlinear nanophotonics. In particular, Lončar is recognized for his work on the development of diamond and thin film lithium niobate nanophotonic platforms. Lončar has co-authored more than 250 manuscripts in top scientific journals and has given more than 300 invited talks and seminars. He has received NSF CAREER Award in 2009, Sloan Fellowship in 2010, Marko Jarić Foundation Award in 2020, and Microoptics Conference Award in 2023. In recognition of his teaching activities, Lončar has been awarded Harvard University Levenson Prize for Excellence in Undergraduate Teaching (2012), and has been named Harvard College Professor (2017 – 2022). Lončar is Fellow of Optical Society of America and IEEE. He is co-founder of HyperLight Corporation (Cambridge, MA), VC backed startup commercializing lithium-niobate technology.

Xinrui Zhu

Xinrui (Anna) Zhu received her Bachelor of Arts degree in Physics and Astronomy from Mount Holyoke College in 2021. She is currently a Ph.D. candidate in Electrical Engineering at the John A. Paulson School of Engineering and Applied Sciences (SEAS), Harvard University, where she also earned a Master of Science degree in 2025. She conducts her research under the supervision of Professor Marko Lončar, focusing on the development of ultra-high-Q thin-film lithium niobate resonators. Her broader research interests include integrated photonics, optical communication, and quantum information processing.

Yaowen Hu

Assistant Professor, School of Physics,

Peking University, China

Yaowen is an assistant professor at the School of Physics, Peking University. He received his Bachelor of Science in Physics at Tsinghua University in 2018 and PhD in Physics at Harvard University in 2023. After graduating, he was the postdoctoral fellow at Harvard University School of Engineering and Applied Science during 2023-2024. He joined Peking University in 2024. His research is focused on nanoscale photonics and electro-optics, including nonlinear optics and frequency combs, photonic computing and communications, quantum optics, non-Hermitian and topological photonics, etc. He has been awarded the Peking University Weiming young scholar, Peking University Boya young scholar, MIT Technology Review 35 innovators under 35 China.