image: Fig. 1 The waveguide loss evaluation for polymer-loaded waveguide. (a) The schematic of the polymer-loaded waveguide. (b) The waveguide loss of different optical modes vs. waveguide width w at 1550 nm. (c)The electric field (|Ey|) profiles for TM00 and TM01 modes. (d) The electric field (|Ez|) profiles for TE00 and TE01 modes.
Credit: OEA
A new publication from Opto-Electronic Sciences; DOI 10.29026/oea.2025.240114, discusses an ultra-high-Q photonic crystal nanobeam cavity for etchless lithium niobate on insulator (LNOI) platform.
In the field of integrated photonics, enhancing light-matter interactions is one of the most effective approaches for achieving efficient photon manipulation. Photonic crystal nanobeam cavities (PCNBC) play a crucial role in this process of development due to their high quality factors (Q) and small mode volumes (V), enabling significant confinement of photon energy at sub-wavelength scales, thus markedly enhancing light-matter interactions. When combined with the next-generation photonic material, LiNbO3 (LN), which features ultra-low absorption losses and superior electro-optic, acousto-optic, piezoelectric, and second-order nonlinear properties compared to traditional silicon, PCNBCs have the potential to provide photonic resonator devices with new functionalities and characteristics.
However, the realization of high-quality etching for LN has proven challenging across various etching processes, especially for fabricating sophisticated structures such as PCNBCs. To avoid etching LN, a novel route has been proposed that involves spinning and patterning a low-refractive-index polymer (~1.5) on top of the thin-film lithium niobate on insulator (LNOI) substrate. This approach simplifies the fabrication process and reduces scattering loss associated with the imperfect etching process. On this platform, various intriguing physical phenomena and high-performance key optical components have been demonstrated by creating polymer-loaded waveguides. Nevertheless, most devices primarily operate in transverse magnetic (TM)-polarized modes produced by photonic bound states in the continuum (BIC). Moreover, TM mode exhibits a poor overlap with the LN layer, which hinders the potential of the outstanding material properties of LN and results in poor energy efficiency, particularly for devices used as a modulators and nonlinear devices. Therefore, the PCNBC, operating in an optical mode that is robust to waveguide width and has a larger overlap with LN, is desired for efficient photon manipulation.
In order to address these above-mentioned challenges, the authors of this article achieved a significant milestone in design and fabrication of ultra-high Q-PCNBC, as well as in efficient photon manipulation based on the LNOI platform. This work based on a y-cut LNOI substrate, as shown in Fig. 1(a). On this platform, polymer-loaded rib-waveguide is capable of accommodating both transverse electric (TE) and TM modes. The relationship between the propagation loss and waveguide width for both TM and TE modes is illustrated in Fig. 1(b). The result shows that TE modes are robust to waveguide width. In comparison, most of the optical field of TE modes are confined within the LN layer (60% for TE00 and 38% for TM00), showing Fig. 1(c) and (d). Therefore, TE modes address all the challenges associated with BIC and enhance the modulation capability with an external field, particularly for the TE00 mode, which is crucial for high-efficient photon manipulation.
Based on the discussion of optical losses and waveguide modes above, the researchers designed a TE00-polarized PCNBC, with the schematic of the proposed PCNBC shown in Fig. 2(a). It consists of a series of periodically arranged polymer dielectric blocks that defined by different filling factors. According to the filling factor, the PCNBC can be divided into taper region and mirror region. Fig. 2(b) shows the measured optical transmission of a PCNBC, including the fundamental mode, second-order mode and other high-order modes. The fundamental mode exhibits a Q factor of 0.47×105 and a high transmission of 52%. Fig. 2(c) illustrates the relationship between the Q factor of the fundamental mode and the waveguide width. Combining the propagation loss of the TE00 mode in Fig. 1(b), this result confirms that the designed PCNBC operates in the TE00 mode. In addition, the researchers also investigated the influence on the Q factor and transmission by varying the number of polymer dielectric blocks in both the taper region and mirror region. The results are shown in Fig. 2(d) and (e). Among them, there is the highest Q factor of 1.87×105, which exceed the most similar cavities on LNOI substrate. More importantly, it is over one order of magnitude higher than any previously reported PCNBC operating in BIC. All these results are crucial for the further development of PCNBCs as high-performance photon manipulation devices.
Keywords: photonic crystal nanobeam cavity / LNOI platform / high Q / thermo-optic tuning / bistability / Fano line shapes
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Danyang Yao is an Associate Professor at Xidian University and a Huashan Scholar. His research primarily focuses on integrated microcavity optoelectronics and microwave photonic modulators. He has published over 30 papers in well-known optoelectronics journals such as APL, OL, OE, and IEEE PTL. He holds 14 authorized Chinese patents and one U.S. patent. Professor Yao has led a National Natural Science Foundation of China (NSFC) Youth Project, an industry-sponsored project, and two projects funded by the Fundamental Research Funds for the Central Universities. Additionally, he has participated in several NSFC general and key projects, as well as national key R&D projects.
Personal homepage: https://faculty.xidian.edu.cn/YAODANYANG/zh_CN/index.htm.
Xuetao Gan is a Professor at Northwestern Polytechnical University, specializing in nanophotonics and integrated optoelectronic devices. He has published over 100 papers as the first or corresponding author in internationally renowned journals such as Nature Photonics, Science Advances, and Nature Communications. His research achievements have been positively covered more than 30 times by journals like Nature and various scientific media outlets. Professor Gan has led projects including the National Key R&D Program, the National Science Fund for Excellent Young Scholars, the Major Research Plan for New Physics and Applications of Optical Field Regulation, as well as General and Youth Projects from the National Natural Science Foundation of China. He was recognized as an Elsevier Highly Cited Chinese Researcher for 2021 and 2022.
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Jiang Z, Fang CZ, Ran X et al. Ultra-high-Q photonic crystal nanobeam cavity for etchless lithium niobate on insulator (LNOI) platform. Opto-Electron Adv 8, 240114 (2025). doi: 10.29026/oea.2025.240114