Moulding light on a ring

MEETING REPORT https://doi.org/10.1038/s42005-021-00660-x OPEN Moulding light on a ring 1234567890():,; Drawing around 60 attendees and 20 presenters to a virtual lecture room, April’s CHI-2 Photonics in Microresonators and Beyond conference explored recent progress in the use of microresonators and integrated photonic devices exhibiting second-order nonlinearity for optical frequency conversion. The capability of photonic integrated circuits to convert incoming photons to new colours is a critical tool in modern-day information processing and precision spectroscopy. For light, microresonators act as racetracks with the photons looping around. A long path length and small footprint are the key advantages of on-chip frequency conversion. The material property enabling frequency conversion is called nonlinear susceptibility. The second-order, or chi-2, susceptibility is one of the best-known enablers of frequency conversion. It naturally doubles or halves the light frequency — a pretty large spectral leap on any practical account. Despite this, the majority of recent scientific and technological breakthroughs in microresonator-based frequency conversion have utilised the third-order, chi-3, nonlinear susceptibility. This is due to the challenges in making chi-2-based devices, such as the need to match both the phase and group velocities of photons across a broad spectral range. Over the past few years, frequency conversion in chi-2 microresonators has gradually come out from beneath the shadow of chi-3 resonators. There are very good reasons for this. Primarily, dramatic improvements in fabrication capabilities using chi-2 materials have increased the options for device architecture. This in turn allowed reduced power requirements and the softening of numerous other constraints thanks to new resonator designs, material choices, and pumping arrangements. The event opened with lectures describing recent work on non-monolithic resonators, aimed at generating relatively narrow frequency combs1. Frequency combs contain a regular and equally-spaced pattern of spectral lines, similar to the teeth on a comb. The regularity of these teeth allow them to be used for precision spectroscopy and other applications. Because the comb-teeth separation increases as the resonator radius decreases, the wavelength coverage of the generated spectra can be controlled. Credit: D Puzyrev COMMUNICATIONS PHYSICS | (2021)4:158 | https://doi.org/10.1038/s42005-021-00660-x | www.nature.com/commsphys 1 MEETING REPORT COMMUNICATIONS PHYSICS | https://doi.org/10.1038/s42005-021-00660-x Most of the following lectures considered smaller monolithic resonators, either fabricated mechanically from bulk crystals or lithographically from thin films. Many groups chose to work with lithium niobate as the microresonator material, as it combines one of the strongest chi-2 responses with good compatibility with established fabrication procedures, resulting in low-loss microresonators2. Low losses are associated with high quality factors, which is the main parameter boosting the efficiency of frequency conversion. One impressive result, announced at the meeting by Prof. Ya Cheng from the Chinese Academy of Sciences, was the realisation of on-chip lithium-niobate resonators with quality factors of 1083. Silicon4 and aluminium5 nitrides are less common choices for chi-2 platforms but are now generating growing interest, in part due to the existing use of these materials in electronic devices. Finally, Prof. Christoph Marquardt from the Max Planck Institute captured the attention of the audience with proposals of how to use chi-2 microresonators to generate quantum states of light6 for secure communication on with satellite platforms. The convergence of advanced fabrication methods and fundamental concepts leave no doubt about the continued growth of this research area, and demonstrate that its success is strongly dependent on international collaborations. A video record of the lectures is available online. Dmitry V. Skryabin 1 ✉ & Ingo Breunig2 ✉ 1 Department of Physics, University of Bath, Bath, UK. 2 Department of Microsystems Engineering—IMTEK, University of Freiburg, Freiburg, Germany. ✉email: ; References 1. 2. 3. 4. 5. 6. Ricciardi, I. et al. Optical Frequency Combs in Quadratically Nonlinear Resonators. Micromachines 11, 230 (2020). Zhang, M. et al. Monolithic ultra-high-Q lithium niobate microring resonator. Optica 4, 1536 (2017). Gao, R. et al. Broadband highly efficient nonlinear optical processes in on-chip integrated lithium niobate microdisk resonators of Q-factor above 10^8. https://arxiv.org/abs/2102.00399 (2021). Lu, X. et al. Efficient photoinduced second-harmonic generation in silicon nitride photonics. Nat. Photon. 15, 131 (2021). Bruch, A. W. et al. Pockels soliton microcomb. Nat. Photon. 15, 21 (2021). Otterpohl, A. et al. Squeezed vacuum states from a whispering gallery mode resonator. Optica 6, 1375 (2019). Acknowledgements The event was supported by the EU Horizon-2020, project - 812818, MICROCOMB. Competing interests The authors declare no competing interests. Additional information Correspondence and requests for materials should be addressed to D.V.S. or I.B. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/ licenses/by/4.0/. © The Author(s) 2021 2 COMMUNICATIONS PHYSICS | (2021)4:158 | https://doi.org/10.1038/s42005-021-00660-x | www.nature.com/commsphys  (...truncated)