Xiao and Vollmer Light: Science & Applications (2021) 10:141 Official journal of the CIOMP 2047-7538 https://doi.org/10.1038/s41377-021-00583-w www.nature.com/lsa EDITORIAL Open Access Special Issue on the 60th anniversary of the first laser—Series I: Microcavity Photonics—from fundamentals to applications ✉ Yun-Feng Xiao 1,2 and Frank Vollmer 3 Optical microcavities confine light to small volumes by This special issue covers a series of cutting-edge works resonant recirculation. Because of their ultrahigh quality on advanced physics and applications of optical micro- factors (Q) and small mode volumes (Vm), optical cavities and microlasers, ranging from the study of chaotic microcavities have attracted strong research interests for resonances, microcombs and soliton physics, lasers with their unique property of significantly enhancing tailored orbital angular momentum, coherent light-matter light–matter interaction.1 In recent decades, optical coupling and quantum condensation, optical non- microcavities have become cornerstones for a wide range reciprocity, to multiplexed biochemical sensing. In what of studies and applications, including nonlinear photo- follows, a brief introduction to each topic will be pre- – nics2 4, non-Hermitian5,6 and chaotic physics7,8, cavity sented along with their key implications highlighted. quantum electrodynamics9,10 and cavity optomechanics11, Observation and understanding of the cavity mode optical sensing12,13, and in particular, microlasers14,15. structure and dynamics is the foundation of its engi- Optical cavities serve as an essential building block of neering and applications. While numerous experimental lasers from the date of their birth by enhancing techniques have been established to understand the – 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; photon material interactions and providing optical feed- microcavity internal wave dynamics based on the input- backs. In the past decades, along with the development of output characteristics, direct inspection into the black box various nanofabrication and packaging techniques, is always highly desired yet challenging, especially microcavities with ultrahigh quality factors and minia- regarding the complex chaotic electromagnetic fields of tured sizes have enabled low-threshold and high- asymmetric microcavities. Writing in this special issue, coherence lasers down to chip scale. Meanwhile, laser Wang, S. et al. demonstrate a simple but robust approach physics and applications have been greatly advanced by to directly and rapidly map the internal mode patterns in exploiting the spectral, temporal, and spatial degrees of chaotic microcavities16. With this technique, chaos- freedom of microcavities and their rich interactions with assisted tunneling and its time-reversed process are various materials. Nowadays, microcavity-based lasers experimentally observed in the optical domain with provide not only indispensable tools in applied researches unprecedented certainty. This research offers a new such as biochemical sensing/imaging and high precision pathway in the understanding of the ultrafast and delicate spectroscopy, but also practical devices including atmo- physical processes in optical microcavities. spheric monitors or smart phone and computer chips that The interplays between optical nonlinearity and dis- have the potential for entering diverse aspects of people’s persion, loss and gain give rise to an important branch in daily life. microcavity photonics, i.e., the optical frequency combs, and especially, the dissipative Kerr solitons (DKSs). DKSs are on-chip coherent light sources with equidistant fre- Correspondence: Yun-Feng Xiao ([email protected]) quency lines spanning a wide spectral range, which find 1State Key Laboratory for Mesoscopic Physics and Frontiers Science Center for widespread applications in microwave source generation, Nano-optoelectronics, School of Physics, Peking University, Beijing, China optical spectroscopy, and precision measurements. This 2Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, China special issue brings up a collection of original research Full list of author information is available at the end of the article © The Author(s) 2021 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 linktotheCreativeCommons 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/. Xiao and Vollmer Light: Science & Applications (2021) 10:141 Page 2 of 3 works in the field of soliton fundamentals and applica- of exciton-polariton orbital states with symmetric petal- tions. Wang, H. et al. demonstrated the peculiar char- shaped patterns in real space, resulting from symmetry acteristics of Dirac solitons in microresonators, a new type breaking due to the anisotropic effective potential of the of soliton in microresonators resulting from broadband birefringent perovskite crystals. This work thus demon- modal coupling, by solving the corresponding con- strates the feasibility of precisely control the OAM of light servative coupled Lugiato-Lefever equation (LLE) non- by manipulating both the light and matter degrees of free- perturbatively and using the exact solution as a soliton dom in a strongly coupled system. ansatz for the hybrid system17. The properties of Dirac Optical nonreciprocity is critical for optical commu- optical solitons in microresonators are important at a nication and information processing. Optical non- fundamental level and provide a road map for soliton reciprocal systems enforce one-way transmission of the microcomb generation in the visible band. Application- light signals, which requires the breaking of the Lorentz wise, Wang, B. et al. established a new high-power, high reciprocity theorem. To date, the realization of optical coherence photonic millimeter-Wave (mmWave) plat- nonreciprocity generally requires time-reversal symmetry form through the combination of integrated micro- breaking via a magnetic field or magnetic order, or non- resonator solitons and high-speed photodiodes18. linear optical effects, which requires either complex fab- Importantly, the power level with microresonator solitons rications, stringent experimental conditions, or high approaches the theoretical limit of heterodyne detection, power consumptions. In this special issue, Huang et al.22 which assumes an ideal photodiode with zero power roll- took a step towards solving these issues, by proposing an off in its frequency response, providing a viable path to magnetic-field-free nonreciprocal system in the linear chip-scale, high-power, low-noise, and high-frequency optical regime. The authors took advantages of energy sources for mmWave applications. Qin et al. reported the loss, which is usually regarded as harmful, to generate realization of electrically controllable laser frequency optical nonreciprocity. The energy loss introduces a phase combs in graphene-fiber microresonators with unprece- lag in a resonance mode, which, in an optical resonator dented dynamic tunability19. Such realization of the network, gives rise to different interference conditions in dynamic control and stabilization of the microcomb in a the forward and backward propagation directions, and heterogeneous graphene-fiber microcavity provides a new hence leading to nonreciprocal transmission. This work platform for the interfacing of single-atomic-layer provides a general approach that may open new possibi- optoelectronics and ultrafast photonics, and will pro- lities in designing nonreciprocal devices. mote versatile applications for arbitrary waveform gen- Optical microcavities manifest themselves by virtue of eration, fiber communication, signal processing, and the strongly enhanced light–matter interaction, and thus spectroscopic metrology. have witnessed tremendous progresses in ultrasensitive Orbital angular momentum (OAM) is another important detection of nanoparticles, biomolecules, and physical degree of freedom of light that has drawn a great deal of quantities (temperature, force, pressure, electric and interests recently. Besides their novel interaction physics magnetic fields) over the past decades. However, most with matters, OAM-carrying lights can in principle support existing microsensors were enabled by tracking a single infinite orthogonal optical modes that are feasible for high- optical mode, which limits its multiplexing capabilities. capacity optical communication and information proces- Nowadays, the field is developing towards more sophis- sing. Whispering gallery microcavities are a natural plat- ticated sensing functionalities such as specific identifica- form for generating and studying OAM light, with their tions of mixed particles, real-time monitoring of complex azimuthal mode number directly correlated to the OAM physical processes, and in-situ biochemical imaging, number.