The Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, School of Physics and TEDA Applied Physics Institute, Nankai University, Tianjin 300071, People's Republic of China

*Email: jjxu@nankai.edu.cn

Nonlinear optics is a branch of optics that describes the behaviors of light in media, in which polarization density P responds nonlinearly to the electric field E of the light as . The quantity is responsible for the linear optical response and is the permittivity of free space. The and are knowns as the second- and third-order nonlinear optical (NLO) susceptibilities, respectively. The birth of the nonlinear optics. The birth of the nonlinear optics is often taken to be the discovery of second-harmonic generation (SHG) by Franken et al in 1961, shortly after construction of the first laser by Maiman in 1960. As a matter of fact, before the development of the lasers, some nonlinear effects had already been observed without realizing their nonlinear nature. Two examples are DC Kerr effect and Pockels effect discovered in the 19th century. In these effects, the refractive indices of media were found to vary as a result of the external electrostatic fields. The invention of the laser provides a new tool to generate strong optical frequency electric fields comparable to that inside atom (∼108 V m−1), making the nonlinear perturbation noticeable, thus ushering in a golden-rush era of nonlinear optics.

The nonlinear optics developed at an explosive rate in its first decade (1961–1969). During this period, most of the fundamental principles were established, and most of the nonlinear phenomena known today were observed. These include optical harmonics (1961), sum-and difference-frequency generation (DFG) (1962, 1963), multiphoton absorption (1961), stimulated Raman/Brillouin scattering (1962, 1964), self-focusing (1964), optical breakdown (1964), photon echo (1964), optical parametric amplification and oscillation (OPA and OPO) (1965), coherent anti-Stokes Raman spectroscopy (CARS, 1965), photorefractive effect (1966), electromagnetically induced transparency (1967), optical nutation (1966), and optical bistability (1969), etc.

During the 1970s and the following few decades, the establishment of new principles and the discovery of new effects became rare; nevertheless, the understanding of the known effects was dramatically improved. With the amazing advances in tunable and pulsed lasers, people started to exploit nonlinear optics at an astonishing rate in new materials, in new systems, in new configurations, and especially on challenging spatiotemporal scales. During this period, in addition to focusing on its own development, nonlinear optics has penetrated other related disciplines (e.g. solid state physics, plasma physics, integrated optics, acoustics, mechanics, chemistry, biology, etc.).

The golden-rush of nonlinear optics is said to have ended in the 1960s [1, 2]. However, in the recent decades, some things that are truly novel and hardly predicted in the 1960s have appeared. For example, the emergence of photonic crystals, and metamaterials represents an important trend towards artificial designer media with exotic nonlinear performance surpassing the natural materials. Two-dimensional materials, such as graphene, certainly provides a nonlinear framework with ultimate thin thickness, etc.

In this roadmap, we do not intend to outline the global picture of the nonlinear optics, but rather focus on the developments in China. Inspired by the landmark paper of 'Infrared and optical masers' by Schawlow and Townes in 1958, scientists from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) began to conceive of optical quantum amplifiers, and successfully built the China's first laser in 1961. In the following 15 years, different lasers were built [3], such as He–Ne laser (1963), Nd–glass laser (1963), GaAs semiconductor laser (1963), Ar+ laser (1965), CO2 laser (1965), dye laser (1974) and excimer laser (1977). The nonlinear effects that are related to improving the performance of high-power lasers were well studied, such as optical breakdown, self-focusing, optical filamentation, parasitic oscillation, etc. However, the researches on nonlinear frequency conversion and nonlinear spectroscopy were largely hindered by the lack of high-quality nonlinear crystals before 1980 in China [4].

In the 1980s, researches of nonlinear crystal flourished with fruitful results. Periodically poled lithium niobate (PPLN) crystals were successfully grown in Nanjing University and the quasi-phase matched SHG was demonstrated experimentally, which provided a new framework for harmonic generation. Nankai University, in cooperation with Norla Institute of Technical Physics, found the high-resistance of optical damage in Mg doped lithium niobate (LN), which opened a new horizon for applications of LN in high power laser and integrated optics. Scientists of the Fujian Institute of Research on the Structure of Matter synthesized barium metaborate (BBO) and lithium triborate (LBO) crystals, which enable the lasers with ultraviolet emission. Potassium titanyl phosphate (KTP) was successfully grown in Shandong University. In the meantime, Chinese researchers made great efforts to catch up with the international counterparts, and nearly all NLO phenomena that had been observed abroad were successfully reproduced in China in a short period of time. Until the middle of 1980s, Chinese researchers have achieved remarkable progresses in such as optical bistability, four-waves mixing, and stimulated Raman scattering (SRS), etc. By the turn of 21th century, nonlinear optics has become an important academic topic in China. A series of breakthroughs have been made in photorefractive effect and its applications in optical storage and computing, as well as the quasi-phase matching (QPM) frequency conversion and tunable lasers, etc. More progresses of nonlinear optics in China can be found in several review articles and books [5–9].

In this Roadmap, we highlight the status, current and future challenges, and emerging technologies in several research areas of nonlinear optics in China. This roadmap is divided into several topics, grouped thematically.

The first part of the roadmap covers the current progress of lasers, which is an essential tool to produce nonlinearities. In article 1, Pengfei Lan and Peixiang Lu describe the developments of attosecond lasers, which is an ideal tool to study the ultrafast nonlinear dynamics in matters. In article 2, Zhi-Yuan Li and Li-Hong Hong demonstrate an all-spectrum white laser, which has an extremely broadband spectrum like solar radiation but with coherent light. In article 3, Yulei Wang and Zhiwei Lv present an overview of the stimulated Brillouin scattering (SBS) effect and its application in manufacturing lasers with designer performance. The realization of quantum light sources based on nonlinear optics is discussed in article 4 by Zhiyuan Zhou and Baosen Shi.

The second part of the roadmap addresses the field of nonlinear materials. In article 5, Yong Zhang, Shining Zhu and Min Xiao discuss the state-of-the-art three-dimensional artificial microstructures, which is a solid step towards nonlinear optics in three-dimensional space. In article 6, Satoshi Aya and Yanqing Lu introduce emerging polar liquid crystals (LCs) with tunable polarization structures, which are the first class of second-order nonlinear materials in the liquid form. In article 7, Huixin Fan, Min Luo and Ning Ye describe the design of the nonlinear materials for SHG in ultraviolet and deep ultraviolet (DUV). Zeyuan Sun, Weitao Liu and Shiwei Wu present in article 8 the atomically thin two-dimensional materials, which provide a unique opportunity to study various NLO phenomena. Furthermore, this part of the roadmap ends by Qingyun Li and Hui Hu with an introduction to the current status and perspectives of the lithium niobate thin films (LNTFs), which is a promising platform for the next generation integrated photonics.

The third part of the roadmap deals with new nonlinear effects and behaviors. In article 10, Yuanlin Zheng and Xianfeng Chen show the classical and quantum nonlinear frequency conversion by LNTF, which is essential to build all-optical functional chips. In article 11, Xiaoyong Hu surveys the research on ultrafast and giant third-order nonlinearity, which is essential to achieve high-speed and low energy consumption all-optical processing system. Chuanshan Tian describes in article 12 the surface-specific NLO spectroscopy and its implementation in probing the microscopic structure and dynamics at surfaces and interfaces. In article 13, Zixian Hu and Guixin Li review the spin–orbit interaction in the NLO processes, which has been proven as an efficient method to generate and control the angular momentum of the harmonic waves. In the last article of this part, Yi Hu and Jingjun Xu introduce some counterintuitive phenomena by nonlinear light interactions, which manifest synchronized acceleration of optical beams breaking the action-reaction symmetry.

The final part of the roadmap discusses the applications of the nonlinear optics. Kun Huang and Heping Zeng discuss in article 15 how parametric upconversion imaging acts as a promising strategy for mid-infrared (MIR) imaging, where infrared photons are detected by high-performance visible detector. In article 16, Zhenze Li and Hongbo Sun present the status of ultrafast laser nonlinear manufacturing, which represents a promising method to construct next-generation integrated photonic system. In article 17, Lei Dong introduces the application of photoacoustic (PA) effect in sensitive gas spectroscopic sensing. The whole roadmap concludes by Runfeng Li, Wenkai Yang and Kebin Shi with an important application of NLO microscopy for bio-imaging, which is an unparalleled tool for observing biological dynamics in-vivo with high spatiotemporal resolution and biomedical specificities.

Nonlinear optics is a flourishing field, which has grown far beyond what we expected at its birth. It not only provides us with new understanding or essence to the light-matter interactions, but also new technique to harness light. We shall not limit the nonlinear optics as an academic subject, but also recognize its power in driving economic development. Its advances have fostered innovations across a broad spectrum of applications in a diverse array of economic sectors. For example, the electro-optical effect, which enables information routing at a hyper-speed in fiber networks, makes the Internet economy thrive. Furthermore, NLO spectroscopy has become a standard method for material inspection, playing a vital role in many aspects of microelectronic industries. There is no doubt that the nonlinear optics will offer greater societal impact over the coming decades, but as highlighted in the following sections there are certainly many new challenges to overcome. The nonlinear optics in China has achieved remarkable achievements during the past decades. To maintain this encouraging trend and meet the challenges ahead, greater investment in national policy, financial support and intellectual resources are highly desirable in the future.