Lasers, or “light amplification by stimulated emission of radiation,” are one of the most important research areas in modern physics. The uniqueness of lasers lies in the multiple degrees of freedom (DoFs) of photons, including amplitude, frequency, orbital angular momentum, and polarization. Understanding and controlling these DoFs are central to laser applications. For a given photon’s DoF, we can define distinctive modes to distinguish different beam states. Gradually, our understanding of laser cavities has expanded, and laser control techniques have become more sophisticated, sparking interest in multimode lasers. Multimode lasers transcend the traditional limitation of single-mode operation, offering richer physical phenomena and expanding the applications of lasers. For example, when multiple longitudinal modes coexist in a resonant cavity and are correlated in phase, they lock in the frequency domain, producing ultrashort pulses (ranging from 10-12 to 10-15 s) in the time domain, which are valuable for scientific research and industrial applications. Similarly, multimode lasers in spatial DoFs have been garnering increasing interest, broadly bridging the fields of complex optics, wavefront shaping, optical simulators, computational imaging, and many-body physics.

Here, we review research progress on the recent designs and applications of multi-transverse-mode lasers, with a focus on solid-state lasers. We begin with a brief introduction to the frequency-degenerate cavities used for generating complex structured light (Fig. 1), where stringent cavity boundary conditions must be satisfied. We place more emphasis on degenerate cavity lasers with a 4-F system within the resonators (Fig. 2), which are particularly interesting because arbitrary-shapes transverse modes are naturally supported in such a setup. The discussion is then expanded to include random multimode lasers, chaotic microcavities, and photonic network lasers (Figs. 3‒5), which differ but share the same property of multimode lasing. Alongside explaining their basic principles, we highlight several promising approaches for controlling them (Fig. 5); here, the pump beam profile is elaborately designed. Although these controlling strategies may be task-dependent, we envision that the existing demonstrations will be inspiring for further advancements in other multimode lasers. These unconventional laser designs enable numerous exciting applications, as summarized in Section 3. A notable example is speckle-free imaging, made possible by spatial-coherence-tunable degenerate cavity lasers (Figs. 6 and 7). In experiments, researchers could optimize the coupling strengths or distributions among the supported modes, thereby achieving the high-tunability of spatial coherence while preserving a decent laser power. Owing to the large bandwidth, fast dynamics, and high parallelism of light, such degenerate cavity lasers have been recently used in optical computing, such as in finding ultrafast solutions to iterative problems (Fig. 8) or acting as physical system simulators (Fig. 9). In particular, the dynamics of the cavity can be perfectly mapped to the Kuramoto model; the lasing output corresponds to a minimum Hamiltonian of an XY model, thus defining a solution to a combinatorial optimization problem. This is especially intriguing as the laser obtains the solution in an ultrafast and automated manner. Interestingly, by reverse conceptualizing the lasing process, degenerate cavity lasers have been extended to coherent perfect absorbers that can, in principle, absorb arbitrary wavefronts (Fig. 10). This innovative application of a laser has led to several creative absorber designs, such as random absorbers derived from anti-random lasers. We also introduce several application examples in random number generation, sensing, and topological self-healing (Figs. 11 and 12). Further developments in multimode laser control technologies, such as deep learning, and potential new applications are also discussed.

This review focuses on advancements in multi-transverse-mode lasers, introducing the design principles of resonant cavities and their corresponding control techniques. By discussing the transition from single-mode to multimode lasers, we highlight the innovations and challenges involved. This review emphasizes five novel categories of multi-transverse-mode lasers and delves into their principles to explore the diverse applications of these unconventional lasers, providing readers with a fresh perspective on the evolution and new developments of laser technology. Finally, we offer a perspective on the promising research directions of the mode number, control, and applications of multimode lasers. Specifically, we believe artificial intelligence can play an important role in designing new multimode lasers, as evidenced by the exciting scientific discoveries made by these models. Likewise, it remains promising to optimize neural network models, acting as policy makers, to control the intermodal couplings of these lasers. In parallel, new technology breakthroughs, such as new laser materials and optoelectronic devices, are important to these applications. For example, spatial light modulators with ultra-number pixels, response speed, and high threshold power can potentially elevate laboratory investigations to real commercial scenarios. In conclusion, with the advancement of machine learning technologies, intelligent multimode laser systems are becoming a new trend in research and applications. This interdisciplinary integration spans laser physics, many-body physics, combinatorial optimization, nonlinear dynamic systems, and machine learning. Consequently, we believe customized laser designs through multidisciplinary collaboration will significantly advance intelligent laser technologies. This progress will overcome current challenges and bottlenecks, broaden the applicability of novel lasers in scientific research and industrial applications, and open new avenues for future development.