Progress The rapid evolution of information technology imposes continuous demands on information transfer speed, the energy consumption of devices, and anti-interference performance. In this context, photons, which serve as information carrier, exhibit greater capability in terms of information processing compared with electrons. Photonic crystals, which are recognized as quintessential structures for manipulating photons, have garnered substantial research interest. The periodic arrangement of materials with varying refractive indices in photonic crystals results in advanced optical modes, including photonic band gaps and slow light modes (Fig.1). The conceptualization of photonic crystals has significantly advanced investigations into micro/nano optics and optical devices, thus promoting the development of optical communication, information displays, and integrated photonics. Optical components fabricated using one- and two-dimensional photonic crystals have been investigated comprehensively and applied extensively. However, the effective utilization of 3D photonic crystals, which are characterized by periodic structures in all three orthogonal directions, remains hindered by limitations in micro/nano manufacturing technology. The creation of 3D photonic crystals with attributes such as high precision, minimal random defects, high yield, and controllable 3D profiles presents several challenges. To propel the progress and practical application of photonic crystals, comprehensive investigations into 3D micro/nano processing technology are warranted.

Diverse techniques are employed to fabricate photonic crystals, among which mask-projection lithography, nanoimprint lithography, and electron/ion-beam lithography are the predominant methods used for creating planar structures. Complex structures achieved via these methods typically involve the layered stacking of planar components, which presents challenges in realizing arbitrary 3D structures. Self-assembly is typically conducted to form densely packed particles, which yields large-area samples with random defects, thus rendering it difficult to control the overall 3D contour of the structure. As an effective micro/nano manufacturing technology for fabricating arbitrary 3D structures, TPL is the preferred method for manufacturing 3D photonic crystals (Fig.2). In this regard, continuous efforts are directed toward advancing the resolution (Fig.3), accelerating the printing speed, and diversifying the material library (Fig.4) for the TPL-based production of 3D photonic crystals. The resulting 3D photonic crystals demonstrate outstanding performance in structural color displays (Fig.5) and many other optical applications (Fig.6).

Conclusions and Prospects The synergy between research pertaining to 3D photonic crystals and advancements in TPL technology is evident. Serving as exemplary 3D structures, photonic crystals function as standardized printing models that facilitate the meticulous evaluation of TPL performance. Concurrently, the evolution of TPL technology streamlines the preparation and experimental investigation of 3D photonic crystals. By leveraging their unique light manipulation capabilities, 3D photonic crystals offer significant potential in the optical domain. Augmenting TPL technology with a diverse range of functional materials will expand the application scope of photonic crystals. The trajectory of future research entails harnessing micro/nano processing technologies such as TPL to fabricate a myriad of photonic crystals and other optical components on a monolithic chip, thereby facilitating the development of high-performance integrated optical circuits. Beyond optics, 3D photonic crystals can be applied to energy, biomedicine, and other fields. From enhancing the efficiency of solar cells to crafting 3D micro-scaffolds for guiding cell growth, their potential applications are expansive. The ongoing research pertaining to TPL technology and 3D photonic crystals can create new possibilities for scientific research activities that benefit daily life.

Investigations pertaining to two-photon polymerization lithography (TPL) and photonic crystals are mutually reinforcing. This review first outlines the concept and typical structures of three-dimensional (3D) photonic crystals, as well as the principles and characteristics of TPL technology. Subsequently, research progress pertaining to the utilization of TPL for printing 3D photonic crystals is introduced, with emphasis on aspects such as resolution, printing speed, and the extension of material library. Additionally, the potential applications of 3D photonic crystals in the field of optics are highlighted. Finally, the existing challenges in the TPL printing of 3D photonic crystals are discussed, and the prospective future research directions are presented.