Structural color arises from the interaction between light and structures. Compared with the colors produced by traditional dyes, structural colors are usually environmentally friendly and resistant to fading. The two-photon polymerization-based three-dimensional (3D) printing technology provides sufficient freedom for the structural design and control of the full three-dimensional space and possesses higher resolution than traditional 3D printing technologies. Therefore, it has been widely used in the preparation of high-resolution structural colors in recent years. This review first briefly describes the principle and characteristics of two-photon polymerization 3D printing technology. We then introduce representative structural color schemes such as 3D printed diffraction gratings, photonic crystals, biomimetic structures, and single nanopillars. We also review 3D and dynamic structural colors for optical anti-counterfeiting, information storage, and optical sensing. Finally, the research status, existing problems, and future research directions of two-photon polymerization 3D printing technology are discussed.

The emergence of two-photon polymerization lithography (TPL) has successfully improved 3D printing resolution to submicron and nanometer levels. In 1997, Maruo et al. used TPL to fabricate micron-scale helical 3D polymer structures for the first time, and the field of two-photon processing was launched. In 2001, Kawata et al. used a near-infrared femtosecond pulsed laser to induce two-photon polymerization of photoresist to manufacture “micro-bull” sculptures. Its spatial resolution reaches 120 nm, which can be regarded as a milestone of progress in TPL, demonstrating that TPL can break the limit of classical optical diffraction and it can be used to fabricate high-resolution nanoscale 3D structures. Since then, research and applications of TPL have proliferated greatly. Although TPL cannot compete with planar manufacturing technologies such as electron beam lithography in terms of fabrication resolution, its key advantage lies in its ability to generate complex structures with arbitrary 3D geometries (Fig. 1). Moreover, stimulated emission depletion and diffusion assistant techniques have been employed to improve the resolution of TPL. In addition, TPL has been extended to active materials, such as liquid crystals, shape memory polymers, flexible elastomers, and hydrogels. By combining active photoresists that can respond to external stimuli, TPL can produce 4D micro/nanostructures whose shape and properties change with time. This further promotes the application of TPL in the preparation of micro/nanoscale functional structures.

The interaction between light and micro/nanostructures produces structural colors. By controlling the geometric parameters of the structures, their optical response in the visible spectrum can be adjusted, and rich colors can be produced even with a single material. So far, structural colors have been produced by light interference in thin films and interfaces, diffraction effects in photonic crystals, surface plasmon resonance in metal micro/nanostructures, and Mie resonances in high-refractive-index nanostructures. Owing to their anti-fading property and unprecedented printing resolution, structural colors have become one of the research hotspots of micro/nano-optics. TPL-based nanoscale 3D/4D printing technology provides great design freedom for micro/nano fabrication and has been widely used in research on structural colors in recent years. Vivid structural colors have been produced by TPL printed gratings (Fig. 2), photonic crystals (Fig. 3), biomimetic structures (Fig. 4), and single micro/nanostructures (Fig. 5). Just as people try to use angle information to realize stereo imaging after storing two-dimensional information in photos, researchers have explored the display and storage of stereoscopic structural color information through TPL 3D printing (Fig. 6). Dynamic structural colors (Fig. 7) have also been demonstrated in TPL-printed active devices and used for optical anti-counterfeiting and encryption.

This study presents the progress of structural color research based on two-photon polymerization 3D printing technology. The principle of TPL 3D printing technology and its characteristics are introduced. By preparing different photonic structures and tuning their geometrical parameters, vivid 2D and 3D structural colors can be achieved in polymer nanopillars, diffraction gratings, woodpile structures, and similar structures. Most of the produced structural colors are strongly angle-dependent, while angle-independent structural colors can be realized in some biomimetic structures. Dynamic modulation of structural colors can be achieved by combining TPL with responsive stimulating materials. Although TPL-based 3D structural color printing has been used in various optical applications, its potential has not been fully explored. It is necessary to further increase the fabrication speed while maintaining or further improving the printing accuracy. The pursuit of 3D-printed devices with better structural precision, surface smoothness, and optical performance has never stopped. These demands impose stronger requirements for the development of novel TPL functional materials and manufacturing technologies. Follow-up research on dynamic structural color should develop novel methods to rapidly and actively control the morphology and optical properties of structures. Future research work could further explore the TPL technique to manipulate more degrees of freedom of light, such as orbital angular momentum, to attain greater achievement in the fields of optical communication, optical encryption, and data storage. With its further development in various applications, TPL constitutes a powerful tool for studying light-matter interactions that could lead us to a revolution in optics and photonics research.