Significance Micro-nano optical devices, resulting from the integration of photonics and micro-nano technology, are rapidly growing within the optoelectronics industry. These devices can effectively modulate light fields according to their design and have been widely used in fields such as photonic integrated chips, optical communications, optical storage, sensing imaging, display, solid-state lighting, biomedicine, and photovoltaic energy. As the applications for micro-nano optical devices continue to expand, there is an increasing demand for the development of micro-nano manufacturing technologies to support their production. A range of micro-nano manufacturing techniques is required to meet the writing requirements of different micro-nano optical devices. These techniques include direct laser writing (DLW), interference lithography, mask projection lithography, nanoimprint lithography, electron beam lithography, and ion beam lithography. Interference lithography can achieve rapid large-area writing through interference exposure but is limited in its ability to customize writing patterns, and it is difficult for writing aperiodic structures. Both mask projection lithography and nanoimprinting are well-suited for the fast fabrication of high-resolution features but require the prior fabrication of templates and offer limited flexibility in structure writing. Writing with focused charged particles (ions and electrons) can achieve resolution below 10 nm, but both electron beam lithography and ion beam lithography require a vacuum environment. They have high system costs and are not well-suited for writing three-dimensional structures. DLW technology based on two-photon polymerization (also known as two-photon direct writing) has emerged as an important technology for manufacturing micro-nano optical devices due to its submicron resolution, ability to write arbitrary three-dimensional structures, and cost-effectiveness. Two-photon polymerization exhibits nonlinear characteristics that can confine photochemical conversion to the central region where excitation light is focused. Complex three-dimensional structure writing can be achieved through the relative movement of any three-dimensional position between the laser focus and substrate. The exposed area of photosensitive material will undergo polymerization while unexposed areas can be dissolved in the developer and washed away during development, resulting in a microstructure on the substrate after exposure and development. Devices fabricated using two-photon direct writing primarily consist of polymer materials, with their composition largely dependent on the photoresist used. Photoresists can be liquid, gel, or solid and can be composed of organic molecules or organic-inorganic hybrids. Many types of polymers can be combined with new or active materials to achieve specific functions. The high degree of freedom in polymer material design and the flexibility of two-photon direct writing exposure have led to a widespread study of micro-nano optical devices based on two-photon direct writing. These devices have played an important role in diffractive optics, imaging optics, fiber optics, color optics, integrated optics, and optical data storage. As the applications for micro-nano optical devices based on two-photon direct writing continue to expand, there are increasing demands for improved performance from two-photon direct writing technology. In terms of writing resolution, two-photon direct writing is primarily limited by the diffraction effect of the optical system and the proximity effect in photoresist material. To further improve writing resolution, researchers have proposed a super-resolution DLW technology based on peripheral photoinhibition (PPI) inspired by stimulated emission depletion (STED) microscopy. This technology can overcome the diffraction effect and reduce the proximity effect. Super-resolution DLW technology can advance the fabrication dimension of two-photon direct writing to the nanometer scale and has been successfully applied in fields such as photonic metamaterials, optical data storage, and biological applications. In terms of writing throughput, two-photon direct writing is primarily limited by its single-focus serial writing mode. Compared with that of projection lithography, the throughput of two-photon direct writing is lower, and two-photon direct writing is not well-suited for large-area rapid manufacturing. Utilizing multi-channel parallel writing techniques can significantly enhance the writing throughput of two-photon direct writing processes. This advancement facilitates large-scale printing capabilities for micro-nano optical devices. The ability to achieve cross-scale fabrication with nanometric resolution and centimetric size is crucial for the mass production of large-area and high-quality micro-nanostructure devices. Progress Building upon two-photon direct writing technology, the State Key Laboratory of Extreme Photonics and Instrumentation of Zhejiang University along with Zhejiang Laboratory have developed several advanced techniques to achieve super-resolution and high-throughput writing in two-photon direct writing. These techniques include single-color PPI lithography, two-focus parallel PPI lithography, and high-speed parallel two-photon direct laser writing lithography. We focus on the applications of these techniques in the fabrication of micro-nano optical devices. First, we provide an overview of the principles of two-photon direct writing (Fig. 1) and its applications in diffractive optical devices (Figs. 2-3) and optical fiber integrated devices (Figs. 4-5). We then introduce the principles of single-color PPI lithography (Fig. 6) and discuss our research on the applications of super-resolution direct laser writing in nanophotonic devices, including metasurfaces (Figs. 7-8) and photonic crystals (Fig. 9). We also summarize the current fabrication methods and printed sizes of photonic crystals (Table 1), indicating the extremely high resolution of single-color PPI lithography. Finally, we present the technical advantages of high-precision and high-throughput DLW technology for manufacturing centimeter-scale micro-nano optical devices. Conclusions and Prospects The technology proposed and developed by our group exhibits nanoscale resolution and centimeter-scale cross-scale processing capabilities. This technology has been successfully applied to manufacture various micro-nano optical devices. We continue to explore new technologies and methods to better meet the demands of micro-nano optical device manufacturing. We anticipate that DLW and its enhancement technologies will play a crucial role in advancing the field of micro-nano optical device fabrication in the near future.
