## 2 Fabrication Strategy for Functional Materials with 3D Periodic Nanostructures
### Preparation of 3D Nanostructured Polymer Templates via Optical 3D Lithography  
Since the early 2000s, various optical 3D nanofabrication techniques have been developed to experimentally implement 3D photonic crystals requiring precise structure control at the nanoscale. In principle, optical 3D lithography techniques are originated from photolithography in semiconductor processes and treat photoreactive polymers in different ways. Essentially, the resolution and reproducibility of 3D nanostructures formed by optical 3D lithography are superior compared to other self-assembly-based 3D nanofabrication techniques. In addition, optical 3D lithography has a high degree of freedom of 3D nanostructures that can be produced and has the potential to advance into mass production technology. Such optical 3D lithography techniques can be largely divided into two types: (1) serial process and (2) parallel process. One of the serial processes is DLW, which uses a highly focused laser beam to draw a 3D feature in photoresist. Interference-based lithography is a representative parallel process that generates multilayered, complex 3D nanostructures in a single exposure. First, DLW is also called two-photon lithography or stereolithography. In principle, DLW can be regarded as a type of 3D printing technology capable of constructing 3D nanostructures with sub-100 nm resolution [16]. An ultrafast, high-power laser beam is tightly focused through a high-magnification objective lens within the volume of a transparent photoreactive polymer, and the exposed region is locally polymerized as a result of absorbing two photons simultaneously. Based on drawings designed through computer-aided design (CAD) or computer-aided manufacturing (CAM), the laser beam can travel along a given path to create a continuous 3D networked polymer. Unpolymerized residues can be removed by a developing solvent. DLW is very advantageous when arbitrary 3D nanostructures with high resolution are required. However, since DLW is based on a serial process, it inherently has a fatal disadvantage in that the production speed becomes extremely slow as the overall volume of the product increases or the size of the unit structure decreases [54]. It takes approximately several days to fabricate a 3D periodic nanostructure with a unit cell of several hundred nanometers in cm size. The development of photoresist capable of absorbing two photons is challenged, and the power threshold for causing two-photon polymerization is very high as well. On the other hand, MBIL, also called holographic lithography, which uses 3D interference patterns of light generated by overlapping multiple coherent beams at one point, can produce 3D periodic nanostructures in mm or cm size with a single, short exposure [55; 56; 57; 58]. The exposure time depends on the sensitivity of the photoreactive polymer and the intensity of the laser, which is within a few seconds. In order to create 3D periodic nanostructures, interference of at least four beams is required, and various symmetric structures can be generated by controlling the intensity, incident angle, or polarization of each beam. In comparison with DLW, although the resolution of MBIL is slightly lower, it definitely has improved productivity. However, MBIL requires a very complex configuration of expensive optical components because at least four coherent beams have to be superimposed, and the reproducibility is extremely poor due to low vibration tolerance. To solve these issues, a single beam-based interference lithography using a well-designed microprism has been developed, but the degree of structural freedom is still somewhat inferior [59]. PnP, also called phase-mask interference lithography, which uses diffraction and interference generated from a transparent phase mask, can be regarded as a high-resolution interference lithography technique that has evolved to be closer to a mass production process [29; 30; 31; 32; 34; 35; 60; 61].