## 3 Progress in Grating Fabrication Technology
### Nanoimprinting  
Nanoimprinting is a new micro-nano-pattern processing method. Compared with other preparation methods, nanoimprint lithography (NIL) technology does not rely on optical exposure, is not affected by diffraction, and only relies on the imprint template to deform the upper imprint glue. Nanoimprinting has a series of advantages, such as high resolution, low cost, high yield, and easy operation, which have led to the wider use of nanoimprinting technology, taking the manufacture of gratings to a new height. NIL technology was first proposed by [PERSON] [76] of Princeton University in the United States and has been applied in the field of grating fabrication, laying the foundation for the research of many disciplines and practical spectroscopy.  
In 2005, [PERSON] et al. of the LG Corporation successfully fabricated an aluminum grating with a line width of 50 nm on a glass substrate using nanoimprinting technology [77]. The grating area reached 5.5 \(\times\) 5.5 cm, which is the same area as that of a microdisplay panel produced by LG. The grating was used as a metal-grating polarizer, and the polarization extinction ratio exceeded 2000. It had a transmission of 85% for light with a wavelength of 450 nm, and it is currently the best metal-grating polarizer reported. In addition to the expensive imprint templates made by laser interference lithography, the fabrication cost of the monolithic polarizer is low and the efficiency is high. It can already be controlled within the cost range of general parts for consumer electronics. Figure 37 shows a silicon imprint template made by LG and a metal-grating polarizer made with NIL technology.  
In 2011, [PERSON] et al. [78] in Canada prepared molds with rigid UV-curing properties on elastic poly(dimethylsiloxane) (PDMS) scaffolds using three different methods. The first method involves direct imprinting at temperatures above 200 \({}^{\circ}\)C. The second method is imprinting onto a curable resist at temperatures below 100 \({}^{\circ}\)C, then using reactive ion-beam etching to transfer the pattern into PMGI and write directly by electron-beam lithography. The third method is to use hybrid nanoimprint soft lithography to obtain sub-10 nm feature-size pattern replication, which is short and flexible.  
In 2021, [PERSON], [PERSON], et al. [79] in Ireland proposed a simple method to fabricate silver gratings using silver nanoparticle solutions and nanoimprint lithography  
Figure 37: SEM image of an aluminum grating.  
and found that gratings in solutions of silver nanoparticles nanoimprinted directly onto glass substrates have similar properties to those produced by thermally evaporating silver on nanoimprinted PMMA. It was confirmed that this solution-based method does not require special equipment and is more cost-effective than the traditional method of NIL used on PMMA and then thermally evaporated Ag film. In the same year, Professor [PERSON] et al. from the Institute of Precision Optical Engineering of Tongji University used a combination of nanoimprint lithography, atomic layer deposition, and reactive ion-beam etching to fabricate a rectangular grating-groove profile with HfO\({}_{2}\) as the material on top of the surface. The PDMS mold was imprinted onto the resist (PMMA), spin-coated on the multilayer dielectric film mirror by UV nanoimprint lithography, and the resist grating structure opposite to the target HfO\({}_{2}\) grating structure was obtained. A rectangular diffraction grating with a diffraction efficiency of 95% (from 714 to 865 nm) and a damage resistance threshold of 0.59 J/cm\({}^{2}\) (40 fs) was fabricated [80].  
After the introduction of nanoimprint lithography, certain breakthroughs have been made in grating fabrication, bringing low-cost, high-yield, large-area, and high-density gratings closer and closer to reality.