## 3 Progress in Grating Fabrication Technology
### Holographic Exposure  
With the advent of lasers, holographic lithography has been researched and applied, and grating preparation technology has entered a new stage of development. The principle of holographic lithography is that two coherent plane waves overlap in space to form a standing wave. Then, the periodic interference light-field distribution is recorded with photoresist and other photosensitive materials to realize the fabrication of periodic pattern masks. The graph period expression is:  
\[\Lambda=\lambda/(2\sin\theta)\]  
where \(\lambda\) is the wavelength of the incident laser and \(\theta\) is the incidence angle of the corresponding laser beam. Different incidence angles can result in grating structures with different sizes. Compared with traditional mechanically scribed gratings, diffraction gratings fabricated by holographic lithography have the advantages of no ghost lines, high signal-to-noise ratios, short fabrication periods, low consumption costs, diverse grating grooves, and large area exposures. Holographic lithography can also be combined with the deposition of multilayer dielectric films to improve diffraction efficiency. In addition, holographic lithography technology combined with ion-beam etching technology can produce blazed gratings with different angles, which significantly improves the diffraction efficiency of the gratings. Therefore, holographic lithography technology has gradually replaced the application of mechanical scribing technology in some fields. In 1990, scientists such as [PERSON] and [PERSON] of the Massachusetts Institute of Technology used achromatic holographic lithography combined with oxygen-plasma-reactive etching technology to fabricate a holographic grating with a period of 100 nm; an anti-reflection coating was used to suppress the standing wave effect before exposure [67]. In 1999, [PERSON] and [PERSON] of Taiwan National University first proposed the fabrication of sub-quarter-micron gratings with periods as small as 0.22 \(\upmu\)m by combining deep-ultraviolet (DUV) holographic lithography with silanization [68]. In 2000, researchers such as [PERSON] and [PERSON] in Singapore proposed a simple UV holographic lithography technology, in which a double-reactive surface-relief grating was fabricated in a single step by holographic lithography on silica-titania glass. Compared with traditional photoresist-based microfabrication techniques, the single-step fabrication process does not require additional exposure and etching steps or expensive etching equipment [69]. In 2003, researchers in the Czech Republic proposed a new doped-layer structure formed by alternating Ag and As\({}_{30}\)S\({}_{70}\), and then used holographic lithography (\(\lambda\) = 514 nm) to fabricate a phase grating with a period of 2 \(\upmu\)m; Ag can be completely dissolved in the chalcogenide film after exposure, and the refractive index change leads to the achievement of maximum diffraction efficiencies in multilayer and bilayer systems of 3% and 7%, respectively. The multilayer structure greatly increases the sensitivity of the system to incident light. The sensitivity of the system is five times that of the two-layer system [70]. In 2017, [PERSON], [PERSON], et al. [71] fabricated a grating layer with variable line spacing based on the RCWA method and holographic lithography, with a grating duty cycle of 0.5 and a grating-groove depth of 120 nm. The holographic lithography system used a cylindrical lens placed between the collimating mirror and the sample. The laser wavelength was 442 nm, and the holographic grating period changed monotonically on the surface of the device to obtain resonant-wavelength-tunable features. Under TE polarization, the main reflection peak ranged from 762.1 to 908.3 nm, and the grating period increment in the 22 mm range was approximately 108 nm.