## 2 High Diffraction Efficiency and Damage-Resistance Threshold Gratings
### Metal-Film Reflection Gratings  
American researchers developed the first diffraction gratings using metal wires in the 18 th century. The surface of a typical reflection grating with a high diffraction efficiency is gold-plated, making use of the strong metal conductivity to achieve high reflectivity and increased reflection and diffraction efficiencies. [PERSON] et al. [16] created a single-layer tungsten-silicide-metal grating using double-beam interference lithography and dry etching in 2008. The period of this grating was 400 nm, the groove depth was 350 nm, and the duty cycle was 0.6. Above 5 m, the transverse magnetic (TM) transmission exceeded 50%, and the estimated extinction ratio was approximately 20 dB. Figure 1 provides a scanning electron microscope (SEM) image of the infrared metal-grating surface morphology, along with the polarization characteristics. In 2010, [PERSON], [PERSON], and [PERSON] [17] created a subwavelength metal polarization grating for polarization imaging in the visible-light range. Figure 2 depicts a schematic diagram of the grating construction. Finite-difference time-domain (FDTD) simulation was used to determine the ideal grating parameters: the period was 200 nm, the aluminum thickness was 260 nm, and the duty cycle was 0.4. The transmission efficiency in the visible-light band was higher than 50%, with a maximum value of 70%. The extinction ratio was greater than \(3.5\times 10^{3}\), with a maximum value of approximately \(5.5\times 10^{4}\). In 2012, [PERSON], [PERSON], et al. [18] used a two-beam interference lithography procedure combined with a reactive-ion etching approach and a nanoimprinting process to produce a single-layer aluminum-metal grating on a chalcogenide-glass substrate. The metal grating had a period of 500 nm and a height of 130 nm. The structure comprising a 120 nm Al layer and a 50 nm anti-reflection Cu layer was established with resist-substrate reflectivity and the ultimate performance of the polarizer in mind. The TM transmission efficiency was greater than 71%, and the extinction ratio was greater than 25 dB. The TM transmission was greater than 60% in the 5-9 m range, and the extinction ratio was greater than 20 dB in the 2.5-11 m range. [PERSON] et al. [19] at Wuhan Huake University proposed a metal grating with a sandwiched Au-Si-Au structure in 2014 and obtained a TM transmission performance greater than 90% through simulation calculations. [PERSON], [PERSON], et al. [20] researched and fabricated gold-coated gratings for pulse compression in 2017. The grating structural parameters were: period 574.7 nm, duty cycle 0.82, groove depth 232 nm, and a diffraction efficiency greater than 90% in the 700-1000 nm range. A Ti-sapphire laser system was used for a one-on-one laser-damage experiment on the diffraction grating. Figure 3 illustrates the damage morphology and laser energy density measured at 60 fs under 0.95 J/cm\({}^{2}\). Figure 1: Surface topography SEM image of the infrared grating (groove depth = 350 nm, period (\(\Lambda\)) = 400 nm, and duty cycle (f) = 0.6) and its polarization characteristics. Figure 2: Structural model of subwavelength metal polarization grating. [PERSON], [PERSON], et al. [21] designed a gold-coated grating in 2018. The period of the grating was 574.7 nm, the groove depth was 232 nm, the duty cycle was 0.82, and the \(-1\) order of the grating was constructed in the 700-1000 nm band. The diffraction efficiency was greater than 90%. For damage investigations, a CPA Ti-sapphire laser system was implemented, and the laser-damage threshold of the manufactured grating was 0.62 J/cm\({}^{2}\). As shown in Figure 4.  
Due to their absorption properties, metal gratings have low anti-laser-damage thresholds and diffraction efficiencies [22]. There have been few reports in recent years on the production and application of metal gratings. A summary of the above analysis is shown in Table 1. No photoresist coating is utilized in the grating construction to optimize and improve the performance of the metal grating.