## 3 Experiment and Results  
According to the abovementioned theories about polarization modulation, we used half-wavelength plates to realize the optimal combination of initial polarization status. We used a linear polarizer to measure the initial orientation of polarization of the laser source, and use a half-wavelength plate to modulate it with 0 degrees; we marked it as s. We employed two HWPS, which is half-wavelength plates, to alter the orientation of polarization of beams 2 and 3. However, the basic requirement is that optical elements after the collimation lens are as few as possible, to keep a good wavefront and a relatively large effective exposure area. To a certain extent, if the degree of orthogonality is small enough as mentioned above, we can ignore the interference between beam 2 and 3. Therefore, experiments with half-wavelength plates were carried out to verify the effects of polarization modulation. Meanwhile, for the purpose of obtaining maximum fabrication area and simplification of the optical system, we conducted a bold attempt that abandoned employing the two half-wavelength plates. It is worth noting that the reflected beam had different polarization status with the change of incident angle according to [PERSON]'s theories. For the sake of simplification, all the experiments were carried out under the constant condition of incident angle \(\theta=71.8^{\circ}\). The fabricated 2D gratings could have the same grating pitch at two orthogonal directions (g\({}_{12}=g_{13}=1\)\(\upmu\)m). Figure 5 shows the fabricated gratings without and with polarization, i.e., under polarization state of (s, s, s) and (s, \(-33.5^{\circ}\), \(33.5^{\circ}\)), respectively. It can be seen that the area of 2D grating fabricated without polarization status, i.e., initial polarization of the laser source (s, s, s) is larger than the modulated polarization status (s, \(-33.5^{\circ}\), \(33.5^{\circ}\)). The area of the 2D grating fabricated with the initial polarization status (s, s, s) and (s, \(-33.5^{\circ}\), \(33.5^{\circ}\)) are 25 mm \(\times\) 25 mm and 10 mm \(\times\) 10 mm, respectively. The enlarged grating area will be beneficial for grating interferometry application. The fabricated grating shapes were then evaluated by using an atomic force microscope (AFM). The AFM (Bruker Dimension Icon) worked on a peak force tapping mode with a height sensor data density of 128 \(\times\) 128 of an AFM image with a size of 10 \(\upmu\)m \(\times\) 10 \(\upmu\)m. The grating surface material is photoresist, and AFM tip material is SiC and the shape aspect ratio is 1. The peak force setpoint and amplitude are 0.05 and 300 nm, respectively. Figure 6a,b show the images of the fabricated 2D  
Figure 5: Photographs of the fabricated 2D gratings under the initial polarization status of (**a**) (s, s, s) under conditions of exposure time 30 s, development time 8 s and (**b**) (s, \(-33.5^{\circ}\), \(33.5^{\circ}\)) under exposure time 30 s, development time 8 s.  
gratings under the initial polarization status of (s, s, s) and (s, \(-\)33.5\({}^{\circ}\), 33.5\({}^{\circ}\)), respectively. First of all, it can be seen that the grating shapes under these two polarization status are all regularly uniform, a near pillar shape, and no obvious elliptical shape as illustrated in [13] occurred. More than ten samples have been fabricated and results were highly consistent. Furthermore, the grating periods were evaluated as shown in Figure 6, B-B'. The grating periods are all consistent with the designed value of 1 \(\upmu\)m. It should be noted that, the period consistence is more importance than the absolute grating period value. From the A-A' sectional data, we can see that the grating depth are about 500 nm, (s, s, s) is slightly lower than that of the (s, \(-\)33.5\({}^{\circ}\), 33.5\({}^{\circ}\)), which are also consistent with the grating shape demand.