# An Optically Transparent Metasurface for Microwave Amplitude-Phase Manipulation
## 3 Discussion  
In practical applications, oblique incidence scenarios are common. Thus, it is essential to analyze the incident-angle stability of the proposed meta-atoms. Figure 7a,b present the cross-polarized reflection amplitude and phase versus rotation angle under different incident angles of \(\theta\) = 0\({}^{\circ}\), 30\({}^{\circ}\), and -30\({}^{\circ}\). We note that the monotonically increasing amplitude and almost unchanged phase versus the rotation angle are observed at large incidence angles of \(\pm\)30\({}^{\circ}\). In addition, as can be seen from Figure 7c,d, the cross-polarized reflection amplitudes of the selected meta-atoms #1-8 are above 0.9 when the incident angles are \(\pm\)30\({}^{\circ}\), whereas the phase modulation with a \(\pi\)/4 interval is almost insensitive to the incident angle. Thus, the independent and complete manipulation of amplitude and phase responses of the proposed meta-atoms can be preserved under oblique incidence scenarios of \(\theta\) = \(\pm\)30\({}^{\circ}\), showcasing a remarkable property of oblique-incidence stability which is essential in practical applications. Without loss of generality, broadband performance validation across the Ku-band (12-18 GHz) is presented in Figure 8. It can be seen that a holographic image of \"WH\" appears at the imaging planes of 85, 100, 130, and 160 mm under the frequencies of 12, 14, 16, and 18 GHz, whereas another holographic image of \"UT\" is observed at distances of 200, 240, 270, and 290 mm under the corresponding frequencies, demonstrating the excellent broadband holographic imaging performance of the proposed metasurface. We also note that the observing plane increases with the operating frequency, which can be easily explained by the different phase accumulation of wave propagation related to different frequencies [39, 45]. Figure 6: (**a,b**) Experimental setup for 2D near-field measurement. (**c,d**) Target, (**e,f**) calculated, (**g,h**) simulated, and (**i,j**) experimentally measured holographic images at the imaging planes of \(z_{1}\) = 130 mm and \(z_{2}\) = 270 mm, at a frequency of 16 GHz. Table 1 compares the performance of the proposed metasurface with previously reported counterparts. First, amplitude-phase manipulation was realized by refs. [28; 29; 30; 31] by using metal-based meta-atoms. However, the optically opaque properties restrict their applications in optical windows. Next, refs. [46; 47] focused on realizing optical transparency by replacing the metal resonator with ITO configuration, whereas their wavefront control capabilities were limited to a single degree of freedom, i.e., the phase-only modulation. In contrast to previous studies, our proposed metasurface not only enables independent and complete control of amplitude and phase in the microwave region, but also maintains relatively high optical transparency. Additionally, the metal-frame-based resonator fabricated by the mature and low-cost FPC technology is particularly beneficial for practical applications, which may promote the industrialization of metasurfaces. Figure 8: **(a,b)** Simulated cross-polarized electric field intensity distributions of the proposed metasurface at the corresponding imaging planes at 12 GHz. **(c,d)** Simulated holographic images at the corresponding imaging planes at 14 GHz. **(e,f)** Simulated holographic images at the corresponding imaging planes at 16 GHz. **(g,h)** Simulated holographic images at the corresponding imaging planes at 18 GHz. Figure 7: **(a,b)** Simulated amplitude and phase of \(r_{yx}\) versus the rotation angle \(\beta\) under different incident angles of \(\theta=0^{\circ}\), \(30^{\circ}\), and -\(30^{\circ}\). **(c,d)** Simulated amplitude and phase of \(r_{yx}\) for the selected meta-atoms #1–8 under different incident angles of \(\theta=0^{\circ}\), \(30^{\circ}\), and -\(30^{\circ}\). Despite the achievements of the proposed metasurface, the existing limitations lie in the non-tunable performance and limited operating channels in the microwave region.