# An Optically Transparent Metasurface for Microwave Amplitude-Phase Manipulation
## 2 Results
### Multiplane Complex-Amplitude Hologram
In addition, an ultrathin optically clear adhesive was used for interlayer adhesion between the PET and PMMA substrates. Figure 5a illustrates the photograph of the fabricated metasurface with an overall dimension of 360 \(\times\) 360 mm\({}^{2}\), through which the logo below the sample can be clearly seen, indicating its visual clarity. Next, a UV/Vis/NIR spectrometer (Lambda 750) was used to quantitatively analyze the optical transmittance of the fabricated sample in the wavelength range of 400-800 nm, as shown in Figure 5d. Also, the optical transmittance spectra of PMMA, top layer with PMMA, and bottom layer with PMMA are plotted for a clear comparison. Owing to the low filling ratio of the designed metal frame, the top layer exhibits a high transmittance of over 86%. Meanwhile, the average transmittance of the bottom metal mesh layer attains ~71%. For the entire metasurface, an average optical transmittance of 63.7% is realized at 400-800 nm, showcasing the efficacy of our proposed meta-atom consisting of a metal frame and mesh for realizing visible transparency. Figure 4: (a) Schematic of the designed multiplane hologram. (b,c) Calculated continuous amplitude distribution and digitized eight-level phase distribution of the metasurface with 45 \(\times\) 45 number of pixels at 16 GHz. In the microwave region, the performance of the designed metasurface was first verified by numerical simulations using the time-domain solver in CST Microwave Studio 2020. In the simulations, open (add space) was used as boundary conditions in the \(x\)-, \(y\)-, and \(z\)-directions, and the metasurface was illuminated by a normal incident plane wave with the electric field polarized along the \(x\) direction. Another \(y\)-polarized probe operating at 16 GHz was used to record the near-field intensities. The simulated cross-polarized electric field intensity distributions at the distance of \(z_{1}=130\) mm and \(z_{2}=270\) mm are shown in Figure 6g,h, respectively, indicating good agreement with the theoretical calculations obtained from MATLAB R2017b (Figure 6e,f). Next, a two-dimensional (2D) near-field scanning system was used to experimentally determine the metasurface performance, as shown in Figure 6a,b. To match the operating frequency (16 GHz) of the designed metasurface, an emitting WR-62 waveguide antenna connected to a vector network analyzer (VNA, Keysight N5227B) was used to generate a linearly polarized quasi-plane wave for illuminating onto the metasurface sample with the help of a large-diameter dielectric lens. In addition, a movable probe connected to the VNA was used to detect the electric fields with a step of 1 mm. The entire scanning plane of the probe system was placed between the emitting antenna and the sample, with two distances of 130 mm and 270 mm away from the sample. It is worth mentioning that the whole measurement setup was surrounded by microwave-absorbing materials to eliminate reflection losses. The obtained experimentally measured cross-polarized electric field intensity distributions at the observation planes of \(z_{1}=130\) mm and \(z_{2}=270\) mm are plotted in Figure 6i and Figure 6j, respectively. Specifically, a holographic image displaying \"WH\" appears at the first imaging plane of 130 mm (Figure 6i), while at a distance of 270 mm, a holographic image of \"UT\" is clearly observed (Figure 6j). We note that the measured holographic images closely match the corresponding calculated and simulated results. Further, a signal-to-noise ratio (SNR), which is defined as the ratio of the peak intensity in the holographic image to the standard deviation of the  
Figure 5: (**a**) Photograph of the fabricated metasurface sample. The inset shows the amplified view of the metal-frame-based meta-atoms. Scale bar, 8 mm. (**b**,**c**) Microscopic images of the top metal frame layer and the bottom metallic mesh layer. Scale bar, 200 μm. (**d**) Measured optical transmittance spectra of the fabricated metasurface in the wavelength range of 400–800 nm.