# High-Aspect-Ratio Silicon Metasurfaces: Design, Fabrication, and Characterization. _Applied_
## 4 Optical Characterization  
We utilize THz time-domain spectroscopy (TDS) to characterize the polarization conversion performance of the fabricated samples of different quality to demonstrate the importance of the meta-atom design and the fabrication method. We evaluate the PCR by measuring the transmission amplitude and phase with x- and y-polarized incident THz waves, respectively. Here, we take the transmission amplitude through a fixed aperture without a sample as the reference signal for both polarizations. Figure 7b-d present the measured spectra of the transmission amplitude for x- and y-polarized waves and phase difference between two cross-polarized waves for the three samples with different fabrication quality. The SEM images of these fabricated samples are presented by the insets in Figure 7b-d, with the etch profiles shown by cases #1-3 shown in Figure 5. As shown in Figure 7a, the PCR of sample #1 with a straight sidewall (i.e., aspect ratio (AR) is about 12:1) and a correct vertical thickness distribution, nearly reach 100% at the working frequency of 0.65 THz. Here, we can readily find that our experimental and simulation results are in good agreement for the sample #1 with good fabrication quality. At the working frequency of 0.69 THz, the low PCR caused by resonance can be remedied by optimizing the geometrical parameter of the micro-resonators. The discrepancy between the experimental and simulation results on the Q-factor of resonance at ~0.67 THz might  
Figure 6: Schematics of the multi-stepped etching process with a gradual decrease in cycle number and a gradual increase in platen power for the fabrication of tri-layered metasurfaces. be related to the fabrication imperfection of our device, including the imperfect thickness control of the micro-pillar and connecting layers or/and the etch profile of the micro-pillar. In addition, the working bandwidth of our MHWP can be further improved by carefully designing the periodicity of the structure (see Figure S2 in Supplementary Materials). In contrast, sample #3 without precise quality control on the VTD exhibits very poor polarization conversion capabilities with a PCR of \(\sim\)25%, as shown by the red tri-angles in Figure 7a. From the measured spectra shown in Figure 7b-d, one can notice the phase difference between the x- and y-polarized transmitted THz waves decrease to 90\({}^{\circ}\), and the transmission amplitude for both polarizations also drops significantly at the designed working frequency. The main reason might be related to the distorted anisotropy properties of meta-atoms, due to the presence the micro-grass effect, as illustrate by the inset in Figure 7d. Due to the serious sidewall slope issue for sample #2 (e.g., inverted cone shape or collapse of the micro-pillars), its anisotropic property can be completely ruined (see the inset in Figure 7c). The blue dotted lines in Figure 7b-d illustrate that the measured transmission amplitude spectra for x- and y-polarized THz waves are of similar line shape, and the phase difference between the two crossed-polarizations is only about 20\({}^{\circ}\). Both facts illustrate that the serious sidewall slope issue almost totally destroys the anisotropic property of the designed meta-atom. Here, since the presence of the micro-grass effect and the sidewall slope issue are too complex to numerically simulated, we cannot compare the experimental results with the simulated ones for samples #2 and #3. From our optical characterization, it can be concluded that our tri-layered silicon meta-atom design is an ideal candidate to design transmissive THz meta-devices, which simultaneously exhibit high transmittance with a large enough transmission phase coverage. Additionally, the adopted fabrication methods, especially the balanced etching process, can help to realize silicon metasurfaces with high AR.