# High-Aspect-Ratio Silicon Metasurfaces: Design, Fabrication, and Characterization. _Applied_
## 1 Introduction  
The arbitrary control on a terahertz (THz) wave is highly desired due to tremendous application demands, such as biological sensing, security, information communications, etc. However, traditional THz devices usually suffer from issues such as low working efficiency, bulky sizes, limited wave-control functionalities and difficulties for optics integration. The inherent reasons are due to the weak interactions between THz waves and their weak interaction with naturally existing materials, which exhibit limited variation range in permittivity and permeability. Metasurfaces, 2D metamaterials constructed from planar subwavelength units (e.g., meta-atoms) with pre-designed electromagnetic (EM) responses globally arranged in certain sequences, have attracted immense research interest due to their strong capabilities in manipulate EM waves at different wavelength ranges. Many fascinating wave-manipulation effects have been demonstrated with metasurfaces exhibiting certain phase distributions [1, 2], such as anomalous deflection [3], conversion from propagating waves to surface waves [4], meta-holograms [5, 6], meta-lens imaging [7, 8, 9], polarization manipulation [10, 11, 12] and many others [13, 14, 15, 16, 17, 18, 19, 20]. Metasurfaces were also proposed as ultra-thin and efficient THz devices, such as wave-plates [21], meta-lenses [13, 22], beam deflectors [23],THz special beam generators [24, 25] and so on [26]. However, THz meta-devices made from metals are usually of limited working efficiencies due to inevitable metallic losses. To overcome such an issue, researchers proposed to use low-loss dielectric materials to construct metasurfaces, and realized many high-efficiency meta-devices in different frequency regimes [7, 8, 14, 15, 27]. Some high-efficiency polarization controls of propagation wave, plane waves and guided waves [10, 11, 12] have been achieved with all-dielectric meta-surfaces. At THz frequencies, silicon has been considered as an ideal material to construct meta-devices, thanks to its low-loss properties and mature COMS fabrication technology. Through reviewing the performances of silicon meta-surfaces realized so far in the THz regime [17, 24, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38], most of them still suffer from the issues of low working efficiency and narrow working bandwidth, especially for transmissive meta-devices. The inherent reasons are that the adopted meta-atoms are usually not well designed since an optimized design is difficult to realize due to restrictions on sample fabrication. As discussed in our recent work [39], in order to realize the desired wave-front manipulation on THz waves, each meta-atom should exhibit a wave-range of phase modulation and a high transmission efficiency simultaneously, which requires that the constitutional silicon meta-atom should possess a high aspect-ratio (AR) and a good fabrication quality. Unfortunately, making such high-quality silicon meta-structures is challenging with current fabrication technologies. Here, we develop a fabrication technology to realize high-performance silicon meta-devices for controlling THz waves with diversified functionalities. We first design a silicon meta-atom consisting of a high-AR micro-pillar (see Figure 1a), and then systematically study how the fabrication quality can affect the final performance of a meta-device constructed from such meta-atoms. We next examine how two crucial steps in deep-reactive-ion-etch (DRIE) technology [40, 41, 42] (i.e., the etch phase and the passivation phase) influence the quality of the fabricated structure, based on which we develop an optimized Bosch process to fabricate silicon meta-devices with the best qualities by balancing the two working phases. Finally, we employ the developed Bosch process to fabricate a silicon THz meta-device with an AR of 12:1, and experimentally characterize its polarization-control performance.