Terahertz (THz) waves encompass the 0.1–10 THz frequency band and combine the advantages of both photonics and electronics, offering significant potential applications in high-speed wireless communications, security inspection, medical diagnosis, THz imaging, and other fields. However, traditional THz devices suffer from inherent drawbacks such as bulky size, low efficiency, and limited bandwidth. Although all-dielectric metasurfaces can achieve wavefront manipulation in subwavelength scale, high-aspect-ratio deep etching processes for creating them usually introduce various structural defects such as sidewall tilting, dimensional deviations, and etching depth errors. These issues restrict device performance and practical application. Therefore, achieving high-efficiency, broadband, and high-fidelity terahertz wave manipulation at the subwavelength scale, while overcoming high-precision micro/nanofabrication challenges, has become a critical issue that needs attention in this field.
Recently, the team led by Professor Shulin Sun from Fudan University, in collaboration with the team led by Professor Junsuk Rho from Pohang University of Science and Technology (POSTECH), published a research paper entitled "High Aspect-Ratio Meta-device for Broadband and High-Efficiency Terahertz Wave Manipulation" in Light: Advanced Manufacturing. This work advances the fabrication technology for high-quality, high-yield deep silicon etching. They successfully produce multiple high-aspect-ratio silicon-based metasurface devices, enabling broadband, high-efficiency, and customizable polarization and wavefront manipulation of terahertz waves. These advancements offer essential device solutions for applications such as terahertz communications, security inspection imaging, and biosensing.
This study presents a design and fabrication method for high-performance terahertz all-dielectric metasurface devices. The research team systematically analyzed the influence of etching depth deviation and lateral morphology distortion in the Bosch process on device performance, and established an error compensation mechanism. By finely tuning the etching–passivation cycle parameters, the team successfully fabricated high-aspect-ratio silicon-based meta-atoms with vertical sidewalls and precise dimensions, along with excellent fabrication repeatability, achieving a maximum aspect ratio of 19.2:1. To address the pattern shrinkage caused by photoresist baking, the team pre-compensated the lateral dimensions at the layout design stage, significantly improving the device fabrication yield and process stability. To address the high reflectivity of silicon, the team introduced antireflective silicon-based microstructures at the bottom of the structures. By optimizing parameters such as thickness and width, the overall reflection loss was effectively reduced. This synergistic design significantly improved the device transmission efficiency and increased the operating bandwidth while ensuring a large phase modulation range.
Based on the above design and fabrication techniques, the team successfully fabricated several metasurface wave-plates and achieved broadband, high-efficiency polarization manipulation in the terahertz band. The fabricated metasurface half-wave plate operates stably within the 0.6–0.8 THz bandwidth, with a relative bandwidth of 28.57% and a peak polarization conversion efficiency as high as 0.915 at 0.73 THz; the metasurface quarter-wave plate covers an operating bandwidth of 0.5–0.8 THz, with a relative bandwidth of 46.15% and a peak polarization conversion efficiency approaching 0.99 at 0.66 THz. The experimental results agree well with the simulations, validating the reliability of the design and fabrication.
Building on the Pancharatnam–Berry geometric phase mechanism, the team further developed a metalens with an aperture size of 10 × 10 mm² and a numerical aperture (NA) of 0.8. Over the 0.6–0.8 THz operating band, the metalens achieved an average focusing efficiency—defined as the ratio between the integrated power carried by the focal beam carrying the cross-polarization and that of transmitted THz beam—of 85.56%. Experimental measurements, full-wave simulations, and theoretical predictions were in excellent agreement, confirming the superior performance of the fabricated silicon-based metasurface devices for terahertz wavefront manipulation. This approach can be extended to the design and fabrication of other optical metadevices for terahertz wavefront control.
This study introduces a fabrication strategy for highly stable, high-quality, high-aspect-ratio silicon-based terahertz metasurfaces. Two fundamental types of functional devices for terahertz manipulation, meta-wave-plates and metalens, were successfully demonstrated. These devices offer several advantages, including broadband operation, low loss, high efficiency, and excellent reproducibility. This research advances the high-precision fabrication of terahertz metadevices and strongly supports the transition of metasurfaces from laboratory demonstrations to engineering applications. These achievements are expected to play a significant role in various scenarios, such as terahertz communications, imaging, security inspection and nondestructive testing, as well as biomedical sensing.
