High-Performance Terahertz Antireflective Structures on Silicon

• Applicants: Dr. Benjamin Röben; Dr.-Ing. Thomas Siefke
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 563444588

Terahertz (THz) technology has come a long way from first demonstrations to applications in fundamental research and industry, and is expected to play a major role in wireless communications beyond 5G and in the bio-medical sector. However, further progress is hampered by the lack of effective methods to reduce the surface reflectance of THz optics, which are required for virtually all THz devices and instruments. This limitation stands in sharp contrast to the visible spectral range, where antireflective coatings are essential for optical technologies. For high-resistivity silicon - one of the best materials for THz optics - the problem of surface reflections is particularly severe. Since antireflective coatings have been of limited success in the THz range, our approach for the reduction of surface reflections is based on fabricating antireflective structures into the surfaces of silicon-based THz optics. Within this project, we will lay the foundations required for the design and fabrication of THz antireflective surfaces with peak efficiencies and the broadest-possible spectral bandwidths. For that purpose, we will compare three different approaches to antireflective structures on silicon with respect to their performance. To fabricate these antireflective surfaces, we will employ inductively coupled plasma reactive ion etching - one of the most powerful techniques to structure silicon. We will enable the fabrication of the envisioned structures by further developing etching processes beyond the state of the art so that the required deep trenches, high aspect-ratios and sophisticated sidewall profiles of the structures can be achieved. The THz characterization will be carried out using Fourier transform and time-domain spectroscopy. While the capabilities for the characterization in transmission mode at normal incidence are already available, a metrology-grade goniometric setup will be designed and built which enables us to reliably measure the transmittance and reflectance at variable angles of incidence and angles of emergence/reflection. As an important goal of the project, we aim for a comprehensive understanding of the relationship between structure and antireflection performance through modeling and full-wave simulations, which are validated by structural characterizations and the metrology-grade THz measurements of the samples. This understanding will lead to optimal antireflective structures for THz optics, which will act as a catalyst for the progress of THz technology towards unprecedented performance and novel applications. Furthermore, the progress in reliable THz characterizations will be beneficial for a variety of solid samples, while the advancements in silicon fabrication technology and the comprehensive understanding of antireflective surface structures will be highly valuable even beyond the THz range.

• DFG Programme: Research Grants