Tunable Frequency Selective Surfaces for Terahertz Applications

Between the microwave and infrared spectrums, the range of 0.1 to 10 terahertz (THz) is often referred as the “terahertz gap” as there are relatively few commercially available sources and detectors for this region. While still in its infancy, terahertz technologies have entered a phase of unprecedented interest and expansion, offering opportunities for new engineering paradigms to fill the void between the lower frequencies of electronics and the higher frequencies of photonics. This emerging field has been recognized as of extreme importance for many scientific and engineering applications in future high data-rate wireless communications, security screening, chemical and biological spectroscopy and sensing, and inspection and quality control in manufacturing, etc.. These applications call for novel research activities and technological developments such as THz tunable signal sources and filters, which are the major efforts in the State Key Laboratory of Millimeter Waves at City University of Hong Kong.We have been continuously working on microwave and optical filters in the form of frequency selective surfaces (FSSs) for over 25 years. Our review article published in the Proceedings of IEEE in 1988 remains one of the most popular Proceedings articles with 355 citations per Xplore and 12,635 downloads. Dr. Alexander B. Magoun, IEEE Historian, has recently written an article to be published in the Proceedings that focuses on the history of this work. With the acquisition of the state-of-the-art equipment, it is the time for us to take advantage of our rich experience to work on tunable filters in the terahertz regime.Tuning of the frequency response of FSSs can be accomplished by thermal, optical, electronic or magnetic external stimulus. Each of the tuning approaches has different tuning range, fabrication process, cost, and area of application. A pilot study has been carried out in cooperation with Dr. Wei-Chih Wang of the University of Washington on using a fishnet FSS configuration with the substrate consisting of polymer dispersed liquid crystal (PDLC) encapsulated in a thin polyimide layer. The voltage-controllable FSS was fabricated and tested at CityU, achieving a tunable range of 10 GHz at the resonant frequency of 0.87 THz. While the concept of tunable FSS with PDLC has been proven and a negative refractive index of the fishnet FSS achieved, the results obtained also revealed some of the underlying problems that need to be addressed, such as the narrow tuning range, breakdown of the PDLC at certain locations of the fishnet FSS, etc. In this project, we propose to study electrical properties of different FSS unitcell geometries, different weightings of polymer and liquid crystal, pre-alignment of liquid crystal in the PDLC matrix, optimization of layer thickness and fabrication processes to improve the tuning range and lower the tuning voltage and creating new devices such as patternless gradient structure for wave shaping and manipulation in THz. The fabricated designs will be tested with our Agilent PNA-X vector network analyzer with 0.75 to 1.1 THz extension module form Virginia Diodes and EKSPLA terahertz spectrometer up to 3.5THz.Based on our preliminary results, we strongly believe that the proposed research will contribute to the development of THz technology for real-world applications, and could create the basis for a new THz technology industry in Hong Kong.