Deeply Subwavelength Metasurface Resonators for Terahertz Wavefront Manipulation

Metasurfaces offer a highly flexible platform for controlling the propagation and localization of electromagnetic waves. Due to the relatively large size of commonly used resonators, various undesirable effects including spatial dispersion and spurious diffraction occur, thus limiting the metasurface performance. To overcome these problems, one straightforward approach is to utilize deeply subwavelength metaunits. In contrast to conventional approaches that minimize the resonator size by reshaping the metallic patches, the capacitive gaps are reshaped, an approach which is more robust to material loss, minimizing the problem of overdamping. As an example, a novel design based on interdigital capacitors (meander gap) is introduced with extremely subwavelength gaps for use in the terahertz frequency range. The size of the new resonator can be reduced to below lambda/30 in a reflective-type terahertz metasurface, while maintaining the 2 pi phase shift required for full wavefront control. Using an advanced electron-beam lithography technique, a proof-of-concept experiment is performed and a 5 mm x 5 mm beam deflector is fabricated, with the capacitive gaps as small as 300 nm (approximate to lambda/1130). The device performance is characterized using angle-resolved time-domain spectroscopy. The study provides useful insight for ultracompact metadevices based on deeply subwavelength metaunits working at terahertz frequencies and beyond.