Dispersion-engineered metasurfaces for spoof surface plasmon polaritons at terahertz frequencies

Abstract

The field of photonic integration in system-on-a-chip devices (SoC) has become an enormous focus of research in recent years. Such highly integrated systems are not only of utmost importance for the development of highly integrated smart sensors, but also play an increasingly important role in communication technology, for example in the miniaturization of antenna systems in mobile devices. In many cases, photonic integration relies on the exploitation of the strong energy confinement of surface plasmon polaritons (SPPs) that allow the transport of energy from point to point within highly restricted (electromagnetic) space. While SPPs can be readily generated on flat metal surfaces at optical frequencies, it is much more difficult to obtain and observe SPPs in the terahertz frequency range. The reason for it is the lack of dispersion at terahertz frequencies, which inhibits a strong confinement of the SPPs. In order to increase the energy concentration of SPPs at lower frequencies, it is necessary to engineer the dispersion of the metal surfaces by geometric structuring. In the most general sense, such surfaces can be understood as metasurfaces. The propagating surface waves on such surfaces mimic the properties of SPPs and are called spoof surface plasmon polaritons (SSPPs). Here, we report the design, fabrication and experimental testing of dispersion-engineered metasurfaces that guide and manipulate SSPPs at will. Specifically, we investigate both the out-of-plane and in-plane confinement of the SSPPs during propagation along straight and curved routes on the metasurface. For this purpose, we tailored metasurface pathways of different width and shape. The metasurfaces were fabricated in the cleanroom. The electric field of the propagating SSPPs was measured in amplitude and phase by terahertz near-field spectroscopy, so that we obtained full information about the temporal dynamics of the SSPP propagation. We observed that the SSPPs maintained a strong out-of-plane and in-plane field confinement over the complete propagation distances of several millimeters. In addition, we designed patch antenna arrays that enable a frequency-selective coupling of SSPPs to free space. In combination with specifically implemented sensor properties of the metasurface structure, such combined metasurface/antenna systems can serve as smart, compact SoC devices in the terahertz frequency range.