KEYWORDS: Surface plasmon polaritons, Radio propagation, System on a chip, Wave propagation, Terahertz spectroscopy, Telecommunications, Metals, Dispersion, Antennas, System integration
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.
KEYWORDS: Surface plasmon polaritons, Terahertz spectroscopy, Terahertz radiation, Scattering, Terahertz technology, System on a chip, Spectroscopy, Radio propagation, Phase measurement, Near field
In the last decades, highly integrated electronic circuits have paved the way towards compact, smart electronic devices with excellent computing power. Considering the benefits of electronic integration, it is compelling to apply similar integration methods to shrink the size of photonic on-chip devices in the terahertz and optical regime. Here, we numerically and experimentally investigate the guiding, routing and manipulation of strongly confined spoof terahertz surface plasmon polaritons (terahertz SSPPs) on metasurface pathways. The pathways are composed of single-, two- or three-cut wires that define the subwavelength width with respect to the SSPP wavelength. We measured the spatio- and spectro-temporal dynamics of the electric field of the SSPPs by electro-optic imaging. We observed that the terahertz SSPPs exhibit a strong out-of-plane and in-plane confinement, even when they propagate on curves of subwavelength path width. The spatio- and spectro-temporal behavior of the terahertz SSPPs evidences that they can be tightly guided within subwavelength space on metasurfaces without loss of the out-of-plane confinement. Due to these beneficial electromagnetic properties, metasurface pathways of subwavelength width seem to be ideally suited for the implementation of on-chip terahertz networks and sensor systems.
Integrated circuits revolutionized electronics long time ago and paved the way towards minimized microprocessors today. In analogy, plasmonics aims at the creation of highly integrated optical networks on a small chip that enable the implementation of ultra-small sensors or optical processors. In the terahertz frequency regime, we investigate the propagation of tightly bound pure surface waves on specifically designed meta-surfaces. While most presented metasurfaces on a thin film in the literature support waveguide mode propagation in the thin film substrate, whose evanescent electromagnetic fields form the surface waves at the waveguide boundaries, we observed pure surface waves that are not coupled to a waveguide mode in the thin film. Such meta-surfaces are particularly advantageous for use as surface sensors, since the surface waves carry most of their energy in the space between the surface and air and almost no energy in the thin film substrate. This is in strict contrast to most of the presented meta-surfaces in literature so far, which guide a significant part of unusable energy in the inaccessible region of the substrate. Furthermore, we study structures of Goubau lines and meta-surfaces that combine excellent spectrally broadband terahertz surface wave guiding with frequency-selective meta-surface areas and meta-surface sub-wavelength resonators on a chip. In detail, we investigate the coupling efficiency between Goubau lines and meta-surfaces.
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