Daniel Headland, Withawat Withayachumnankul, Ryoumei Yamada, Masayuki Fujita, Tadao Nagatsuma,
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| Luneburg lens concept, showing the reciprocal principle of operation that relates a point on the circumference to a plane wave on the opposite side. It is noted that this behavior is represented with a ray-tracing diagram, but in practice, the lens must be significantly larger than a wavelength for this to be truly valid. |
https://aip.scitation.org/doi/abs/10.1063/1.5060631
Recent years have seen the emergence of efficient, general-purpose terahertz photonic-crystal waveguides etched from high-resistivity silicon. Systems founded upon this platform will require antennas in order to interface with free-space fields. Multi-beam antennas are desirable to this end, as they are capable of interacting with a number of distinct directions simultaneously. Such functionality can be provided by Luneburg lenses, which we aim to incorporate with the terahertz photonic crystal waveguide. A Luneburg lens requires a precisely defined gradient-index, which we realize using effective medium techniques that are implemented with micro-scale etching of silicon. Thus, the photonic crystal waveguides can be integrated directly with the Luneburg lens and fabricated together from the same silicon wafer. In this way, we develop a planar Luneburg-lens antenna with a diameter of 17 mm and seven evenly spaced ports that cover a 120° field of view. Numerical and experimental characterization confirm that the antenna functions as intended over its operation bandwidth, which spans from 320 to 390 GHz. The Luneburg-lens antenna is subsequently deployed in a demonstration of terahertz communications over a short distance. The device may therefore find applications in terahertz communications, where multiple point-to-point links can be sustained by a given transceiver node. This form of terahertz beam control may also be useful for short-range radar that monitors several directions simultaneously.
