Abstract
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.
Highlights
In recent years, an integrated platform that makes use of high-resistivity silicon photonic crystal slabs has been demonstrated in the terahertz range.[1,2,3,4,5] This platform confines radiation by exploiting a photonic bandgap material, which in this case is implemented using a lattice of through-holes in the silicon slab
The index cannot be reduced to unity, as we must maintain enough silicon such that the antenna remains mechanically stable. With these constraints in mind, the maximum hole diameter is set to 110 μm, which yields an edge index that ranges from n0 = 1.2 to 1.35 across the operation bandwidth
In aid of mechanical stability, we extend the effective medium at the circumference of the Luneburg lens such that it meets with the photonic crystal, thereby uniting the two components within a single dielectric slab
Summary
An integrated platform that makes use of high-resistivity silicon photonic crystal slabs has been demonstrated in the terahertz range.[1,2,3,4,5] This platform confines radiation by exploiting a photonic bandgap material, which in this case is implemented using a lattice of through-holes in the silicon slab. We have demonstrated a terahertz dielectric resonator antenna that was integrated directly into a photonic crystal waveguide.[37] Both the waveguide and the antenna were fabricated from a single silicon slab, resulting in a highly compact and efficient antenna that is free from frequency-scanning. The Luneburg lens is integrated directly with seven photonic crystal waveguides, resulting in an efficient multi-beam antenna with an operation bandwidth from 320 GHz to 390 GHz. Aside from field confinement, the use of photonic crystals provides benefits to mechanical stability, which renders the device entirely free-standing and self-supporting. This, combined with the fact that a terahertz wavelength is smaller than a millimeter, results in a highly manageable and practical antenna
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