The terahertz range holds potential for ultra-wideband wireless communications applications, including 6G and beyond, due to large and under-utilized spectral bandwidth. Terahertz communications will benefit from channelization as individual channel bandwidths will be reduced, simplifying baseband and intermediate-frequency (IF) circuits. Furthermore, passive multiplexers enable power combining of several independent sources at different frequencies, which is beneficial to terahertz technology because factors including limited power and strong path loss restrict the achievable transmission distance.
Researchers have introduced the concept of gratingless tunneling multiplexers and employed it to realize a diplexer, triplexer, and 4×1 multiplexer for terahertz waves—all of which are both highly compact and efficient owing to in-slab beamforming technique.
The simplicity of this technique offers versatility. A designer may specify a desired in-slab magnitude distribution mediated by a single physical parameter: waveguide-to-slab separation. Furthermore, the curvature of the waveguide may be optimized, e.g., using Gerchberg–Saxton hologram-like techniques to project near-arbitrary beam shapes into the slab.
The development of all-dielectric platforms for terahertz waves opens the door to established passive nanophotonic multiplexers, such as arrayed waveguide gratings (AWG). Devices of this sort employ a dielectric slab as a free-propagation region to which several waveguides may connect. However, AWGs are electrically large, requiring two free-propagation regions connected in parallel by an array of curved waveguides.
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