Terahertz communications is a booming field in rapid development. While in most of the existing terahertz communication systems, modulated THz carrier wave is transmitted via free-space communication channels, the THz waveguide-based integrated solutions can be of great utility both at the transmitter and receiver ends. Thus, at the transmitter end they can be used for steering, beam forming, and multiplexing of the THz signals. At the receiver end, terahertz waveguide-based solutions can be used as reliable interconnects (especially in the geometrically complex environments, ex. intra-vehicle communications), as well as for real-time analogue signal processing such as filtering and demultiplexing. More generally, waveguide-based THz optical circuits are indispensable for miniaturization and mass production of cost-effective THz communication systems. In this work, we present comprehensive numerical, fabrication and optical characterization studies of a new type of modular THz integrated circuits based on the micro-encapsulated two-wire plasmonic waveguides. Particular attention is payed to the design of optimized components such as waveguides, couplers and waveguide Bragg gratings to realize easy to handle, highly reconfigurable terahertz circuits capable of complex functionalities such as multiplexing and demultiplexing. The basic element of all the developed subcomponents is a low-loss low-dispersion two-wire waveguide suspended inside of a protective micro-sized enclosure (cage) using deeply subwavelength dielectric supports. The high resolution stereolithography 3D printing and wet chemistry metal deposition techniques are employed to fabricate such waveguides where the THz light is mainly confined in the air gap between the two wires. First, the straight waveguides are characterized using continuous-wave THz spectroscopy system with the measured transmission loss and group velocity dispersion (GVD) of 6 m-1 and -1.5 ps/THz·cm respectively at the carrier frequency of 140 GHz. Next, waveguide bends and a Y-coupler based on the two coalescing waveguide bends are studied. We find that due to the presence of a cage, the curved two-wire waveguides show smaller bending loss than the free-standing two-wire waveguides of similar geometry. Additionally, we find that relatively tight bends of ~5cm-radius can be well tolerated by adding less than ~10 m-1 propagation losses to the curved waveguide propagation loss. Next, we design and fabricate the two-wire waveguide Bragg gratings by hot stamping a periodic sequence of metal strips onto a paper sheet and inserting it into the air gap between the two-wire waveguides. The geometry of the grating featuring a Bragg frequency of 140 GHz is studied theoretically and numerically, and the optimal waveguide gratings are then realized experimentally. Such structures can have bandwidths as high as ~20 GHz. Finally, using thus developed modular components, a two channel THz Add-Drop Multiplexer (ADM) is demonstrated for the operation at 140 GHz carrier frequency and featuring a spectral width of 2.8 GHz. We believe that the reported modular platform based on the micro-encapsulated two-wire waveguides can have a strong impact on the field of integrated optical circuits for THz signal processing and potentially sensing due to ease of device fabrication (standard 3D printers and wet chemistry), modular design and high degree of reconfigurability, low-loss and low-dispersion of the underlying waveguides, as well as high potential for the real-time tunability of the optical circuits due to ease of access of the modal fields inside the controlled in-cage environment.
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