A research group at Institut national de la recherche scientifique (INRS) in Quebec has achieved broadband terahertz (THz) signal processing by directly engineering the wire surfaces of metal-wire waveguides. The approach allowed multiscale-structured Bragg gratings to be directly etched on metal wires without the need to introduce additional materials. When paired with diverse waveguide designs, the INRS team’s approach could provide a structurally simple platform for achieving exceptional signal processing capabilities in the THz regime. This is the first time such an approach has been applied to THz frequencies, the researchers reported. The team was led by Roberto Morandotti. “By engraving judiciously designed grooves with multiscale structures directly on the metal wires, we can change which frequencies are reflected or transmitted — that is, a THz Bragg grating — without adding any material to the waveguide,” researcher Junliang Dong said. The approach succeeded because THz guidance in metal-wire waveguides is based on the propagation of THz surface plasmon polaritons (SPPs) along the metal-air interface — which is extremely sensitive to the conditions of the metal surface. The metal-wire waveguides offer structural simplicity and bendability, and their affinity to cables makes connections efficient and straightforward. Scientists who participated in the research on metal-wire waveguides for broadband terahertz signal processing and multiplexing, from left: Roberto Morandotti, Junliang Dong, Giacomo Balistreri, and Pei You. The team envisions that its platform can be applied for multichannel transmission of uncompressed ultrahigh-definition videos, and ultrahigh-speed short-distance data transfer between THz network elements. Courtesy of Robin Helsten. As a proof of concept, the researchers demonstrated a versatile metal-wire waveguide topology called a four-wire waveguide (FWWG). The FWWG geometry supports the low-loss and low-dispersion propagation of polarization-division multiplexed THz signals. The FWWG can sustain two independent, orthogonally polarized modes. Since the modes do not interfere with each other, the FWWG is able to act as a broadband polarization-division multiplexer. By integrating Bragg gratings on metal wires into the FWWG, the researchers demonstrated the independent manipulation of polarization-division multiplexed THz signals. The FWWG device could provide a way to enable polarization-division multiplexing in waveguides, while processing multiplexed signals over a broad THz frequency range. It could support the independent manipulation of signal channels multiplexed in both frequency and polarization, the researchers believe, and could enhance the capacity of THz systems, eventually achieving data rates of about Tb/s in future THz networks. “Our device represents the first THz waveguide architecture, with a new metal-based design, which supports polarization-division multiplexing,” Morandotti said. “In particular, the capability of realizing such a complex signal-processing functionality — that is, the independent manipulation of multiplexed THz signals — has never been achieved elsewhere.” Further, the approach to the manipulation of THz pulse propagating within metal-wire waveguides could inspire new ways to boost the capacity and spectral efficiency of future THz networks — providing a path, for example, to “holographic messaging” in 6G networks. It also provides a universal platform for attaining broadband THz signal processing. It could, for example, lead to the multichannel transmission of uncompressed ultrahigh-definition video; ultrahigh-speed, short-distance data transfer between devices; and chip-to-chip communications. The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-27993-7).