Waveguides
Guided terahertz systems replace part of the free-space path with a controlled propagation structure. This can simplify coupling, bring the sensing head into a confined region and support probe-like measurements at a distance from the source and receiver. The guide, however, becomes part of the measurement response and must be characterised for attenuation, dispersion and modal content.
Work on Sommerfeld waves investigated coupling and propagation on wires at 100 and 300 GHz. Differential or discontinuous phase structures were used to couple a free-space beam to a wire mode, enabling local access to the longitudinal electric field. Related microscopy experiments reported subwavelength resolution on metallic test structures.
A later pulse-reflectometry architecture combined two photoconductive antennas with a hollow-core silica guide. Guided FMCW configurations and a solid-immersion element at the guide end were also studied for imaging and sensing in constrained environments. These results position waveguides as an integration strategy, while making propagation calibration inseparable from image interpretation.
Guided propagation, Sommerfeld-wave structures, passive probes, and reflectometry approaches at terahertz and millimeter-wave frequencies.
Waveguides in the terahertz measurement chain
This technology forms one part of a larger measurement chain that includes sources, detectors, optics or antennas, positioning, acquisition, calibration, and data processing. Its value depends on how well those elements are matched to the sample and to the information that must be recovered.
Performance figures must therefore be read in context. Frequency range and bandwidth affect material contrast and depth resolution; aperture and working distance affect lateral resolution; dynamic range determines which weak interfaces remain measurable; and acquisition strategy controls speed, stability, and the amount of data available to a reconstruction algorithm.
Design constraints and performance limits
Propagation, coupling losses, coherent reflections, dispersion, alignment, and calibration can dominate an experiment even when the individual components perform well. Research on this technology combines modelling and measurement so that limitations are identified rather than hidden by post-processing. The final criterion is whether the recovered quantity remains reproducible and useful for the intended scientific or application question.
Related publications
- Linear to radial polarization conversion in the THz domain using a passive system — DOI
The work presents a compact, passive device that transforms a conventional linearly polarized terahertz (THz) beam into a radially polarized one, a field configuration that offers superior focusing, enhanced longitudinal fields, and improved coupling to near‑field probes. By adapting a proven optical mode‑selection technique to the THz regime, the authors employ a circular metallic waveguide that supports only the fundamental TE11 and the radially polarized TM01 modes. A discontinuous phase element placed at the waveguide entrance inverts the polarization over half the beam, converting…
- Theoretical and experimental studies of metallic grids absorption: Application to the design of a bolometer — DOI
The study delivers a comprehensive, validated framework for designing metallic grid absorbers with precisely tailored electrical resistivity, enabling the creation of efficient, room‑temperature bolometers and other thermal detectors. By treating structured metal layers as equivalent homogeneous films whose resistivity depends on geometry, the authors derived analytical expressions for transmission, reflection, and absorption that incorporate skin‑depth effects and diffraction when the grid period approaches the wavelength. Numerical simulations and experimental measurements at 0.3 THz and in the RF band confirm the model’s accuracy, demonstrating that…
- Near-field wire-based passive probe antenna for the selective detection of the longitudinal electric field at terahertz frequencies — DOI
The work presents a novel passive probe antenna that can be operated at terahertz (0.1 THz) frequencies using a simple, purely passive structure. The antenna consists of a slender metal wire backed by a discontinuous phase plate that converts an ordinary linearly‑polarized free‑space beam into a radially polarized guided mode on the wire, with an estimated coupling efficiency of about forty percent. By exploiting the Sommerfeld wave that travels along the wire, the device can create a highly confined, longitudinal electric field at the…
- Continuous‐wave scanning terahertz near‐field microscope — DOI
The work reported by Guillet, Chusseau, Adam, Grosjean, Penarier, Baida and Charraut describes the development of a continuous‑wave terahertz (THz) near‑field microscope that exploits Sommerfeld surface waves guided along metallic wires. By combining differential phase plates, a Y‑splitter and a sharp, tapered needle probe, the authors created an imaging system that can be coupled to any linearly polarized THz source and detector. The key achievement is the demonstration of sub‑micrometre‑scale resolution—roughly a third of the probe tip radius, or about 10 µm—while retaining sensitivity…
- Coupling and Propagation of Sommerfeld Waves at 100 and 300 GHz — DOI
The study demonstrates that millimetre‑wave guided modes—known as Sommerfeld waves—can be efficiently launched and transported along simple metallic wires at 100 GHz and 300 GHz. By inserting a straightforward differential phase plate in front of the wire, the researchers achieved a theoretical coupling efficiency of about 32 percent, and confirmed experimentally a comparable value of roughly 23 percent. The wire acts as a low‑loss waveguide, with propagation losses measured at about 0.13 dB per metre for a 20 cm section, a figure that matches…