Terahertz Imaging
Terahertz imaging is not a single modality. A raster-scanned time-domain system, a coherent FMCW radarFMCW radar transmits a continuously swept frequency and measures the beat signal produced by delayed reflections, enabling distance, thickness, and depth-resolved imaging with compact coherent hardware. More, a full-field camera and a near-field microscope record different observables and operate at different spatial scales. The choice of architecture therefore begins with the contrast of interest: an interface echo, a thickness variation, a complex refractive index, a local absorption change or the morphology of an internal structure.
Work carried out at IMS spans this range of geometries. At 2.5 THz, real-time full-field data have been used for volumetric reconstruction and subsequent segmentation, component labelling and skeletonization. In another direction, near-field systems have bypassed the millimetre-scale diffraction limit of conventional far-field imaging in tightly controlled sample configurations. These examples illustrate complementary capabilities rather than interchangeable levels of performance.
The central experimental question is not simply whether an object can be seen, but whether the observed contrast remains stable under changes in alignment, surface condition, water content, thickness and acquisition settings. A robust imaging workflow therefore includes calibration, artefact analysis and comparison with an independent description of the sample.
Contactless imaging with pulsed, continuous-wave, camera-based, and radar systems, from experimental acquisition to interpretation.
Terahertz Imaging across the measurement chain
The workflow can include requirement definition, instrument selection or development, calibration, acquisition, signal processing, reconstruction, and interpretation. The first decision is rarely the choice of an instrument. It is the identification of the physical quantity that could answer the research question: an interface delay, a spectral feature, a complex refractive index, a local field component, a surface profile, or a volumetric morphology.
Once that quantity is defined, source bandwidth, detector architecture, numerical aperture, scan geometry, dynamic range, sample environment, and reference measurements can be considered together. This system-level approach is particularly important in the terahertz range, where propagation loss, diffraction, atmospheric absorption, coherent artefacts, and material dispersion may all influence the same dataset.
Calibration, interpretation and validation
A capability is meaningful only when its limits are explicit. Work therefore asks which contrast mechanism is physically interpretable, what bandwidth and geometry are required, how repeatability is measured, and which independent method can serve as a reference. Reconstruction may improve access to phase, depth, or morphology, but it does not remove the need to test model assumptions and uncertainty.
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…