Computational Imaging

Computational Imaging

Computational imaging treats acquisition and reconstruction as a coupled system. Instead of asking the hardware to form a complete image directly, the experiment records a set of fields, intensities, projections or coherent echoes from which the desired quantity is estimated. In the terahertz range, this approach can compensate for limited detector formats, constrained numerical aperture or the difficulty of measuring phase directly.

The portfolio includes several distinct inverse problems. Tomographic methods reconstruct a volume from angular or depth-dependent measurements. Phase-retrieval methods estimate a complex wavefront from intensity data recorded in one or more planes. Synthetic-aperture processing uses coherent measurements over a trajectory to improve lateral resolution, while shape-from-focus estimates surface depth from a stack of differently focused images.

The algorithms must remain tied to their measurement assumptions. A reflection phase-retrieval experiment at 0.287 THz used complementary lock-in settings to extend usable signal quality before reconstructing a surface-height map. A later multiplane method inpainted saturated detector regions and recovered amplitude and phase comparable to a high-dynamic-range reference. These are examples of acquisition-aware reconstruction, not evidence that computation removes the need for dynamic range, calibration or validation.

Tomography, holography, phase retrieval, SAR, inverse problems, and signal processing designed for terahertz data.

Computational 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…

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