THz-TDS
A THz-TDS instrument generates a short terahertz transient and samples its electric field as a function of time. Because the field is measured coherently, the Fourier-domain result contains both amplitude and phase. This supports the estimation of delay, refractive index, absorption and complex dielectric properties over a broad band.
The temporal waveform can also separate reflections from different interfaces in a layered sample. The achievable separation depends on pulse duration, bandwidth, material index and signal-to-noise ratio. In strongly absorbing or irregular samples, later echoes may be attenuated or mixed with reverberation, so interpretation requires an explicit propagation model and reference measurement.
THz-TDS is therefore both a spectroscopic and an imaging technology. Its value lies in the amount of physical information contained in the waveform, while its practical limits arise from acquisition time, alignment, atmospheric absorption and the need for stable phase-sensitive calibration.
Broadband pulsed terahertz measurement of amplitude and phase for spectroscopy, imaging, thickness analysis, and material characterization.
THz-TDS 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
- Propagation beam consideration for 3D THz computed tomography — DOI
The study introduces a new physical model that captures the real behaviour of terahertz (THz) radiation when used for three-dimensional tomographic imaging. Unlike conventional X‑ray methods that treat the beam as a straight, uniform ray, the authors model the THz pulse as a Gaussian beam whose intensity spreads during propagation. This model is incorporated into a realistic acquisition simulator, allowing researchers to predict how the beam will illuminate an object from different angles and to produce more accurate projection data—sinograms—than those obtained with the…
- Aeronautics composite material inspection with a terahertz time-domain spectroscopy system — DOI
- Review of Terahertz Tomography Techniques — DOI
Terahertz (THz) imaging exploits the unique ability of 0.3–10 THz radiation to penetrate a wide range of dielectric materials while providing spectroscopic fingerprints of many chemical species. The reviewed work demonstrates that, beyond conventional two‑dimensional transmission or reflection pictures, a suite of tomographic techniques—computed tomography, tomosynthesis, time‑of‑flight, diffraction tomography, holography, Fresnel‑lens depth encoding, synthetic aperture, and time‑reversal—can reconstruct three‑dimensional internal structures with sub‑millimetre resolution. Each method offers distinct trade‑offs: CT delivers full volumetric data but is limited by absorption and slow acquisition; tomosynthesis provides…
- Ordered subsets convex algorithm for 3D terahertz transmission tomography — DOI
This research introduces a practical, high‑performance technique for three‑dimensional terahertz (THz) tomography that is designed to meet the stringent demands of non‑destructive inspection in industrial and cultural heritage contexts. The method refines the maximum‑likelihood reconstruction framework originally developed for X‑ray computed tomography, integrating a realistic Gaussian beam propagation model that captures THz diffraction and intensity variation across the sample. By incorporating direct measurements of the system’s blank‑scan background and dark‑field signals into the algorithm, the approach delivers robust estimates of material attenuation without the…
- Low-frequency noise effect on terahertz tomography using thermal detectors — DOI
Terahertz computed tomography (THz‑CT) offers a powerful, non‑contact imaging modality for security screening, material inspection and biomedical diagnostics, yet its practical deployment has been limited by the low photon energy of terahertz waves and the high noise levels inherent to thermal detectors. This study investigates how low‑frequency (pink) noise, which dominates the detector output at frequencies below a few hertz, degrades the quality of 3‑D reconstructions obtained with pyroelectric and Schottky diode sensors. By recording real noise traces from a continuous‑wave millimeter‑wave scanner and…