Terahertz Spectroscopy of Granular Materials

Granular media—collections of discrete particles whose dimensions are comparable to terahertz wavelengths—present unique challenges and opportunities for spectroscopic analysis. In these systems, multiple scattering, diffraction and internal reflections can mask intrinsic absorption features, complicating the interpretation of raw spectra. Terahertz spectroscopy, particularly time-domain and frequency-domain modalities, probes low-frequency vibrational modes and dielectric responses of materials, yielding insights into composition, morphology and packing structure. Recent instrumental advances, including ultra-broadband emitters, high-dynamic-range detectors and multi-channel coupling schemes, have extended measurement sensitivity and spatial resolution. Parallel progress in computational methods—employing wavelet multiresolution analysis, cepstral filtering and statistical metrics—now enables the separation of true material resonances from scattering-induced artifacts. Applications span pharmaceutical quality control (where granule size and moisture affect drug performance), geological and soil analysis, additive manufacturing inspection and stand-off detection in security and environmental monitoring. By integrating experimental design with sophisticated signal processing, terahertz spectroscopy is emerging as a versatile, non-destructive tool for characterising granular materials in both laboratory and field settings.

Research from Nature Portfolio

One landmark study has introduced a diffuse terahertz spectroscopy approach for highly scattering granular samples, combining wavelet multiresolution analysis with a novel bimodality coefficient spectrum. By quantifying skewness and kurtosis across spectral images, researchers achieved simultaneous material identification at detection angles up to 90°, attaining over 95% accuracy without requiring prior knowledge of sample signatures. This advance demonstrates that incoherent, diffused terahertz beams can yield reliable absorption spectra in turbid, particle-rich media and paves the way for flexible emitter–detector geometries in complex real-world environments.

Terahertz Spectroscopy of Granular Materials publication trend

The graph below shows the total number of articles in terahertz spectroscopy of granular materials across all publications each year (not limited to Nature Index journals).

Technical terms

Terahertz time-domain spectroscopy (THz-TDS): A method that generates broadband terahertz pulses in the time domain and measures material responses to extract frequency-dependent absorption and refractive index.

Wavelet multiresolution analysis: A technique for decomposing a signal into components at multiple scales, enabling the isolation of absorption features from noise and scattering artifacts.

Bimodality coefficient spectrum: A statistical tool combining skewness and kurtosis across spectral images to differentiate true absorption signatures from multiple scattering effects.

Cepstrum filtering: A signal-processing approach that converts spectral data into the quefrency domain to separate and attenuate scattering-induced echoes, enhancing the clarity of intrinsic resonances.

References

1. Diffuse terahertz spectroscopy in turbid media using a wavelet-based bimodality spectral analysis. Scientific Reports (2021).
2. Mitigating the effects of granular scattering using cepstrum analysis in terahertz time-domain spectral imaging. PLOS ONE (2019).
3. Chemical Identification in the Specular and Off-Specular Rough-Surface Scattered Terahertz Spectra Using Wavelet Shrinkage. IEEE Access (2021).
4. Translation-Invariant Zero-Phase Wavelet Methods for Feature Extraction in Terahertz Time-Domain Spectroscopy. Sensors (2022).