Terahertz time-domain spectroscopy is a specifically appropriated technique to analyze layered structure composition and dimension. Inverse electromagnetic problems are commonly solved to extract, from a recorded terahertz-signal, the distinctive layer dielectric properties and thicknesses. However, main origins and formation routes of the signal are left unassessed while it could be of great value to deepen and to enlarge stratified material terahertz-probing property knowledge and understanding. In this work, a new numerical method to reconstruct and to analyze terahertz experimental signals is reported. It consists of an iterative algorithm implementing a connected propagation tree where each node of the tree denotes the occurrence of the incident pulse division. Descendent pulses are individually monitored and their carried proportion of the incident power can be evaluated. Therefore, it provides a flowchart of the predominant optical paths contributing to the structure response. On this basis, a simplified global transfer function is automatically derived by the algorithm. The effectiveness of the numerical procedure is demonstrated through the reconstruction and the analysis of a reflected terahertz-signal from an aerospace coating structure with individual and different thicknesses of several tens of microns. The recovered signal is firstly discussed as a function of the sum of pulses detected at each iteration. Then, the power proportion distribution foe each iteration is studied to delimit the meaningful number of required algorithm iteration. Finally, within each iteration are sorted the most contributing optical paths and a simplified global transfer function is derived. The present algorithm allowed to identify the main propagations inside this stratified material giving arise to the recorded signal and to reduce by 98.23 the number of considered and calculated optical paths compared to the standard signal reconstruction procedure.

Terahertz-light imaging is attracting great attention as a new approach in non-invasive/non-staining biopsy of cancerous tissues. Positively, terahertz light has been shown to be sensitive to the cell density, the hydration content, and the chemical composition of biological samples. However, the spatial resolution of terahertz imaging is typically limited to several millimeters, making it difficult to apply the technology to image biological tissues which have sub-terahertz-wavelength-scale inhomogeneity. For overcoming the resolution, we have recently developed a terahertz near-field microscope with a spatial resolution of 10 µm, named scanning point terahertz source (SPoTS) microscope. In contrast to conventional far-field terahertz techniques, this microscope features the near-field interactions between samples and point terahertz sources on a sub-terahertz-wavelength scale. Herein, to evaluate the usefulness of terahertz imaging in cancer tissue biopsy in greater detail, we performed terahertz near-field imaging of a paraffin-embedded human-breast-cancer section having sub-terahertz-wavelength-scale inhomogeneity of the cancer cell density using the SPoTS microscope. The observed terahertz images successfully visualized local (~250 µm) inhomogeneities of the cell density in breast invasive ductal carcinoma. These results may bypass the terahertz limitation in terms of spatial resolution and may further motivate the application of terahertz light to cancer tissue biopsy.

Terahertz imaging is an emerging candidate to diagnose breast cancers in a label-free manner. However, detailed terahertz analysis of early stage breast cancers is difficult to achieve owing to its low spatial resolution. In this study, utilizing a probe-less terahertz near-field microscope named scanning point terahertz source microscope, we visualize an unstained comedo ductal-carcinoma-in-situ including an architectural structure (comedo necrosis) measuring ∼ϕ500 µm, which is known as highly-malignant early-stage breast cancer, in terahertz images for the first time. The outcome is a critical step toward the label-free diagnosis of single early stage cancer lesions with terahertz waves.

The angular spectrum method can be applied for the analysis of light diffraction on thin films. In model we have used the Debye coefficients instead of complex dielectric permittivity that making our calculations more rapid. In this work, we have conducted a simulation of the image reconstruction of histological slides, consisting of normal and cancer tissue.