Terahertz diffractive imaging with saturated data inpainting.

Multiplane iterative phase retrieval is a promising approach to diffraction imaging, which accurately determines the topographic and internal characteristics of various objects. Nevertheless, the detection systems used often have a limited dynamic range, resulting in overexposure of recorded intensity distributions. In this Letter, we present a novel, to the best of our knowledge, reconstruction algorithm that inpaints saturated areas on the measured intensity datasets and reliably retrieves wave complex amplitude. The proposed technique can be used in various spectral ranges, while we have tested it in the terahertz frequency range, where the problem of sources and detectors is most acute. We show that retrieved amplitude and phase distributions have a quality comparable to that of the images reconstructed from the reference high dynamic range technique. Herewith, the proposed approach seriously simplifies the process of data acquisition, what expands the possibilities in the design of measurement tools and studies of dynamic scenes.

Terahertz Nondestructive Testing with Ultra-Wideband FMCW Radar.

This paper presents the development, performance, integration, and implementation of a 150 GHz FMCW radar based on a homodyne harmonic mixing scheme for noncontact, nondestructive testing. This system offers high-dynamic-range measurement capabilities up to 100 dB and measurement rates up to 7.62 kHz. Such interesting characteristics make this system attractive for imaging applications or contactless sensing. Numerous samples of different materials and geometries were imaged by taking advantage of the radar's performance. By taking into account the nonionizing capability of the system, new applicative fields such as food industry and pharmaceutical packaging were explored.

Terahertz refractive index-based morphological dilation for breast carcinoma delineation.

This paper reports investigations led on the combination of the refractive index and morphological dilation to enhance performances towards breast tumour margin delineation during conserving surgeries. The refractive index map of invasive ductal and lobular carcinomas were constructed from an inverse electromagnetic problem. Morphological dilation combined with refractive index thresholding was conducted to classify the tissue regions as malignant or benign. A histology routine was conducted to evaluate the performances of various dilation geometries associated with different thresholds. It was found that the combination of a wide structuring element and high refractive index was improving the correctness of tissue classification in comparison to other configurations or without dilation. The method reports a sensitivity of around 80% and a specificity of 82% for the best case. These results indicate that combining the fundamental optical properties of tissues denoted by their refractive index with morphological dilation may open routes to define supporting procedures during breast-conserving surgeries.

Terahertz phase retrieval imaging in reflection.

Terahertz phase retrieval is a promising technique able to assess the complex diffracted wave properties through an iterative processing algorithm. In this Letter, we demonstrate the implementation of this technique in reflection geometry with a continuous wave acquisition system working at 0.287 THz. To ensure a high signal-to-noise ratio in the measured dataset, we proposed a double parallel recording scheme with one detector and two lock-in amplifiers operating with the complimentary sensitivity setting. This provided a higher numerical aperture than conventional raster-scanning focal plane imaging. A specialized digital interferometric postprocessing procedure was applied to obtain a surface height map from the reconstructed phase distribution in the object's irradiated area.

A Versatile Illumination System for Real-Time Terahertz Imaging.

Terahertz technologies are attracting strong interest from high-end industrial fields, and particularly for non-destructive-testing purposes. Currently lacking compactness, integrability as well as adaptability for those implementations, the development and commercialisation of more efficient sources and detectors progressively ensure the transition toward applicative implementations, especially for real-time full-field imaging. In this work, a flexible illumination system, based on fast beam steering has been developed and characterized. Its primary goal is to suppress interferences induced by the coherence length of certain terahertz sources, spoiling terahertz images. The second goal is to ensure an enhanced signal-to-noise ratio on the detector side by the full use and optimized distribution of the available power. This system provides a homogeneous and adjustable illumination through a simplified setup to guarantee optimum real-time imaging capabilities, tailored to the sample under inspection. Working toward industrial implementations, different illumination process are conveniently assessed as a result of the versatility of this method.

Guided terahertz pulse reflectometry with double photoconductive antenna.

Developments toward the implementation of a terahertz pulse imaging system within a guided reflectometry configuration are reported. Two photoconductive antennas patterned on the same LT-GaAs active layer in association with a silica pipe hollow-core waveguide allowed us to obtain a guided optics-free imager. Besides working in a pulsed regime, the setup does not require additional optics to focus and couple the terahertz pulses into the waveguide core, simplifying the global implementation in comparison with other reported guided terahertz reflectometry systems. The system is qualified for imaging purposes by means of a 1951 USAF resolution test chart. An image resolution, after a 53 mm propagation length, by about 0.707 LP/mm over the 400-550 GHz integrated frequency band, was obtained, thus providing a promising basis to pursue efforts toward compact guided pulse imagers for sample inspection within the terahertz range.

Terahertz Spectroscopy and Quantum Mechanical Simulations of Crystalline Copper-Containing Historical Pigments.

Terahertz spectroscopy, a noninvasive and nondestructive analytical technique used in art conservation and restoration, can provide compelling data concerning the composition and condition of culturally valuable and historical objects. Terahertz spectral databases of modern and ancient artists' pigments exist but lack explanations for the origins of the unique spectral features. Solid-state density functional theory simulations can provide insight into the molecular and intermolecular forces that dominate the observed absorption features as well as reveal deviations from simple harmonic vibrational behavior that can complicate these spectra. The characteristic terahertz spectra of solid azurite, malachite, and verdigris are presented here, along with simulations of their crystalline structures and sub-3.0 THz lattice vibrations. The powerful combination of theory and experiment enables unambiguous spectral assignment of these complex materials and highlights the challenges that anharmonic peak broadening in organic-containing materials may present in the construction of reference pigment databases.

Pilot study of freshly excised breast tissue response in the 300-600 GHz range.

The failure to accurately define tumor margins during breast conserving surgery (BCS) results in a 20% re-excision rate. The present paper reports the investigation to evaluate the potential of terahertz imaging for breast tissue recognition within the under-explored 300 - 600 GHz range. Such a frequency window matches new BiCMOS technology capabilities and thus opens up the opportunity for near-field terahertz imaging using these devices. To assess the efficacy of this frequency band, data from 16 freshly excised breast tissue samples were collected and analyzed directly after excision. Complex refractive indices have been extracted over the as-mentioned frequency band, and amplitude frequency images show some contrast between tissue types. Principal component analysis (PCA) has also been applied to the data in an attempt to automate tissue classification. Our observations suggest that the dielectric response could potentially provide contrast for breast tissue recognition within the 300 - 600 GHz range. These results open the way for silicon-based terahertz subwavelength near field imager design, efficient up to 600 GHz to address ex vivo life-science applications.

Terahertz frequency modulated continuous wave imaging advanced data processing for art painting analysis.

Reflection terahertz frequency modulated continuous waves scanner (300 GHz) has been proficiently optimized for imaging two easel paintings of different age. The information content of the obtained THz images has been fully inspected by selecting the appropriate THz image parameter. At the same time, a new data processing has been developed for improving the level of detail held by the axial parametric THz images by means of Gaussian fit of the reflected signals. By carefully weighting the reflected signals as a function of the optical path, the reflected amplitude has been corrected for the positioning of the object surface with respect to the beam focal point. The artifact affecting the THz images recorded from an uneven painting surface have been resolved and the obtained images fairly represent to the original painting.

Ultra-flexible multiband terahertz metamaterial absorber for conformal geometry applications.

Standard optical lithography relying on clean room and microelectronic facilities is used to fabricate a thin-flexible metamaterial absorber, designed to operate at submillimeter wavelengths over the 0.1-1 THz frequency band. Large terahertz absorption has been demonstrated numerically and through experimental measurements with a maximum level of about 80%. We put emphasis in this present work on the use of single-sized "meta-cells" to achieve multiple absorption peaks. Furthermore, the use of a thin-flexible dielectric spacer makes it promising for stealth technology applications in order to disguise objects and make them less visible to radar and other detection methods.