Search

In this work, we capitalize on the unique properties of terahertz radiation to investigate nanoparticle-assisted laser-tissue interaction. We demonstrate the promising capability of the proposed technique to perform the simultaneous monitoring of laser-tissue interaction and the three-dimensional visualization of the induced photothermal damage in a non-invasive and noncontact manner.

International audience; By taking advantage of the interaction between terahertz polarization and carbonfiber orientation, impact-induced intra-and inter-laminar damages at the same interface are differentiated via terahertz polarization-resolved imaging in three dimensions.

In view of minimally-invasive clinical interventions, laser tissue soldering assisted by plasmonic nanoparticles is emerging as an appealing concept in surgical medicine, holding the promise of surgeries without sutures. Rigorous monitoring of the plasmonically-heated solder and the underlying tissue is crucial for optimizing the soldering bonding strength and minimizing the photothermal damage. To this end, we propose a non-invasive, non-contact, and non-ionizing modality for monitoring nanoparticle-assisted laser-tissue interaction and visualizing the localized photothermal damage, by taking advantage of the unique sensitivity of terahertz radiation to the hydration level of biological tissue. We demonstrate that terahertz radiation can be employed as a versatile tool to reveal the thermally-affected evolution in tissue, and to quantitatively characterize the photothermal damage induced by nanoparticle-assisted laser tissue soldering in three dimensions. Our approach can be easily extended and applied across a broad range of clinical applications involving laser-tissue interaction, such as laser ablation and photothermal therapies.

We exploit a single-pixel imaging approach to obtain multi-dimensional (space, time/frequency) images at terahertz frequencies by reconstructing the temporal waveform in each pixel. Moreover, we apply compressive sensing to reduce the acquisition time.

The consequences of a four-point bending test, up to 12 mm, are examined by emitting 1 MHz ultrasonic guided waves in woven carbon fiber reinforced polymer specimens, using coda wave interferometry (CWI), revealing a potential use for nondestructive evaluation. It is known that CWI is more sensitive to realistic damage than the conventional method based on the first arriving time of flight in geophysical, or in civil engineering applications such as concrete structures. However, in composite materials CWI is not well established because of the involved structural complexity. In this paper, CWI is investigated for monitoring the occurrence of realistic defects such as micro-cracks in a woven carbon fiber composite plate. The micro-cracks are generated by a four-point bending test. The damage state is stepwise enhanced by gradually increasing the load level, until failure initiation. The damage is monitored, after each loading, using ultrasound. It is demonstrated that CWI is a powerful tool to detect damage, even low levels, in the sample. Two damage indicators based on CWI, i.e. signals correlation coefficient and relative velocity change, are investigated and appear to be complimentary. Under significant loading levels, the normalized cross-correlation coefficient between the waveforms recorded in the damaged and in the healthy sample (reference at 0 mm), decreases sharply; this first indicator is therefore useful for severe damage detection. It is also demonstrated, by means of a second indicator, that the relative velocity change between a baseline signal taken at zero loading, and the signals taken at various loadings, is linear as a function of the loading, until a critical level is reached; therefore this second indicator, is useful for low damage level detection. The obtained evolution of the relative velocity measurement is supported by relative comparison to the evolution of the bending modulus in function of displacement. The relative velocity change exhibits the same evolution as the bending modulus with loading. It could be used to indicate when the material stiffness has decreased significantly. The research is done in the framework of composite manufacturing quality control and appears to be a promising inspection technique. This work is supported by the Région Grand Est.

Polarization-sensitive terahertz imaging is applied to characterize subsurface damage in woven carbon fiber-reinforced composite laminates in this study. Terahertz subsurface spectral imaging based on terahertz deconvolution is tailored and applied to detect, in a nondestructive fashion, the subsurface damage within the first ply of the laminate caused by a four-point bending test. Subsurface damage types, including matrix cracking, fiber distortion/fracture, as well as intra-ply delamination, are successfully characterized. Our results show that, although the conductivity of carbon fibers rapidly attenuates terahertz propagation with depth, the imaging capability of terahertz radiation on woven carbon fiber-reinforced composites can nonetheless be significantly enhanced by taking advantage of the terahertz polarization and terahertz deconvolution. The method demonstrated in this study is capable of extracting and visualizing a number of fine details of the subsurface damage in woven carbon fiber-reinforced composites, and the results achieved are confirmed by comparative studies with X-ray tomography. The authors gratefully acknowledge the financial support of the Conseil Régional du Grand Est of the Fonds Européen de Développement Régional (FEDER), and of the Institut Carnot ARTS.

International audience; Terahertz pulsed imaging has attracted considerable interest for revealing the stratigraphy and hidden features of art paintings. The reconstruction of the stratigraphy is based on the precise extraction of THz echo parameters from the reflected signals. Several historical panel paintings and wall paintings have been well studied by THz reflective imaging, in which the detailed stratigraphy has been successfully revealed. To our knowledge, however, the stratigraphy of oil paintings has not been clearly uncovered by THz imaging, since the paint layers in an oil painting on canvas, especially for the 16 th and 17 th century art works, are usually very thin (~10 m) in the THz regime. Therefore, in order to improve the performance of THz imaging, advanced signal-processing techniques with higher depth-resolution are still needed. In this study, THz reflective imaging is employed to reveal for the first time the detailed stratigraphy of a 17th century Italian oil painting on canvas. The paint layers on the supporting canvas are very thin in the THz regime, as the THz echoes corresponding to the stratigraphy totally overlap in the first cycle of the reflected THz signal. THz sparse deconvolution based on an iterative shrinkage algorithm is utilized to resolve the overlapping echoes. Based on the deconvolved signals, the detailed stratigraphy of this oil painting on canvas, including the varnish, pictorial, underdrawing, and ground layers, is successfully revealed. The THz C-and B-scans based on the THz deconvolved signals also enable us to reveal the features of each layer. Our results thus enhance the capability of terahertz imaging to perform detailed analysis and diagnostics of historical oil paintings on canvas with foreseen applications for the study of the artist's technique and for authentication.

The process by which art paintings are produced typically involves the successive applications of preparatory and paint layers to a canvas or other support; however, there is an absence of nondestructive modalities to provide a global mapping of the stratigraphy, information that is crucial for evaluation of its authenticity and attribution, for insights into historical or artist-specific techniques, as well as for conservation. We demonstrate sparsity-based terahertz reflectometry can be applied to extract a detailed 3D mapping of the layer structure of the 17th century easel painting Madonna in Preghiera by the workshop of Giovanni Battista Salvi da Sassoferrato, in which the structure of the canvas support, the ground, imprimatura, underpainting, pictorial, and varnish layers are identified quantitatively. In addition, a hitherto unidentified restoration of the varnish has been found. Our approach unlocks the full promise of terahertz reflectometry to provide a global and detailed account of an easel painting’s stratigraphy by exploiting the sparse deconvolution, without which terahertz reflectometry in the past has only provided a meager tool for the characterization of paintings with paint-layer thicknesses smaller than 50 μm. The proposed modality can also be employed across a broad range of applications in nondestructive testing and biomedical imaging.

Laser diodes with optical feedback can exhibit periodic intensity oscillations at or near the relaxation-oscillation frequency. We demonstrate optoelectronic oscillators based on external-cavity semiconductor lasers in a periodic dynamical regime tunable over the entire $X$ -band. Moreover, unlike standard optoelectronic oscillators, we need not employ the time-dependent optical intensity incident on a photodiode to generate the microwave signal, but rather have the option of generating the electrical microwave signal directly as a voltage $V(t)$ at the laser-diode injection terminals under constant current operation; no photodiode need be involved, thus circumventing optical-to-electrical conversion. We achieve a timing jitter of $\lesssim 10$ ps and a quality factor of $\gtrsim 2\times 10^{5}$ across the entire $X$ -band, that ranges from 6.79 to 11.48 GHz. Tuning is achieved by varying the injection current $J$ .

International audience; Terahertz reflective imaging is applied to characterize the failure modes in a polymer coating on a steel plate. The coating was initially scratched, then after accelerated aging, several types of failure have occurred. In order to resolve the thin coating (∼50 µm), terahertz frequency-wavelet domain deconvolution is implemented. With the deconvolved signals, the temporally overlapping echoes of the incident, roughly singlecycle terahertz pulse are clearly resolved, and three important failure modes, viz. corrosion, delamination, and blistering, are characterized quantitatively. Terahertz images in three dimensions clearly exhibit the coating thickness distribution across the entire damaged coating, highlighting the terahertz features associated with different failure modes, thus demonstrating that terahertz imaging can be considered as an effective modality for characterizing damage mechanisms in polymer coatings on metals.

International audience; Terahertz sparse deconvolution based on an iterative shrinkage algorithm is presented in this study to characterize multilayered structures.With an upsampling approach, sparse deconvolution with superresolution is developed to overcome the time resolution limited by the sampling period in the measurement and increase the precision of the estimation of echo arrival times. A simple but effective time-domain model for describing the temporal pulse spreading due to the frequency-dependent loss is also designed and introduced into the algorithm, which greatly improves the performance of sparse deconvolution in processing time-varying pulses during the propagation of terahertz waves in materials. Numerical simulations and experimental measurements verify the algorithms and show that sparse deconvolution can be considered as an effective tool for terahertz nondestructive characterization of multilayered structures.

We present an implementation of the single-pixel imaging approach into a terahertz (THz) time-domain spectroscopy (TDS) system. We demonstrate the indirect coherent reconstruction of THz temporal waveforms at each spatial position of an object, without the need of mechanical raster-scanning. First, we exploit such temporal information to realize (far-field) time-of-flight images. In addition, as a proof of concept, we apply a typical compressive sensing algorithm to demonstrate image reconstruction with less than 50% of the total required measurements. Finally, the access to frequency domain is also demonstrated by reconstructing spectral images of an object featuring an absorption line in the THz range. The combination of single-pixel imaging with compressive sensing algorithms allows to reduce both complexity and acquisition time of current THz-TDS imaging systems.

International audience; This Letter presents a method for enhancing the depth resolution of terahertz deconvolution based on autoregressive (AR) spectral extrapolation. The terahertz frequency components with a high signal-to-noise ratio (SNR) are modeled with an AR process, and the missing frequency components in the regions with low SNRs are extrapolated based on the AR model. In this way, the entire terahertz frequency spectrum of the impulse response function, corresponding to the material structure, is recovered. This method, which is verified numerically and experimentally, is able to provide a “quasi-ideal” impulse response function and, therefore, greatly enhances the depth resolution for characterizing optically thin layers in the terahertz regime.

Terahertz reflective imaging is applied to characterize the damage mechanisms in the coating on metal substrate after aging. THz frequency-wavelet deconvolution is designed to resolve the overlapping echoes due to the optically thin coating. With the decovolved THz data, different types of damages including corrosion, blisters and delamination are characterized quantitatively in three dimensions.

International audience; Terahertz imaging is applied to characterize a hybrid fiber-reinforced composite laminate subject to low-velocity impact in this study. The hybrid fiber-reinforced composite laminate comprises unidirectional glass/epoxy and carbon/epoxy plies with a cross-ply stack pattern. Both impact-induced intra- and inter-laminar damages are successfully detected, and the damage evolution throughout the thickness is also evaluated. The interaction between the terahertz polarization and carbon-fiber orientation is investigated in detail. Inter-laminar damage at the interface and the intra-laminar damage close to the same interface can be differentiated via polarization-resolved imaging. With a parameter fitting method based on multiple regression analysis, delamination is characterized quantitatively. Terahertz C- and B-scan images clearly exhibit the propagation of the damage from the top to the bottom surface in three dimensions.

Terahertz (THz) reflection imaging is applied to characterize a woven glass fibre-reinforced composite laminate with a small region of forced delamination. The forced delamination is created by inserting a disk of 25- μ m-thick Upilex film, which is below the THz axial resolution, resulting in one featured echo with small amplitude in the reflected THz pulses. Low-amplitude components of the temporal signal due to ambient water vapor produce features of comparable amplitude with features associated with the THz pulse reflected off the interfaces of the delamination and suppress the contrast of THz C- and B-scans. Wavelet shrinkage de-noising is performed to remove water-vapor features, leading to enhanced THz C- and B-scans to locate the delamination in three dimensions with high contrast.

A hybrid fiber-reinforced composite laminate with impact damage has been studied via terahertz imaging. Both of the impact-induced intra- and inter-laminar damages are successfully detected. Terahertz C-and B-scans in time domain show the damage evolution throughout the thickness. Intra- and inter-laminar damages at the same interface can be differentiated via polarization-resolved imagings, which can also be located with THz C- and B-scans in frequency domain.