Dr. Yannick Abautret Profile

Dr. Yannick Abautret

at Institut Fresnel

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Publications (3)

Proceedings Article | 12 September 2021 Presentation

Thin film terahertz reverse engineering to analyse vegetal tissues

Yannick Abautret, Dominique Coquillat, Ryad Bendoula, Daphné Héran, Bruno Grèzes-Besset, Claude Amra, Myriam Zerrad, Xavier Buet, Gabriel Soriano, Frédéric Chazallet

Proceedings Volume 11872, 1187209 (2021) https://doi.org/10.1117/12.2597056

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Thin film techniques also find applications outside their classical sectors, and among them is the terahertz field1. These waves allow to penetrate a large variety of materials opaque to optics, and find applications in security and defence, automobile and avionics, medicine … THz waves provide new imaging techniques (transverse mode) but can also be used to probe in detail the depth of samples2 in the form of single layers or multilayers, which is the scope of this paper. Actually we take profit from thin film design procedures (usually developed for visible and infrared ranges) in order to address reverse engineering in the THz range. We first emphasize some key differences due to the fact that most broad-band THz sources are pulsed sources (here the THz pulse duration was around 3ps). Hence conversely to optics where optical properties are intensity data issued from spectrophotometric measurements, THz pulses directly allow to record the temporal signals with and without sample interaction, which gives the modulus and phase of the spectrum in the frequency domain. The consequence is that we operate the reverse engineering procedure in the complex plane (in opposition to the real axis of photometry), which involves more data. Here the pulse duration is around 3ps, and the frequency domain with acceptable noise is limited to [0.2 THz – 3,5 THz]. A few classical (inorganic) etalon samples are first analysed and their echoes are exploited to reveal their thicknesses under the assumption of negligible absorption. Then we use reverse engineering to take account of absorption and fit all data in the THz range, which confirms the previous results but with more accuracy. The resulting thicknesses are compared with success to the provider data. In a last step we investigate vegetal tissues (sunflower leaves), which is a much more complex task3. This study falls within the context of the optimization of plant production in regard to global warming and increasing demography, a challenge which requires to analyse and control the hydric stress of plants. Actually there is a growing demand to develop non-contact techniques to analyse leaves microstructure and understand their interaction with the surrounding medium. However the vegetal leaf is highly heterogeneous and cannot be analysed with optics, due to high diffuse reflectance of transmittance (no specular beams). A solution is provided by the THz waves, due to their much larger wavelengths which reduces scattering and the weight of heterogeneities. We show that in this THz regime, the sunflower leaf indeed behaves like a homogeneous multilayer, and this allows to use reverse engineering to extract the leaf design. Results emphasize a 8-layer stack including trichomes, cuticules, epidermis and mesophyll layers4. For each layer we extract the thickness and complex index. To our knowledge this is the first time the leaf multilayer structure is extracted with accuracy with non-contact techniques.

Proceedings Article | 12 September 2021 Presentation

Probing of the multilayer structure of sunflower leaf

Yannick Abautret, Myriam Zerrad, Dominique Coquillat, Ryad Bendoula, Gabriel Soriano, Daphné Héran, Bruno Grèzes-Besset, Frederic Chazalet, Claude Amra

Proceedings Volume 11856, 118560T (2021) https://doi.org/10.1117/12.2600295

KEYWORDS: Tissue optics, Terahertz radiation, Reverse engineering, Agriculture, Absorption, Yield improvement, Tissues, Statistical analysis, Scattering, Satellites

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New techniques for agriculture science are widely explored since several decades in order to improve production yield. Measurements of optical properties at different scales of the crop are investigated and exploited to assess different parameters of interest such as state of stress. For instance, nowadays, there exists acquisition systems embedded in drones, mobile machines and satellites that are able to collect huge amount of hyperspectral imaging data. Identification of optical signature extracted from these techniques can help agronomist with adapting irrigation or distinguishing different plant varieties. These techniques allow to improve greatly the agricultural management, however they do not provide information about the internal structure of the plant leaf and their interaction with electromagnetic fields. Knowing precisely the plant leaf structure can bring critical information that can lead to the development of new techniques for phenotyping and precocious stress detection. To do this it is necessary to probe the plant at the leaf scale using THz instead of optical frequencies because the scattering sensitive phenomenon for plants is more drastic at optical frequencies. To find out how the light interact with the leaf, in a deterministic way, we can model the vegetal tissue as a stack of different physical layers characterized by the thickness and the optical index. In this study, funded by ANR project OptiPAG, we use a well-known reverse engineering technique to retrieve leaf architecture from the reflection data. In time domain, a short Terahertz pulse illuminates a multilayer sample that reflects a part of the signal carrying information about the sample structure. Using a numerical fit in the frequency domain allows to identify each layer and deduce the respective optical index over the input frequency range. We use a few classical (inorganic) etalon samples and analyze the echoes to reveal their thicknesses under the assumption of negligible absorption. Then, we use reverse engineering technique to fit the data in the THz range by taking into account the absorption, making an excellent agreement with the previous results with more accuracy. The measured thickness of the samples correspond very well with the manufacturing specifications. And finally we use this technique with vegetal tissues (sunflower leaves), that poses a much more complex situation. Results emphasize a 8-layer stack including trichomes, cuticules, epidermis and mesophyll layers and for each layer we extract the thickness and the complex index. To our knowledge this is the first time that the leaf multilayer structure is extracted with accuracy using a non-contact techniques.

Proceedings Article | 2 March 2020 Presentation + Paper

Optical thickness of a plant leaf measured with THz pulse echoes

Yannick Abautret, Dominique Coquillat, Myriam Zerrad, Ryad Bendoula, Gabriel Soriano, Daphné Héran, Bruno Grèzes-Besset, Frédéric Chazallet, Claude Amra

Proceedings Volume 11279, 112790L (2020) https://doi.org/10.1117/12.2543710

KEYWORDS: Terahertz radiation, Data modeling, Spectroscopy, Microscopes, Absorption, Reflection, Terahertz spectroscopy, Thin films, Biological research, Multilayers

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