Une étude sur la caractérisation de bois tropicaux (la grenadille) pour la facture d’instruments à vent est en cours dans le cadre d’une collaboration entre Henri SELMER Paris et le LMGC. Les recherches portent actuellement sur la diffusion de l’eau dans des échantillons de bois de différentes espèces et présentant des formes variées (cylindriques, parallélépipédiques, autres...), la modélisation de la teneur en eau à l’équilibre (EMC) sous diverses conditions atmosphériques, la mesure des coefficients de retrait et gonflement du bois au cours de changement de teneur en eau, et la validation d’un modèle de diffusion en utilisant l’imagerie TéraHertz (THz).

In the implementation of field effect transistor (FET) terahertz (THz) detectors, the integration of properly designed planar antennas could effectively enhance the coupling efficiencies between the transistors and THz radiation, thus improving the responsivities of THz detectors. A method to design the planar antenna which is based on the simulation of channel electric field at the gate edge of FET is reported here. This method is suitable for the situation where the input impedances of FETs may not be conveniently obtained in the THz regime. The validity of this method in the antenna design is confirmed by the measurements of the fabricated GaN/AlGaN FET THz detectors. The maximum responsivities of the bowtie detector and the dual-dipole detector are obtained at 170.7 GHz (1568.4 V/W) and 124.3 GHz (1047.2 V/W) respectively, which are close to the simulation results of channel electric field at the gate edge of the bowtie detector and the dual-dipole detector.

Nowadays high frequency electronics uses two distinct families of semiconductors-based transistors: field effect transistors and heterojunction bipolar transistors (HBTs). They compete reaching impressive cut off frequencies going up to THz range. Except their usual functions related to switching and amplifying of current or voltage both have been demonstrated as efficient direct THz radiation detectors. Indeed, both types of the transistors have shown that once equipped with antennas, they can capture THz radiation from the open space and deliver the voltage/current proportional to incoming THz radiation power (THz rectification).Most of the work was dedicated to the field effect transistors that rectify THz radiation by plasma related nonlinearities. After pioneering work of [1-2] only very small attention was devoted to HBTs. In this work, we present experimental studies of THz detection by different HBTs fabricated using InP double HBT (DHBT) technologies [3]. Different devices were investigated: single-finger devices and multi-finger devices formed using equally spaced parallel single-transistor fingers [4]. We have evaluated the room temperature detection performances of the devices in the sub-THz range from 250 GHz up to 650 GHz and analyse in details the physical mechanisms of THz detection. Finally, THz domain is an excellent non-contact probe of water content in biological tissues [5]. We also show that the sensitive HBTs detectors can be used for THz spectroscopy and 2D THz imaging to study the water dynamics of sorghum (a grass species cultivated for its grain) by monitoring the dehydration kinetics of its leaves.

Water movement and diffusion in wood is a well-documented research topic. It is strongly linked to the physical and mechanical properties of wood, and thus it has a high degree of importance and prominence for real-life application, particularly in wood drying. Here we employed the terahertz (THz) spectroscopy and imaging techniques to study the diffusion of water in wood. For our first experiments, we used balsa, a species of wood with a relatively homogenous structure (at the tissue level) and a low density that turns into fast diffusion rates and low absorption of the terahertz beam. This enables us to conduct several consecutive observations from various conditions (angles, diffusion directions, etc.) in a short time. These preliminary studies confirm the feasibility of terahertz spectroscopy and imaging techniques as a tool to observe the water diffusion in wood as a function of the direction (Fig. 1). We have also measured the evolution of the attenuation coefficient of the terahertz beam on wet and drying wood at different times. A model of the diffusion behavior based on these measurements, and according to the water content, will be developed and compared with other conventional water diffusion models based on gravimetry methods for the identification of the diffusion coefficients.

We present experimental studies of THz detection by different Heterojunction Bipolar Transistors (HBTs) fabricated using InP technology. We also show that the sensitive HBTs detectors can be used for THz spectroscopy and 2D THz imaging to study how the water content affects the polarization rate and biattenuation in fibrous sorghum leaf.

We consider a terahertz detector based on a graphene ballistic rectifier integrated with an antenna. The device demonstrates an extrinsic responsivity of hundreds of V/W in the range 70-440 GHz at room temperature with no indications of a cut-off frequency up to 0.44 THz. The device also demonstrates linear response for more than 3 orders of magnitude of power input due to its zero threshold voltage and its high saturation current.