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International audience; We apply reverse engineering techniques (RET) to analyze the dehydration process of a sunflower leaf with terahertz time-domain spectroscopy. The multilayer structure of the leaf is extracted with accuracy during the entire process. Time variations of thickness and the complex index are emphasized for all leaf layers (2 cuticules, 2 epiderms, and 2 mesophylls). The global thickness of the sunflower leaf is reduced by up to 40% of its initial value.

The low-frequency (terahertz) vibrational spectroscopy of two chlorophyll species, Chl-𝑎 and one of its magnesium derivatives (Chl-Mg-Na), has been investigated experimentally. The combination of terahertz time-domain spectroscopy and Fourier transform infrared spectroscopy has enabled a broad frequency range to be covered (0.2 to 18 THz). For Chl-Mg-Na, the terahertz spectra show clear and well-marked features at 1.44, 1.64, and 1.83 THz dominated by intermolecular interactions. The frequency dependent refractive index and absorption coefficient of Chl-Mg-Na were determined using the Fit@TDS software. Below 1.0 THz, a refractive index of 2.09 was measured. In order to acquire further understanding of the observed vibrational modes, a detailed study of the temperature dependence of the line positions of the lowest modes in Chl-Mg-Na was performed. As the temperature is increased from 88 K to 298 K, the feature at 1.83 THz experiences a notable red shift of frequency and line shape broadening, whereas the feature at 1.44 THz shows little change. These results suggest that the 1.83 THz feature is dominated by intermolecular motions occurring over the crystalline unit cell of the Chl-Mg-Na molecular crystal. Finally, terahertz time-domain was used to acquire the spectra of an ornamental plant bearing yellow-green variegated leaves (ivy, Aureomarginata variety), the yellow sectors having lower chlorophyll content compared to green sectors. In dehydrated green tissue, the chlorophyll molecules showed well-marked intermolecular vibrational modes at 1.85 THz, indicating that chlorophyll molecules are prone to pack with an ordered molecular arrangement. These results demonstrate the potential application of THz spectroscopy in agricultural sciences.

Terahertz time-domain spectroscopy has been used to investigate the vibrational spectra of polycrystalline sodium magnesium chlorophyllin - one of the natural derivatives of chlorophyll - over the temperature range 88 K–298 K. A number of well-resolved absorption peaks were observed in the frequency range 0.2–2.5 THz, which are interpreted as originating from mixed character of intramolecular and intermolecular vibration modes. As the temperature is increased, the observed absorption features resolve into broader peaks. The peak centered at 1.83 THz shifts towards higher frequencies, indicating that for this feature, significant intermolecular anharmonicity exist.

International audience; 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.

International audience; Thin film techniques also find applications outside their classical sectors, and among them is the terahertz field. 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 samples 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.

International audience; We investigate the use of an InP-based double-heterojunction bipolar transistor as a detector for wireless communications around 300~GHz. The sensitivity of the detector, along with its complex output impedance are characterized for a transistor (un)coupled to a log-spiral antenna, at different biases. A real-time uncompressed high-definition video signal is successfully transmitted at 1.5 Gbps in a heterodyne configuration.

Terahertz (THz) technology has recently attracted wide scientific and technological interests in quantum science, where the need of fast and sensitive photodetectors prospects fascinating impacts in quantum computing, quantum metrology and optical communications. In this work, we demonstrate that quantum dot single electron transistors (SETs) based on InAs/InAs0.3P0.7 axially heterostructured nanowires integrated with planar on-chip nano-antennas, behave as highly sensitive zero-bias photo-thermoelectric detectors at 0.6 THz. The detector photoresponse can be optimized by electrostatic gating, taking advantage of the SET transport characteristics that clearly reflects energy quantization and single-electron tunneling in the dot.

International audience; The terahertz absorption spectra of sodium magnesium chlorophyllin (Chl-Mg-Na) and sodium copper chlorophyllin (Cu-Chl), two major members of the chlorophyll derivative family, have been measured in the range 0.2−3.0 THz (6.6−100 cm-1), at room temperature. Additionally, surface-enhanced Raman scattering spectroscopy was used to supplement data in the higher frequency range. The capability of terahertz spectroscopy for quantitative characterization of Chl-Mg-Na intermolecular vibrations was investigated and the sensitivity of the 1.82-THz feature with degree of hydration by changes in the molecular environment was examined. For Cu-Chl derivative, a broad feature was observed around 1.8 THz which currently hinders clear Cu-Chl identification and quantification.

Low dimensional nano-systems are promising candidates for manipulating, controlling and capturing photons with large sensitivities and low-noise. If quantum engineered to tailor the energy of the localized electrons across the desired frequency range, they can allow devising efficient quantum sensors across any frequency domain. Here, we exploit the rich few-electrons physics to develop millimeter-wave nanodetectors employing as sensing element an InAs/InAs0.3P0.7 quantum-dot nanowire, embedded in a single electron transistor. Once irradiated with light the deeply localized quantum element exhibits an extra electromotive force driven by the photothermoelectric effect, which is exploited to efficiently sense radiation at 0.6 THz with a noise equivalent power < 8 pWHz-1/2 and almost zero dark current. The achieved results open intriguing perspectives for quantum key distributions, quantum communications and quantum cryptography at terahertz frequencies., This is the authors' version of the article submitted to Nano Letters and accepted for publication https://doi.org/10.1021/acs.nanolett.1c02022 (18 Pages, 4 Figures)

International audience; Recent advances in THz detection with the use of CMOS technology have shown that this option has the potential to be a leading method of producing low-cost THz sensors with integrated readout systems. This review paper, based on authors’ years of experience, presents strengths and weaknesses of this solution. The article gives examples of some hints, regarding radiation coupling and readout systems. It shows that silicon CMOS technology is well adapted to the production of inexpensive imaging systems for sub-THz frequencies. As an example paper presents the demonstrator of a multipixel Si-CMOS THz spectroscopic system allowing for chemical identification of lactose. The THz detectors embedded in this system were manufactured using the CMOS process.

We analyze Terahertz (THz) echoes by reflection on a sunflower leaf in order to evaluate the internal leaf structure (geometry, complex indices and thicknesses). The analysis is based on the thin film multilayer formalism in time and frequency domains. A high agreement is emphasized between experiment and theory, and we evaluate how realistic the multilayer solution can be in regard to our knowledge related to the sunflower leaf. A test campaign is performed in Charles Coulomb laboratory, which is equipped with the THz spectrometer.

International audience; Understanding the forces that enable protein interactions is a major scientific challenge. Indeed, Brownian diffusion alone, which is usually considered as the main engine of protein dynamics, does not explain the rapidity and efficiency of the biomolecular reactions at works in cells. In this work, we will present observations of the collective oscillations of a model protein driven out-of-equilibrium by means of optical excitation in aqueous medium using terahertz spectroscopy.

Grapevine trunk diseases affect grapevines worldwide and dramatically shorten the longevity of vineyards. This study ·has successfully demonstrated the use of THz timedomain imaging to identify the grapevine wood tissues degraded by the fungi responsible for grapevine trunk diseases.

Semiconductor physics, quantum electronics & optoelectronics : SQO 22(2), 237-251 (2019). doi:10.15407/spqeo22.02.237, Published by Inst. of Semiconductor Physics, Kyiv

International audience; A graphene ballistic rectifier is used in conjunction with an antenna to demonstrate a terahertz (THz) detector and imager. The device is used to take an image of an optically opaque object at 0.68 THz demonstrating potential in both medical and security imaging applications.

International audience; Increasing the data rates of wireless communicationsrequires to increase the carrier frequency to hundred’s of GHz,thus emitters and detectors that can operate at high data ratesare required. We review a possible source and receiver that canbeen used for THz communications. We present a photonic-basedemitter that provides a coherent THz signal in the 50–700+ GHz,based on the excitation of a uni-travelling carrier photodiodeby a two-frequency semiconductor laser that operates on twotransverse modes. The performance of THz detectors basedon GaAs field-effect transistors are also presented, along withcommunications using direct amplitude modulation at 8.3 Gbpsand an heterodyne communication at 1.5 Gbps.

International audience; An overview of main results concerning THz detection related to plasma nonlinearities in nanometer field effect transistors is presented. In particular nonlinearity and dynamic range of these detectors are discussed. As the main result, we will show some of the first real-world applications of the FET THz detectors: demonstrators of the imager developed for fast postal security and industrial nondestructive quality control.

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.

A graphene ballistic rectifier is used in conjunction with an antenna to demonstrate a rectenna as a terahertz (THz) detector. A small-area (1 μm

International audience; We have evaluated the performance of the InP double heterojunction bipolar transistor and InP-based varactors as a room temperature sensitive detectors for THz computed tomography applications. They were used in transmission mode for the 350 GHz and 650 GHz atmospheric windows.

International audience; We report on the photoresponse of 0.7-μm InP double heterojunction bipolar transistor to THz radiation of low and high intensities, when the collector is unbiased. At low intensities, the photoresponse is linear in radiation intensity. Under intense laser radiation, we observe a saturation at intensities > 15 kW/cm2, sample damaging arising around ∼ 40 kW/cm2. The photoresponse as a function of base-emitter bias dependence does not change with the radiation intensity.

In this paper, an overview of main results concerning THz detection by nanometer field effect transistors (FETs) and heterojunction based transistors (HBTs) is presented. Different GaInAs/InP and GaN/AlGaN nanometer field effect transistors based detectors are presented. We present also first Silicon C-MOS transistors based integrated circuits for wireless communication in sub-THz range. Special attention is given to transistors based focal plane arrays. We show, how these arrays, together with in purpose developed diffractive 3D printed optics lead to construction of the demonstrators of safety and industrial quality control scanners.

Fabrication and experimental tests of materials and optics for Terahertz range used in field effect transistor detectors arrays are described. The method using diffractive optics, fabricated in 3D printing technology, was used for tests with GaN/AIGaN based FETs sub-THz detectors working in 300 GHz atmospheric window. The lens arrays focus energy exactly on the detectors and additionally reduce mutual detector crosstalk. Finally they improve detectors signal to noise ratio by more than one order of magnitude. Moreover such lens arrays are cost-effective, easy reproducible and thin elements.

Design, manufacturing and measurements results for silicon plasma wave transistors based wireless communication wideband receivers operating at 300 GHz carrier frequency are presented. We show the possibility of Si-CMOS based integrated circuits, in which by: (i) specific physics based plasma wave transistor design allowing impedance matching to the antenna and the amplifier, (ii) engineering the shape of the patch antenna through a stacked resonator approach and (iii) applying bandwidth enhancement strategies to the design of integrated broadband amplifier, we achieve an integrated circuit of the 300 GHz carrier frequency receiver for wireless wideband operation up to/over 10 GHz. This is, to the best of our knowledge, the first demonstration of low cost 130 nm Si-CMOS technology, plasma wave transistors based fast/wideband integrated receiver operating at 300 GHz atmospheric window. These results pave the way towards future large scale (cost effective) silicon technology based terahertz wireless communication receivers.

We report on the photoresponse of AlGaN/GaN high electron mobility transistors to the THz radiation of low (15 mW/cm(2)) and high (up to 40 kW/cm(2)) intensities. We show that the response can be described by the Dyakonov-Shur theory in the whole range of radiation intensity. At low intensities, the photoresponse is linear in radiation intensity. Under intense laser radiation, we observe a saturation at intensities >20 kW/cm(2). We explain our results by the change of the channel conductivity under the influence of strong THz field. This mechanism of photoresponse saturation, which is due to the mobility decrease in high ac electric field, should exist for any type of field effect transistor detectors.

International audience; We present the concept, the fabrication processes and the experimental results for materials and optics that can be used for terahertz field-effect transistor detector focal plane arrays. More specifically, we propose 3D printed arrays of a new type - diffractive multi-zone lenses of which the performance is superior to that of previously used mono-zone diffractive or refractive elements and evaluate them with GaN/AlGaN field-effect transistor terahertz detectors. Experiments performed in the 300-GHz atmospheric window show that the lens arrays offer both a good efficiency and good uniformity, and may improve the signal-to-noise ratio of the terahertz field-effect transistor detectors by more than one order of magnitude. In practice, we tested 3 × 12 lens linear arrays with printed circuit board THz detector arrays used in postal security scanners and observed significant signal-to-noise improvements. Our results clearly show that the proposed technology provides a way to produce cost-effective, reproducible, flat optics for large-size field-effect transistor THz-detector focal plane arrays.

Experimental and theoretical investigations of the influence of substrate thickness on the performance of Si field effect transistor terahertz detectors are presented.. We show that in the case of planar metal antennas a significant portion of incoming radiation, instead of being coupled to the transistors, may be coupled to the substrate leading to important responsivity losses..The ways of avoiding these losses are discussed.

We present results of investigations of Plasma Assisted Molecular Beam Epitaxy (PAMBE) - grown GaN/AlGaN High Electron Mobility Transistor (HEMT) structures for fabrication of detectors and emitters operating in THz spectral range. These devices are standard HEMT structures but equipped with additional antennas that ensure THz radiation coupling. THz detectors and emitters were processed using different HEMT designs. The results of emission experiments at room temperature demonstrate emission in the spectral range from 0.5 to 12 THz with a total integrated power of the single transistor of the order of 100 nW. The results of room temperature THz detection (photoresponse experiments) in the ranges 110–170 and 220–330 GHz are presented and discussed. THz experiments, transistor electrical tests and data analysis presented in this work, clearly show that PAMBE grown GaN/AlGaN transistors can used as promising elements of future THz imaging systems.

In this paper, we report on nonresonant detection of terahertz radiation using the rectification mechanism of two-dimensional plasmons in high-electron-mobility transistors using InAIAs/InGaAs/InP material systems. The experiments were performed at room temperature using a Gunn diode operating at 0.30 THz as the THz source. The measured response was dependent on the polarization of the incident THz wave; The device exhibited higher response when the electric-field vector of the incident radiation was directed in the source-drain direction. The 2D spatial distribution image of the transistor responsivity extracted from the measured response shows a clear beam focus centered on the transistor position, which ensures the appropriate coupling of the terahertz radiation to the device. The device also demonstrated excellent sensitivity/noise performances of approximately 125 V/W and approximately 10(-11) W/Hz(0.5) under 0.30 THz radiation.

Resonant frequencies of the two-dimensional plasma in field effect transistors (FETs) increase with the reduction of the channel dimensions and can reach the terahertz (THz) range for micrometer and sub-micrometer channel lengths. Nonlinearity of the gated electron gas in the transistor channel can be used for the detection of THz radiation. The possibility of tuneable narrow band detection in sub-THz and THz range, related to plasma resonances, has been demonstrated for nanometre gate length transistors at cryogenic temperatures. At room temperatures the plasma oscillations are usually strongly damped, but field effect transistors can still operate as an efficient broadband detectors in the THz range. We present an overview of experimental results on THz detection by field effect transistors made of III-V and Si materials, The material issue is discussed and first room applications of FETs for imaging at frequencies above 1 THz are demonstrated. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We device plasmonic, bolometric and thermoelectric antenna-coupled Terahertz photodetectors based on black phosphorus and hybrid van der Walls heterostructures, showing 20000 signal to noise ratios and 100pW/Hz1/2 noise equivalent powers in the 0.3-3 THz frequency range.

International audience; Demand for efficient terahertz (THz) radiation detectors resulted in intensive study of the asymmetric carbon nanostructures as a possible solution for that problem. In this work, we systematically investigate the response of asymmetric carbon nanodevices to sub-terahertz radiation using different sensing elements: from dense carbon nanotube (CNT) network to individual CNT. We conclude that the detectors based on individual CNTs both semiconducting and quasi-metallic demonstrate much stronger response in sub-THz region than detectors based on disordered CNT networks at room temperature. We also demonstrate the possibility of using asymmetric detectors based on CNT for imaging in the THz range at room temperature. Further optimization of the device configuration may result in appearance of novel terahertz radiation detectors.

Phenomena of the radiation coupling to the field effect transistors based terahertz detectors is studied. We show that in the case of flat metallic antennas important part of radiation, instead coupling to the transistors, is coupled to the substrate leading to losses. Experimental and theoretical investigations of the responsivity versus substrate thickness are performed. They clearly show how to minimize the losses by the detector substrate thinning.

We present results of improvement of THz images registered at the frequency of 300 GHz. The improvement is achieved by processing of both phase and magnitude information provided by lock-in amplifier. The active imager uses a continuous-wave tunable radiation source. We investigate capabilities of utilizing phase and intensity information in order to obtain more accurate shape of object. Results of processing images in order to obtain more details are shown. Improvement of the image resolution and quality by composition of selected elements of both amplitude and phase-based images using specific fusion methods is presented.

International audience; We report on the intrinsic responsivity of an asymmetric dual-grating-gate plasmonic detector over 100 kV/W at 200 GHz and 50 kV/W at 300 GHz measured at room temperature with zero source-drain bias. We demonstrate that broadband characteristics of the responsivity depend much on the geometrical parameters of the detectors.

A fully integrated 0.3 THz antenna-coupled plasma-wave detector with 10 GHz (measured) bandwidth is presented. Fabricated in 130nm CMOS technology, the chip is formed of an E-shaped patch antenna, plasmonic based Field Effect Transistor (FET) detector and a wide bandwidth amplifier employing inductive shunt peaking. The open drain mode of operation of the detector achieves an absolute responsivity of 10 V/W with a minimum signal to noise ratio (SNR) of 40 dB over the entire bandwidth. With a drain current of 0.24 mA, the responsivity increases by 10X with a decrease in bandwidth to 3 GHz. The detector is also characterized without the on chip amplifier for imaging applications and shows a measured absolute responsivity of 150 V/W for a drain current of 5 μA at 0.3 THz.

Angular-dependent reflection measurements were used to map out the dispersion relations of the resonant Bloch modes, and the angular dispersion of the phonon modes, in a photonic crystal consisting of selectively grown GaN pyramids containing a single InxGa1-xN/GaN quantum well. The dispersion of the photons exhibits strong photonic crystal characteristics, while the mean indium content of the 2 nm thickness InxGa1-xN layer was extracted from the angular dispersion of the phonon modes.

The growth of semiconductor nanowires (NWs) has recently opened new paths to silicon integration of device families such as light-emitting diodes, high-efficiency photovoltaics, or high-responsivity photodetectors. It is also offering a wealth of new approaches for the development of a future generation of nanoelectronic devices. Here we demonstrate that semiconductor nanowires can also be used as building blocks for the realization of high-sensitivity terahertz detectors based on a 1D field-effect transistor configuration. In order to take advantage of the low effective mass and high mobilities achievable in III–V compounds, we have used InAs nanowires, grown by vapor-phase epitaxy, and properly doped with selenium to control the charge density and to optimize source–drain and contact resistance. The detection mechanism exploits the nonlinearity of the transfer characteristics: the terahertz radiation field is fed at the gate-source electrodes with wide band antennas, and the rectified signal is then rea...

Self-assembled nanowires represent a new interesting technology to be explored in order to increase the cutoff frequency of electronic THz detectors. They can be developed in field effect transistor (FET) and diode geometries exploiting non-linearities of either the transconductance or the current-voltage characteristic as detection mechanism.