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The aim behind this work is to design and manufacture a beam shaping lens for active terahertz imaging systems that boosts their performance in terms of sensitivity and image quality. The proposed beam shaper is based on an adaptation of the original optical Powell lens, where a collimated Gaussian beam is converted into a uniform flattop intensity beam. The design model for such a lens was introduced, and its parameters were optimized by a simulation study conducted using COMSOL Multiphysics software. The lens was then fabricated using a carefully chosen material [polylactic acid (PLA)] through a 3D printing process. The manufactured lens was implemented in an experimental setup to validate its performance using a continuous-wave sub-terahertz source around 100 GHz. Experimental results demonstrated a high-quality flattop beam maintained along the propagation path, which is highly recommended for terahertz and millimeter-wave active imaging systems to produce high-quality images.

We investigate terahertz time-domain spectroscopy (THz-TDS) as a non-destructive and non-contact technique for depth profiling of dopants in semiconductors. THz temporal waveforms transmitted through silicon-ion-implanted semi-insulating gallium arsenide substrates, as-implanted or post-annealed by rapid thermal annealing, were analyzed by assuming a multi-layered Gaussian refractive index profile in the ∼sub-micrometer-thick implantation region. The implantation energy and dosages in this work were 200 KeV, 1014, 5 × 1014, and 1015 ions/cm2, respectively. The average values of real ( n) and imaginary ( κ) parts of refractive indices of an as-implanted sample in the depth range of 0–800 nm are 5.8 and 0.7, respectively, at 0.5 THz and are 6.2 and 0.2, respectively, at 1 THz. On the other hand, the refractive index profile of the post-annealed samples displays a prominent Gaussian-like form, and peak refractive indices ( n ∼ 25 and κ ∼ 32.7 at 0.5 THz and n, κ ∼17 at 1 THz) were found to be at the depth of 210 nm. Reconstructed dopant profiles in as-implanted, implanted, and post-annealed substrates were found to be in good agreement with measurements by secondary ion mass spectroscopy as well as simulation by the Monte Carlo method. We were also able to determine accurately the projected range ( Rp), straggle ( Rs), and concentration of dopants by the analysis of THz-TDS data. The spatial resolution, along the depth direction, of the THz-TDS technique for depth profiling of dopants was estimated to be as small as 8-nm. This work suggests the feasibility of using THz-TDS for nondestructive and non-contact diagnostics for profiling dopants in semiconductors.

By generating terahertz pulses in KTP crystals through optical rectification with a pump photon energy varying from below to above the bandgap, we observe a peak of the THz signal at the bandgap energy but also a second one around half the bandgap. This later one is attributed to a two-photon absorption enhanced nonlinearity, which is validated by the similarity of the two-photon absorption coefficient and THz peak amplitude data versus the pump photon energy. A careful analysis of the KTP sample absorption spectral dependence nearby the bandgap demonstrates that KTP is an indirect bandgap crystal, whose absorption below the bandgap involves emission of a phonon related to the symmetric Ti-O stretching vibration, i.e. the ν

We present a UV-visible pump/THz-probe study to determine the photo-carrier lifetime in GaAs. Three samples ( , and oriented) were pumped from 1.56 eV to 4.15 eV. For the three orientations we observed a strong unexpected peak of the carrier lifetime around 2.6 eV.

In order to increase THz images quality, we show that the effect of multiplicative perturbation can be notably removed by using the concept of phase diagram associated with a Frenet-Serret-based quantification.

We perform THz generation by optical rectification in ZnTe, KTP and RKTP with femtosecond laser pulses whose wavelength ranges from UV to IR. We observe a strong THz emission when the pump photon energy reaches the crystal bandgap. This phenomenon is explained by the resonance of material nonlinear susceptibility around its bandgap. Additionally, for the first time, we report of a two-photon enhanced nonlinearity in KTP/RKTP, which leads to an enhanced THz emission near half the bandgap.

We report on the generation of THz waves through optical rectification in ZnTe of femtosecond laser pulses whose photon energy is tuned from below to above the ZnTe bandgap energy. The THz signal exhibits a pronounced peak at the bandgap energy, at THz frequencies for which losses in ZnTe remain small. This peak is likely due to the resonance of the ZnTe nonlinear susceptibility in the vicinity of the bandgap.

ZnTe is a crystal widely used to generate THz waves through optical rectification (OR) of femtosecond 800 nm laser pulses [1] . It indeed presents interesting characteristics since phase-matching is almost realized, losses are rather weak, and the birefringence issue is ignored. Its nonlinear coefficients are however quite small compared to other used crystals like LiNbO 3 for example. It should be possible to increase those nonlinear coefficients by pumping the crystal closer to its bandgap, since this enhancement has been modelled and measured for second harmonic generation [2] . Unfortunately, when dealing with THz generation, pumping at or above the bandgap excites free carriers that absorb the THz wave, leading to a decrease of THz generation efficiency. The goal of our work is to study the balance between enhanced nonlinearity and free carrier absorption by recording the THz signal versus the pump photon energy.

In this paper a methodology has been proposed that implements RF system on a field programmable gate array (FPGA) device. FPGAs are reconfigurable devices that are mostly used for low frequency digital applications. However in the proposed work we have exploited the re-programmability feature of FPGAs to utilize it for very high frequency range applications. The scheme implemented here is aimed towards development of RF communication subsets using FPGA logic blocks. For its validation, an On-off key (OOK) modulation and demodulation technique has been employed on the FPGA. Based upon the implemented OOK scheme a wireless communication between two FPGAs is established. Also, we have given an example of such FPGA based RF implementation. Lastly, we have introduced the scheme of using FPGA for UHF generation.

Counterfeiting of integrated circuits (ICs) has become a genuine concern of the semiconductor industry. Counterfeiting involves both economic and safety issues. The semiconductor companies and embedded system designers are looking for different solutions to gain confidence in the ICs they use. It is necessary to find a robust solution that is both efficient in terms of cost and implementation to detect and avoid the counterfeiting of ICs. In this article, we have described our studies of the use of an electromagnetic (EM)-based approach that can be utilized to authenticate ICs. Additionally, this article suggests ways in which the proposed EM-based solution is robust against reliability and aging effects. Our proposed scheme details the usability of manufacturing-based process variations (PVs) to create device fingerprints using the EM-based approach. To highlight the distinction between various devices under test (DUT), the resulting EM fingerprints have been subjected to post-processing schemes. To validate our approach, we have performed our experiments across different nanoscale field programmable gate arrays (FPGAs). We have also discussed in brief the implementation and results using this technique for microcontrollers (MCUs).

International audience; In this work, we propose a THz tag structure based on a stack of different dielectric layers made of PE and PE-TiO2 mixture, in which we voluntary randomly deposit metallic flakes. Due to the randomness of the flake distribution (position and size), such a structure exhibits a potential unique THz signature that could be used to address identification using targeted statistical analysis. The signature is obtained by making a 2D THz image of a given area of the structure.

International audience; In this paper, we report on the unitary authentication of identically realized diffraction grating-based tags structures in the THz domain by using multivariate statistical analysis. We proceed to a dimension reduction with Principal Component Analysis (PCA) as a preprocessing step before a Gaussian classification and we evaluate the error rates. We then classify the tags using a Linear Discriminant Analysis (LDA). We demonstrate that PCA gives average error rates lower than 0.5% whereas LDA is able to classify the tags with error rates lower than 6.10-5considering its 3 first axes.

A modal method is developed analytically to investigate the THz optical transmission and reflection of a metallic thin film perforated by a 2D array of rectangular apertures. For subwavelength apertures, this optical model is interpreted in terms of passive electrical circuits, with interface admittances accounting for the THz surface conduction properties of the metallic film. The reactive component of the admittance of the evanescent diffraction cloud is shown to exhibit resonant behavior governed by the shape factors of the array. Interaction of such an electrodynamic resonance with the Rayleigh diffraction orders may alter their standard Fano profiles. Experimental evidence of the resonance is obtained owing to lineshape analysis of transmittance measurements in the THz range on metallic thin films deposited on a dielectric substrate, both above and below the first Wood-Rayleigh anomaly.

International audience; In this paper, a chipless solution dedicated to the authentication of manufactured products is proposed. The proposed technique is an extension of chipless radio frequency identification (RFID) with the prevention of tags’ cloning due to the use of natural randomness in the fabrication process. or the first time, we proposed a methodology to characterize the chipless RFID tags for the purpose of authentication. The proposed method has a high level of security due to the employment of a database of authenticity. With a nonintrusive and nondestructive operation, the proposed method has promising possibilities for authentication applications, for example, the chipless tags can be inserted (or hidden) in the product packaging. The chipless tags are realized two times intermittently, where each realization is constituted of 45 tags. These two different realizations share the same company, same technology but different film mask to ensure the natural dimensional randomness. The similarity analysis is conducted inside each realization and also between two different realizations. With the help of this technique, it is possible to authenticate two chipless RFID tags exhibiting natural dimensional variations in their designs with a low probability of error.

National audience

International audience; In this paper, we report on fast identification of low-cost tags with a commercial terahertz (THz) spectrometer. These tags are designed and dedicated for anti-counterfeiting applications. One-dimensional pattern on a fully dielectric material were designed to enhance the robustness and complex the reverse engineering. Moreover, contrary to barcodes and RFID tags based on a planar surface pattern, this one is buried in the volume and can be then hidden in the product to identify as it is only made of dielectric material, without any metal. The generic tag consists of a quasi-periodic stack of two different polyethylene-based dielectric layers with different refractive indices and thicknesses. As we use THz wave, each layer remains thin (of the order of the wavelength), to get a tag whose thickness is in the millimeter range. We show that a reliable identification of the binary code encoded by the tag is possible in few tens of millisecond, with a bit error rate smaller than 1 percent.

Considering its different interaction forms with cellulose, water is very problematic as we consider high frequencies electromagnetic applications of papers. Thereby, cellulosic materials exhibit high losses in the radiofrequency (RF) domain. In this paper, the RF dielectric behavior of different papers versus their moisture content is described and the huge impact of water upon losses is shown. We also present a three components effective medium theory that is able to describe and predict the impact of this water content.

International audience; Cellulosic materials are very interesting for electronics industry. Cheap, environment friendly, easy to produce and flexible, they could be remarkable substrates. However, the poor properties of cellulose in regard to the ambient humidity rate is a real technological lock to the development of this very promising material. We propose, in this paper, to analyze and quantify the impact of humidity upon the RF properties of cellulose papers. The proposed theoretical model for the contribution of water to losses in the medium is validated by experimental results, and thus can help for the design of transmission lines on paper substrates.

This paper proposes a novel radar based approach for chipless tag discrimination. The proposed technique can be considered as a first step towards chipless authentication of manufactured products. The concept of chipless radiofrequency identification (RFID) is extended to tag discrimination, where each tag produces a unique signature that is very difficult to reproduce even if a clone of the device is made. The proposed technique, using radiofrequency (RF) electromagnetic (EM), keeps non-invasive and non-destructive. This new method to discriminate tag's RF signature is introduced based on the comparison between two resemblance metrics in the frequency domain and time domain. To calculate the resemblance between different signatures entire part of the RF signatures is exploited to utilize the maximum signals' richness. Owing to the chipless RF approach, we show that geometrical variations less than $\pmb{10\ \mu \mathrm{m}}$ (i.e., smaller than $\lambda/1000$ ) can be detected which demonstrate the extreme sensitivity of the method.

As no terahertz signal is transmitted through opaque materials, reflection terahertz time-domain spectroscopy is obviously the dedicated setup. On the other hand, optical properties of transparent and thick samples are extracted more accurately with transmission terahertz time-domain spectroscopy. In this paper, we report the intrinsic frontier above or below which it is preferable to use either reflection or transmission setup for probing with the best accuracy possible a given sample.

We excite surface plasmons (SPs) at the surface of highly doped Si using a prism coupler, and we study the propagation properties of these SPs in order to characterize the terahertz (THz) response of the doped semiconductor. Thanks to the long interaction length of the propagating SP with the substrate material, the method is more sensitive than classical THz time-domain spectroscopy in reflection or in transmission. Moreover, we propose a new technique based on measuring the SP signal, for which the delicate problem of accurately measuring the phase of the signal is solved. All of these different experiment techniques allow us to determine reliably the dielectric function of highly doped Si in the THz range. It appears that the experimentally determined values differ strongly from the ones calculated with a Drude model.

A modified Bakman Technologies PB7200-2000-T portable THz spectrometer mounted to a consumer drone was employed to measure water vapor absorptions ten meters above the ground.

International audience; A free-space-based chipless approach using a second-order bandpass filter model is presented in this letter to detect minimum geometrical variations along the dimensions of a C-folded scatterer. With the help of this model, further randomness inherent in fabrication process can also be analyzed and forecast. Quad C-folded scatterers are used to realize chipless tags and three groups of tags exhibiting distinct arms' length are fabricated. A cosine similarity function is then used to evaluate the similarity rate of electromagnetic signatures reflected by tags coming from different groups. Experimental results performed in a realistic outdoor environment show that it is possible to discriminate two tags which differ from each other by a mean variation of around 25 μm along the arms' length. Finally, the proposed analytical model is capable to dissociate the tags by using merely two extracted aspect-independent quantities: the frequency of resonance and the quality factor.

High frequency paper-based electronics is developing fast, with smart yet low cost applications in view. However, the dielectric losses of paper remain a difficult hurdle to overcome. First, the literature on the topic is reviewed and the contributions from moisture, air, mineral filler, and wood constituents to dielectric losses are highlighted. Then, the guidelines for future research are defined, including the need for systematic comparisons in controlled moisture conditions. In an effort to produce low loss materials for paper-based electronics, it is proposed here to impregnate paper with a low loss substance, after removing residual water.

We report on an additional phase noise induced by the delay line or by the ASOP technique in different commercial terahertz time-domain systems. This additional noise increases the uncertainty on the refractive index extracted from a transmission characterization and on the coefficient of absorption in a reflection scheme. In this latter case, it can make impossible a precise extraction of the complex refractive index.

A Bakman Technologies PB7220-2000-T frequency-domain THz spectrometer is modified to allow it to operate autonomously while attached to a DJI-S1000 consumer drone. The THz spectrum of atmospheric water vapor is recorded during an 8 min flight 10 m above the ground at Paramount Ranch Park, Agoura Hills, CA, USA. The spectrum is then compared to theory and the relative humidity determined.

International audience

This paper investigates the limit of detection of the spectral fingerprints of a material to be identified by terahertz spectroscopy, when the material is bare or concealed by clothes. After a theoretical study to determine the minimum observable peak amplitude versus the refractive index and the coefficient of absorption of the sample material, we discuss the case of detecting the absorption peaks from the amplitude or phase of the transmitted or reflected spectra recorded using a THz timedomain spectroscopy system.

We propose here to experimentally demonstrate original methods, combining terahertz time-domain spectroscopy (THz-TDS) and Kramers-Kronig (KK) relations, to fully characterize in transmission non-classical samples for example exhibiting either low transmission bands, or scattering effect. For such samples, classical THz-TDS extraction process leads to erroneous optical parameters (refractive index and absorption coefficient) whose causal relations are in turn not fulfilled. Therefore, by comparing the measured optical parameters with those calculated using the KKR, we can estimate the error and thus correct it to precisely characterize the sample.

We propose and experimentally demonstrate an original method, combining terahertz time-domain spectroscopy (THz-TDS) and Kramers–Kronig relations (KKR), in order to characterize in transmission materials exhibiting high absorption peaks. At these frequencies, the modulus of transmittance of a sample could be null, or at least smaller than the detector noise, and consequently the signal phase is lost, not only at the peak frequency, but also at higher frequencies. Thus, for these high frequencies, the value of the refractive index estimated from the measured THz-TDS data could be strongly erroneous. Using an optimized version of KKR, namely singly subtractive KKR, we are able to retrieve the phase of the THz transmittance over the whole studied bandwidth and thus to correct the THz-TDS data to precisely determine the refractive index as well as the absorption coefficient of the material over almost the whole THz range.

International audience

Chercheurs et ingenieurs revent d'utiliser les ondes electromagnetiques terahertz (THz), c'est-a-dire appartenant a l'infrarouge tres lointain, dans de nombreuses applications entrevues dans des domaines aussi varies que les telecommunications a tres haut debit, l'imagerie medicale, les capteurs pour l'industrie, etc., parmi lesquels la securite et la defense prennent une part preponderante. Les activites de recherche et developpement dans le domaine THz ont ete stimulees par l'avenement, a la fin des annees 1980, de nouvelles techniques et technologies, basees principalement sur l'optoelectronique, et actuellement mettant aussi en jeu la micro- et la nanoelectronique. Cet article presente les principes de la technologie THz [1] et l'etat de l'art des etudes et developpements pour la securite et la defense. Cet etat de l'art est sans doute incomplet, car nombre des travaux menes sont classes confidentiels et ne sont pas publies.

International audience

We report on the transmission of terahertz (THz) electromagnetic waves through gold grids evaporated over a transparent silicon substrate. We observe a near-unity transmission in the subwavelength regime. When the THz wavelength is comparable to the grid periodicity, the grid serves as a grating coupler that allows us to excite guided modes in the device. These guided modes exhibit an intermediate behavior between a surface plasmon mode and modes of a dielectric slab waveguide.