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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.

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.

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 article, we report on video-rate identification of very low-cost tags in the terahertz (THz) domain. Contrary to barcodes, Radio Frequency Identification (RFID) tags, or even chipless RFID tags, operate in the Ultra-Wide Band (UWB). These THz labels are not based on a planar surface pattern but are instead embedded, thus hidden, in the volume of the product to identify. The tag is entirely made of dielectric materials and is based on a 1D photonic bandgap structure, made of a quasi-periodic stack of two different polyethylene-based materials presenting different refractive indices. The thickness of each layer is of the order of the THz wavelength, leading to an overall tag thickness in the millimetre range. More particularly, we show in this article that the binary information coded within these tags can be rapidly and reliably identified using a commercial terahertz Time Domain Spectroscopy (THz-TDS) system as a reader. More precisely, a bit error rate smaller than 1% is experimentally reached for a reading duration as short as a few tens of milliseconds on an 8 bits (~40 bits/cm2) THID tag. The performance limits of such a tag structure are explored in terms of both dielectric material properties (losses) and angular acceptance. Finally, realistic coding capacities of about 60 bits (~300 bits/cm2) can be envisaged with such tags.

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.

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.

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.

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.

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.

International audience

In the present paper, we propose a new structure of chipless and low-cost tag for data encoding in the terahertz frequency range. The device is based on a multilayer structure in which the thicknesses of the different layers are of the order of the wavelength, i.e., in the submillimeter range. In this device, the information is encoded in the volume of the tag thanks to the adjustable refractive index and low-cost materials, leading to a high level of security. The two main advantages compared to classical radiofrequency identification tags are the absence of metal and the encoding of the information in the volume of the structure, thus limiting the risk of damage during handling and preventing from reverse engineering, for example.

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

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.

This paper investigates the limit of detection of the spectral fingerprints of a material to be identified by terahertz (THz) spectroscopy, when the material is bare or concealed by clothes. We propose 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 time-domain spectroscopy system. We experimentally validate our analysis by studying a pellet sample of glutamic acid, bare or hidden under a layer of various fabrics, both in transmission and reflection configuration. This allows us to investigate the influence of various radiometric responses induced by fabrics when spectra are recorded in realistic conditions. In the reflection scheme, interference effects make the identification difficult, if not impossible.

We study the far infrared electromagnetic response of grating devices by THz time-domain spectroscopy. We flrst show that THz waves are efflciently injected into silicon waveguides using grating couplers. Moreover, changing the waveguide material parameters by white-light illumination allows us to strongly modify the coupling e‐ciency. Then we demonstrate resonant efiects in segmented grating structures that act as perfect mirrors at selected wavelengths even for focused beams. About 10 periods of the grating participate in the phenomenon, nevertheless the resonance frequency width of the device remains narrow. This collection of experiments shows that millimeter-size mock-ups and THz waves can be efiectively used to extrapolate the optical response of micron-size actual devices.

A high-quality smart filter for terahertz range with relative tunability reaching 20% has been demonstrated. The filter is based on a narrow transmission band, which originates from a defect mode that appears due to insertion of a single crystal of KTaO3 into otherwise periodic one-dimensional photonic crystal. Frequencies of defect modes are controlled by the refractive index of the defect: their high tunability is achieved by the strong temperature dependence of the dielectric properties of KTaO3. The low losses of KTaO3 lead to a high peak transmission of the filter. Influence of the defect losses on the properties of the filter is also discussed.

THz waves have recently attracted considerable interest mainly because of their various applications. Efficient and compact CW sources are currently under development. Their optimization requires the most efficient coupling of THz radiation from the intrinsic inner semiconductor source to the outside world. In terms of compactness and planarity, this is best achieved using antennas that are fabricated using common processes in semiconductor technology. Although most antennas are narrow band devices, wide bands are expected from fractal 2D geometries like the Sierpinski gasket. Original results are presented on the design, fabrication and THz transmission of 2D metallic fractal deposited on highly resistive GaAs substrate as well as on thin SiNx membranes. Experimental transmission measurements are compared to theory. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present experimental results as well as modelling concerning terahertz (THz) wave scattering by powder materials with grain size of the order of the wavelength. The studied material is a mixture of high density polyethylene and fructose powders, whose grain size is varied from a sample to another one while the volume concentration is kept constant. Experimental data are recorded with a THz time-domain spectroscopy set up. The scattering contribution to the total propagation loss in such powders shows a resonant behaviour for grain size roughly equal to the THz wavelength, for which it reaches 80%–90% of the total loss. Modelling using Mie theory allows us to well describe the absorption and scattering phenomena.

We measure the refractive index and the absorption of porous silicon layers in the millimetric and submillimetric wavelength range using the terahertz time-domain spectroscopy technique. For the studied range of porosity (55%–76%), the refractive index of porous silicon is rather well described by mixture theories, in which the refractive index of bulk silicon enters as a main parameter.

Measuring the DC conductivity of very thin films could be rather difficult because of the electrical contact issue. This DC conductivity can, however, be extracted from noncontact measurements at GHz and THz frequencies using elaborated conductivity models that nicely fit the experimental data. Here we employ this technique to study the DC conductivity of fragile nanometer-thick films of multiwalled carbon nanotubes and monolayer graphene. The THz response of the films is measured by THz time-domain spectroscopy. We show that the THz conductivity of the samples is well fitted by either Drude-Lorentz model or Drude-Smith model, giving information on the physics of electrical conductivity in these materials. This extraction procedure is validated by the good agreement between the so-obtained DC conductivity and the one measured with a classical 4-point probe in-line contact method.

We determined the electrical sheet conductivity of silver nanowire-graphene hybrid films from transmission and reflection terahertz time-domain spectroscopy measurements. The sheet resistance extracted from noncontact terahertz measurement is in good agreement with one measured with a classical 4-point-probe technique. The conductivity is well described by a Drude-Smith model and is calculated to peak around 10 THz.

We explore the optical and electrical characteristics of monolayer graphene by using pulsed optoelectronic terahertz time-domain spectroscopy in the frequency range of 325–500 GHz based on fast direct measurements of phase and amplitude. We also show that these parameters can, however, be measured with higher resolution using a free space continuous wave measurement technique associated with a vector network analyzer that offers a good dynamic range. All the scattering parameters (both magnitude and phase) are measured simultaneously. The Nicholson-Ross-Weir method is implemented to extract the monolayer graphene parameters at the aforementioned frequency range.

Terahertz Time-Domain Spectroscopy (THz-TDS) is one of the most powerful techniques to characterise the far infrared response of materials and devices. Based on the use of short electromagnetic waveforms generated by rectifying femtosecond optical pulses delivered by a mode-locked laser, it allows, within one single experiment, the determination of the optical constants of materials over a wide frequency spectrum ranging from 100 GHz up to several THz. Thanks to its large dynamics and its ability to give both amplitude and phase of the transmitted, reflected or scattered THz fields, this technique addresses a large panel of scientific studies. In this chapter, one introduces the principles of THz-TDS. First, the characteristics and performances of a classical set-up are described, then the extraction of the refractive index and absorption coefficient of the studied device from its complex experimental transmission or reflection coefficients are explained. As a demonstration of the power of this technique, one presents some results of thin film spectroscopy and recent studies of left-handed metamaterials. Finally, a detailed analysis of the precision of determined optical constants is given. One will treat not only the classical transmission scheme, but also the reflection regime that is mandatory in the study of strongly absorbing media.

A vector network analyser is used to study the electrical properties of multi-walled carbon nanotube (MWCNT) thin films deposited on a fused quartz substrate in the sub-terahertz (THz) frequency ranges of 220-325 GHz (WR3.4) and 325-500 GHz (WR2.2). The experiment is performed in free space. The complex permittivity of the MWCNT thin films is extracted using the Nicholson-Ross-Weir method. The refractive index and conductivity are then determined from the extracted permittivity. The method is validated by comparison with values obtained using THz time-domain spectroscopy.

In security applications, items or substances of interest are generally hidden and therefore, for body inspection, must be identified through clothes. We show that diffractive behavior of fabrics when illuminated by THz wave can complicate such approach. This effect is demonstrated performing an angle-resolved THz time-domain spectroscopy experiment and confirmed by a HFSS numerical simulation.

We propose to measure the THz surface plasmon signal transmitted after a long distance propagation over a conductive sample in view of determining the THz properties of the sample material. We demonstrate this very sensitive method on a highly-doped silicon samples. The surface permittivity differs from the Drude model prediction.

Summary form only given. Terahertz time-domain spectroscopy (THz-TDS) is nowadays commonly used to measure the absorption coefficient and refractive index of materials over a very broad spectral band (typically from 0.1 up to several THz). Usual TDS set-ups include ultra-fast emitter and receiver, like photo-conductive antennas or non-linear crystals. Indeed, their ultra fast response time allows one to generate and/or detect sub-picosecond electromagnetic waveforms, whose spectra spread up to the THz range. Nevertheless, such devices require a whole optical bench to manage a pulsed laser beam in both space and equivalent time. On the contrary, power detectors (e.g. bolometers, Golay cells, pyroelectric detectors...) are more convenient to use, allowing an easy mapping the spatial distribution of THz beams. However, the temporal resolution of these detector remains poor (millisecond at best) as they integrate the incoming THz power. Therefore the THz pulse transient evolution and thus the THz E-field phase are lost. Here we describe a THz Michelson interferometer in which the receiver is a pyroelectric detector (Gentec THz 9B-MT, 200-ms rise time). The interferometer permits us to retrieve the phase of the signal. Transient autocorrelation curves of a picosecond pulse have been recorded and are in a good agreement with analytic expectations. The modulus and phase of the THz signal is obtained by Fourier-transforming the recorded waveforms. 2D-mappings of the THz beam power, of the transient signals and of the frequency spatial distribution have been experimentally achieved. The influence of the impinging THz polarization on the measurement dynamics has also been studied. Finally, we employed the set-up to perform broadband spectroscopy of samples.

In this paper, we propose a method to determine the refractive index and the absorption coefficient of samples showing spectral bands of very low transmission. The method, takes advantages of both transmission and reflection terahertz time-domain spectroscopy (THz-TDS) to obtain more precise optical constants than when only reflection measurement is used. It has been then validated using pellets made of maltose, which exhibits resonant lines within the THz spectral range.

This paper introduces a novel method that allows fast and reliable extraction of material parameters in terahertz time-domain spectroscopy. This method could be applied for most materials and requires neither simplifying assumptions nor samples of different thickness for the extraction. The presented extraction procedure operates either on truncated terahertz signals when temporal windowing is possible, or on full ones otherwise. Some experimental examples covering all practical cases are given. In particular, the extraction procedure treats the tedious case of samples for which internal reflections of the terahertz pulse slightly overlap.

We show experimentally and numerically that cholesteric-type metal-dielectric structures, made of twisted sub-wavelength hole elliptical–shaped arrays, exhibit strong optical activity and circular dichroism. The experimental demonstration is carried out with terahertz time-domain spectroscopy measurements on a three layered structure operating around 0.5 THz, leading to a rotary power as high as 1000°/wavelength. The contribution of the chirality factor (κ), permittivity (ɛ), and permeability (μ) is discussed after the retrieval of effective parameters from the complex scattering ones.

International audience; In this paper, we demonstrate for the first time that a 19-bit chipless tag based on a paper substrate can be realized using the flexography technique, which is an industrial high-speed printing process. The chipless tag is able to operate within the ultra-wide band (UWB) and has a reasonable size ( 7×3 cm 2) compared to state-of-the-art versions. Thus, it is possible to use this design for various identification applications that require a low unit cost of tags. Both the simulation and measurement results are shown, and performance comparisons are provided between several realization processes, such as classical chemical etching, flexography printing, and catalyst inkjet printing.

The cyclic olefin copolymer (COC) has recently demonstrated promising properties for THz applications due to its extremely high transparency in the THz region. Here, we prove that COC can be efficiently used as substrate material for free-space THz devices through the design, fabrication, and characterization of high-pass metal mesh filters. Measurements are in good agreement with calculations, and a transmittance higher than 75% has been measured between 1.5 THz and 2.5 THz for a single-layer filter. In addition, we prove that stacked meshes can be easily embedded to improve their rejection ratio in the stop-band, while preserving a high transparency in the pass-band. The broadband behavior of these filters should extend up to their diffraction limit estimated at around 6.3 THz for the single-layer filter.

Here, we demonstrate the possibility of employing a novel low-loss dielectric material as substrate for free-space THz devices through the design, fabrication and characterization of broadband high-transparency high-pass mesh filters. Time-domain spectroscopy measurements show that a transmittance higher than 75% is achieved over a bandwidth of 1 THz.

In the present paper, we present a new approach for data encoding in the THz frequency domain. We proposed a structure based on a 1D periodic stack of dielectric layers whose thickness is of the order of the wavelength, i.e. in the millimeter range. Such a device exhibits well known forbidden band gap (FBG) behavior when illuminated by an electromagnetic wave. We suggest modifying the periodicity of the stack to create defect levels in the 1st FBG to encode binary information. The layers are made of low cost dielectric materials such as pure low density polyethylene (LDPE) and a mixture of LDPE with mineral charges like CaCO 3 or TiO 2 as respectively low (L) and high (H) refractive index material. We show that such a device can be used for binary data encoding in the frequency range from 150 to 600 GHz with encoding capacities greater than 10 bits.

The electrical properties of carbon nanostructures have been greatly stimulating to use in the nanotechnology for electronic components. In this paper, we study the AC and DC electrical conductivity responses of multi-walled carbon nanotube films, prepared by the vacuum filtration methods, with noncontact terahertz time-domain spectroscopy (THz- TDS) approach utilizing the extrapolation analysis as well as probe-in line technique.