THz generation in GaSe crystal pumped below and around the bandgap

International audienc

Terahertz pulse generation in ZnTe crystal pumped around the bandgap

International audienc

Nouveaux tags RFID sans puce fonctionnant dans le domaine Terahertz

National audienceno abstrac

Terahertz Generation in Nonlinear Crystals Pumped from UV to IR

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A novel machine learning approach in Image Pattern Recognition under invariance constraints

International audienceTwo of the main challenges of image recognition in radar, acoustic or Tray imaging regard the viewpoint variation of the pattern and the feature extraction techniques that must retrieve the most discriminative information about different classes. In this paper, we focus on feature extraction and image classification techniques by using a Rotation Invariant Wavelet Packet Decomposition and a novel entropybased feature extraction technique to characterize an image. The entropy-based characterization described in the paper offers an extended analysis compared to usual approaches such as the energy of the wavelet sub bands. The computed features will be further used to train a Graph Neural Network adapted to a quad-tree decomposition which has the powerful advantage of considering the structural information of the rotationinvariant decomposition. We successfully classified the images with an accuracy of 99.3%. The results are compared to other classic feature extraction techniques such as k-NN, SVM and WPD, proving the increased capability of our method

THz generation in GaSe crystals pumped with laser photon energy below and around the bandgap

International audienceWe study optical rectification in GaSe by performing THz generation with femtosecond laser pulses whose wavelength is tuned from below to above the GaSe bandgap. As expected from a theory, we observed a first THz emission peak at 1.77 eV, where phase matching is realized. A second THz emission peak was recorded, when the pump photon energy reaches the crystal bandgap (2.205 eV). This can be attributed to a resonance of the GaSe nonlinearity. In crystals thinner than the coherence length, the bandgap peak is stronger than the phase-matched one

THz generation in GaSe crystals pumped with laser photon energy below and around the bandgap

International audienceWe study optical rectification in GaSe by performing THz generation with femtosecond laser pulses whose wavelength is tuned from below to above the GaSe bandgap. As expected from a theory, we observed a first THz emission peak at 1.77 eV, where phase matching is realized. A second THz emission peak was recorded, when the pump photon energy reaches the crystal bandgap (2.205 eV). This can be attributed to a resonance of the GaSe nonlinearity. In crystals thinner than the coherence length, the bandgap peak is stronger than the phase-matched one

Electrical characterization of silver nanowire-graphene hybrid films from terahertz transmission and reflection measurements

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.1561sciescopu

Video-Rate Identification of High-Capacity Low-Cost Tags in the Terahertz Domain

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