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Interaction between metal surface waves and periodic geometry of subwavelength structures is at the core of the recent but crucial renewal of interest in plasmonics. One of the most intriguing points is the observation of abnormal strong transmission through these periodic structures, which can exceed by orders of magnitude the classical transmission given by the filling factor of the plate. The actual paradigm is that this abnormal transmission arises from the periodicity, and then that such high transmission should disappear in random geometries. Here, we show that extra energy can be coupled through the subwavelength structure by adding a controlled quantity of noise to the position of the apertures. This result can be modelled in the statistical framework of stochastic resonance. The evolution of the coupled energy with respect to noise gives access to the extra energy coupled at the surface of the subwavelength array., Comment: 12 pages

Carrying digital information in traditional copper wires is becoming a major issue in electronic circuits. Optical connections such as fiber optics offers unprecedented transfer capacity, but the mismatch between the optical wavelength and the transistors size drastically reduces the coupling efficiency. By merging the abilities of photonics and electronics, surface plasmon photonics, or 'plasmonics' exhibits strong potential. Here, we propose an original approach to fully understand the nature of surface electrons in plasmonic systems, by experimentally demonstrating that surface plasmons can be modeled as a phase of surface waves. First and second order phase transitions, associated with percolation transitions, have been experimentally observed in the building process of surface plasmons in lattice of subwavelength apertures. Percolation theory provides a unified framework for surface plasmons description.

International audience; La connaissance déjà acquise sur la dynamique de la liaison hydrogène dans l'eau liquide grâce au développement de laser délivrant des impulsions ultra-courtes dans l'infrarouge moyen nous permet de filmer la rotation de molécules HDO dans une solution D2O. L'expérience réalisée au laboratoire est basée sur la technique de spectroscopie pompe sonde résolue en polarisation. L'anisotropie mesurée permet de détecter en temps réel l'angle de déflexion du moment dipolaire de transition dont le point de départ est la direction du faisceau laser de pompe.

International audience; A pump-probe experiment is described to study femtosecond dynamics of hydrogen bonds in liquid water. The key element of the experimental setup is a laser source emitting 150 fs pulses in the 2.5-4.4 urn spectral region, at 10 nJ power level. The OH-stretching band is recorded for different excitation frequencies and different pump-probe delay times. Time-dependent solvatochromic shifts are observed and are interpreted with the help of statistical mechanics of nonlinear optical processes. Using these spectral data, the OH -O motions are "photographed" in real time. Cop. EDP Sciences.

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International audience; We describe a highly sensitive and stable quantum-cascade laser-based attenuated total reflection (ATR) terahertz sensor for the detection of very low concentration solutions, using a dual-modulation differential approach and ATR geometry. This sensor offers a very high dynamic range and along-term stability of 40 dB, which extends the potential of terahertz radiation for the analysis of liquid and biological samples. The performance is illustrated by measurements on standard solutions of ions, sugars, and proteins, for concentrations down to 1 M.

International audience; Using attenuated total reflection (ATR) in the terahertz domain, we demonstrate non-invasive, non-staining real time measurements of cytoplasm leakage during permeabilization of live epithelial cells by saponin detergent and after electropermeabilization. The origin of the contrast observed between cells and culture medium is addressed by both experimental and theoretical approaches, and gives access to permeabilization dynamics of live cells in real time. We show that terahertz modalities are more sensitive than fluorescence microscopy which is the reference optical technique for electropermeabilization. We propose analytical models for the influx and efflux of non-permeant molecules through permeabilized cell membranes.

International audience; Understanding the relevant parameters of the formation of pores during permeabilization is very challenging for medical applications. Several components are involved: the arrival of the permeabilizing molecules to the membrane, the efficiency of formation of the pores and their specific dynamics, and the flux of molecules through the plasma membrane. Using attenuated total reflection in the terahertz domain, we studied the dynamics of Madine-Darby canine kidney cells after permeabilization by saponin molecules. We developed an analytical model taking into account saponin molecule diffusion, cell geometry, cytosol molecule diffusion, and pore dynamics. We also studied the effect of possible pore overlapping on the cell membrane, introducing a dimensionless quantity that is the ratio between overlapping and diffusive effects. Pores are found to be static within 1 h after their creation, hinting that the diffusion of the saponin molecules to the membrane is the limiting factor in our experiments.

The failure to accurately delineate breast tumor margins during breast conserving surgeries results in a 20% re-excision rate. Consequently, there is a clear need for an operating room device that can precisely define intraoperatively breast tumor margins in a simple, fast, and inexpensive manner. This manuscript reports investigations that were conducted towards the ability of terahertz radiations to recognize breast malignant lesions among freshly excised breast volumes. Preliminary works on terahertz far-field spectroscopy have highlighted the existence of a contrast between healthy fibrous tissues and breast tumors by about 8% in refractive index over a spectral window spanning from 300 GHz to 1 THz. The origin for contrast was explored. Results seem to indicate that the dynamics of quasi-free water molecules may be a key factor for demarcation. On these basis, different methods for tissue segmentation based on refractive index map were investigated. A cancer sensitivity of 80% was reported while preserving a specificity of 82%. Eventually, these pilot studies have guided the design of a BiCMOS-compatible near-field resonator-based imager operating at 560 GHz and sensitive to permittivity changes over breast tissue surface.; La faible précision avec laquelle sont délimitées les marges d'exérèse des adénocarcinomes mammaires se traduit par un recours régulier à un second acte chirurgical. Afin d'en limiter la fréquence, la communauté scientifique tente de définir les grands axes de conception d'un système intraopératif permettant la reconnaissance des lésions mammaires malines. Ce manuscrit de thèse rapporte les investigations menées sur la capacité des ondes térahertz à fournir un contraste entre tissus mammaires sains et malins. Les premiers travaux ont montré l'existence d'un contraste sur l'indice de réfraction entre 300 GHz et 1 THz, évalué en moyenne à 8%. Ce dît contraste semble prendre origine dans la dynamique des molécules d'eau intrinsèques aux cellules cancéreuses. Différentes techniques de segmentation d'image, basées sur l'indice de réfraction des zones tissulaires, ont permis de rapporter une sensibilité au cancer jusqu'à 80% tout en maintenant un taux de spécificité de l'ordre de 82%. L'ensemble de ces études a guidé la conception d'un imageur champ-proche opérant à 560 GHz, dont la réponse des différents senseurs est sensible à la permittivité en surface des tissus du sein.

International audience; Terahertz radiation offers new contrasts with biological systems, without markers or staining, at the molecular, cellular or tissue level. Thanks to technological advances, it is increasingly emerging as a solution of choice for directly probing the interaction with molecules and biological solutions. Applications range from dynamics of biological molecules to imaging of cancerous tissues, including ion, protein and membrane sensors.

International audience; The electromagnetic wavefront diffracted by an object carries information about the shape of the object from which the wave was emitted. Being able to record the phase and intensity of such a wave thus allows to reconstruct the object from the information carried by the wave, even if the object is no longer present. Among the reconstruction techniques, holography plays a big part. However the waves may experience a great variety of distortions on their way from the object to the measurement apparatus. Thus being able to shape the wavefront at will is key in holography. Micromirror light modulators are powerful tools for that matter and are well known for holographic applications. This paper explores the fundamental principles for digitally reconstructing a precise image of an object, but also for digitally correcting an imperfectly shaped wavefront, by exploiting the diffraction properties of light on a reflective surface. The methods presented here have been implemented as part of practical work for 2nd year students at the Ecole Polytechnique (last year of undergraduate program).

This thesis deals with the theoretical and experimental study of the generation and shaping of terahertz (THz) pulses. At first, we have studied the generation of THz pulses by optical rectification of intense femtosecond laser pulses in semiconductor ZnTe crystals. We have demonstrated a strong dependence of the efficiency of the generation process with the intensity of the laser pulse. This is reflected particularly by a progressive shift of the spectrum towards lower frequencies and an abnormal evolution of the THz energy with laser intensity. These behaviors result from a combination of three phenomena: the depletion of the laser pulse during its propagation in the crystal, the absorption of THz wave by free carriers created by two-photon absorption and a change of the phase matching condition induced by hot carriers. Secondly, we have developed an analytical approach to generate THz shaped pulses particularly interesting for coherent spectroscopy or coherent control. In particular, we have generated phase-locked THz pulses pairs, pulse trains and tunable narrow-band THz pulses. This technique is based on optical rectification of shaped femtosecond laser pulses generated by a liquid crystal Fourier filter. To demonstrate the validity of our approach, we have also developed a program giving results in very good agreement with experiment., Cette thèse présente l'étude théorique et expérimentale de la génération et de la mise en forme d'impulsions térahertz (THz). Dans un premier temps, nous avons étudié la génération d'impulsions THz par redressement optique d'impulsions laser femtosecondes intenses dans des cristaux semiconducteurs de ZnTe. Nous avons mis en évidence une forte dépendance de l'efficacité de ce processus de génération avec l'intensité de l'impulsion laser génératrice. Ceci se traduit en particulier par un décalage progressif du spectre vers les basses fréquences et par une évolution anormale de l'énergie THz avec l'intensité laser. Ces comportements résultent d'une combinaison de trois phénomènes : la déplétion de l'impulsion laser au cours de sa propagation dans le cristal, l'absorption de l'onde THz par les porteurs libres créés par absorption à deux photons et une modification de la condition d'accord de phase induite par les porteurs chauds. Dans un deuxième temps, nous avons développé une approche analytique permettant de générer des impulsions THz de formes particulièrement intéressantes pour les expériences de spectroscopie cohérente ou de contrôle cohérent. Nous avons notamment généré des paires d'impulsions verrouillées en phase, des trains d'impulsions, ainsi que des impulsions THz accordables de grande finesse spectrale. Cette technique repose sur le redressement optique d'impulsions laser femtosecondes mises en forme à l'aide d'un masque à cristaux liquides placé dans le plan de Fourier d'une ligne à dispersion nulle. Afin de démontrer la validité de notre approche, nous avons également développé un programme de simulation qui donne des résultats en très bon accord avec l'expérience.