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International audience; Conversion of terahertz radiation into thermal radiation is a low-cost approach for terahertz detection by standard infrared camera. In this work, THz→IR thermo-conversion is performed by the combination of a THz absorber made of MIM antenna and an emissive layer made of carbon nanotubes. Structural and optical characterizations of the membrane are done, and multispectral imaging in the terahertz range is demonstrated.

Infrared (IR) reflectography has been used for many years for the detection of underdrawings on panel paintings. Advances in the fields of IR sensors and optics have impelled the wide spread use of IR reflectography by several recognized Art Museums and specialized laboratories around the World. The transparency or opacity of a painting is the result of a complex combination of the optical properties of the painting pigments and the underdrawing material, as well as the type of illumination source and the sensor characteristics. For this reason, recent researches have been directed towards the study of multispectral approaches that could provide simultaneous and complementary information of an artwork. The present work relies on non−simultaneous multispectral inspection using a set of detectors covering from the ultraviolet to the terahertz spectra. It is observed that underdrawings contrast increases with wavelength up to 1700 nm and, then, gradually decreases. In addition, it is shown that IR thermography, i.e., temperature maps or thermograms, could be used simultaneously as an alternative technique for the detection of underdrawings besides the detection of subsurface defects.

Attosecond electron wavepackets are produced when an intense laser field ionizes an atom or a molecule(1). When the laser field drives the wavepackets back to the parent ion, they interfere with the bound wavefunction, producing coherent subfemtosecond extreme-ultraviolet light bursts. When only a single return is possible(2,3), an isolated attosecond pulse is generated. Here we demonstrate that by modulating the polarization of a carrier-envelope phase-stabilized short laser pulse(4), we can finely control the electron-wavepacket dynamics. We use high-order harmonic generation to probe these dynamics. Under optimized conditions, we observe the signature of a single return of the electron wavepacket over a large range of energies. This temporally confines the extreme-ultraviolet emission to an isolated attosecond pulse with a broad and tunable bandwidth. Our approach is very general, and extends the bandwidth of attosecond isolated pulses in such a way that pulses of a few attoseconds seem achievable. Similar temporal resolution could also be achieved by directly using the broadband electron wavepacket. This opens up a new regime for time-resolved tomography of atomic or molecular wavefunctions(5,6) and ultrafast dynamics.

A high-order harmonic generation process using carrier-envelope phase stabilized few-optical-cycle light pulses is investigated. A time-frequency analysis of generated spectra is used to retrieve direct information about the temporal structure of the generated extreme ultraviolet radiation.

The temporal behaviour and relative contributions of both cascaded second order and third order optical effects to the index variation of cubic crystals are evidenced in the femtosecond regime.

We present theoretical and experimental results on high-order harmonic generation in a low-density few-centimeter-long gas medium (${L}_{\mathrm{med}}\ensuremath{\leqslant}$ 10 cm). We study the dependence with focal length of harmonic efficiency. Theoretically, we consider in detail the generation of the $25\mathrm{th}$ harmonic of a short pulse Ti:sapphire laser in argon. Within the strong-field approximation for the atomic dipole, and a complete account of the macroscopic propagation, we compute the number of photons produced as a function of the medium parameters and the focusing conditions. The simulations show that, at constant intensity, the emission of the $25\mathrm{th}$ harmonic scales with the focal length as $\ensuremath{\sim}$$f$${}^{4}$ at low pressure ($P=2$ Torr) and as $\ensuremath{\sim}$$f$${}^{6}$ at higher pressure ($P=5$ Torr). At constant laser energy, we find that the harmonic signal scales approximately as $f$${}^{2}$ at low pressure and as $f$${}^{4}$ at higher pressure. Those numerical results are compared with experimental data.

A monochromatic millimeter wave imaging system coupled with an infrared temperature sensor has been used to investigate historic objects preserved at the museum of Aquitaine. Especially, 2D and 3D analysis have been performed in order to reveal the internal structure of a nearly 3500 years old sealed Egyptian jar.

International audience; 3D terahertz computed tomography has been performed using a monochromatic millimeter wave imaging system coupled with an infrared temperature sensor. Three different reconstruction methods (standard back-projection algorithm and two iterative analysis) have been compared in order to reconstruct large size 3D objects. The quality (intensity, contrast and geometric preservation) of reconstructed cross-sectional images has been discussed together with the optimization of the number of projections. Final demonstration to real-life 3D objects has been processed to illustrate the potential of the reconstruction methods for applied terahertz tomography.

We defined the Terahertz Thermal Converters concept to design a device for 2D THz imaging. Coupled to an infrared temperature flux sensors, we demonstrate an absolute auto-calibrated Terahertz power-meter (>10µW) enable to characterized THz spatial beam profile of CW sources.

A monochromatic millimeter wave system coupled with an infrared temperature sensor has been used to investigate pictorial artworks. The flexibility of the system makes it possible to rapidly reveal buried layer information such as a graphite handmade sketch covered by paintings.

The potential of terahertz technology has been clearly demonstrated by its large applications in security and defence (remote detection of object). A flexible alternative monochromatic millimeter wave system coupled with an original infrared temperature sensor has been developed to visualize large size 3D manufactured opaque phantoms with different refractive index contrasts. The results clearly illustrate applied terahertz tomography particularities such as boundary effects, refraction and diffraction losses that must be prevented for efficient inspection and detection.

Flexible monochromatic millimeter wave system coupled with an infrared temperature sensor demonstrates large size 3D visualization of manufactured opaque phantoms with different refractive index contrasts. Peculiarities such as boundary effects, refraction and diffraction losses will be discussed.

International audience; Coherent x-ray diffractive imaging is a powerful method for studies on nonperiodic structures on the nanoscale. Access to femtosecond dynamics in major physical, chemical, and biological processes requires single-shot diffraction data. Up to now, this has been limited to intense coherent pulses from a free electron laser. Here we show that laser-driven ultrashort x-ray sources offer a comparatively inexpensive alternative. We present measurements of single-shot diffraction patterns from isolated nano-objects with a single 20 fs pulse from a table-top high-harmonic x-ray laser. Images were reconstructed with a resolution of 119 nm from the single shot and 62 nm from multiple shots.

We demonstrate that the characteristics of the high-order harmonic spectra generated by few-cycle carrier-envelope phase-stabilized pulses can be finely adjusted by controlling the time-dependent ellipticity. The experimental measurements show evidence for the generation of single, pairs, and trains of attosecond pulses by controlling the time window of linear polarization of the driving pulses. The influence of the carrier-envelope phase on the generation process in different confinement configurations is interpreted and analyzed using a nonadiabatic stationary phase model. We show that the xuv emission depends critically on particular aspects of the fundamental electric field that allows us to steer the electron trajectories on the time scale of tens of attoseconds.

International audience; We present a full optimization of the high harmonics wave front owing to the use of a soft X-ray Hartmann sensor. The sensor was calibrated using a high harmonic source with an accuracy of λ/50 root-mean-square (rms) with λ around 30 nm. We observed a high harmonic wave front of λ/7 rms, which is two times the diffraction-limit, astigmatism being the dominant aberration for every condition of generation. By clipping slightly the unfocused high harmonic beam, it is possible to produce a diffraction-limited beam containing approximately 90% of the incident energy.

We have investigated the intensity dependence of high-order harmonic generation in argon when the two shortest quantum paths contribute to the harmonic emission. For the first time to our knowledge, experimental conditions were found to clearly observe interference between these two quantum paths that are in excellent agreement with theoretical predictions. This result is a first step towards the direct experimental characterization of the full single-atom dipole moment and demonstrates an unprecedented accuracy of quantum path control on an attosecond time scale.

High-order harmonic generation (HHG) in aligned molecules is an important area in strong field ultrafast physics [1]–[3]. It has been shown that the dependence of HHG on the molecular structure can give new insights into measuring molecular structure on a subfemtosecond timescale [3]. A unique aspect of HHG in a simple molecular system is the possibility to observe quantum interference phenomena related to the shape of the molecular wavefimction [4],[5]. Recently this effect has been observed in HHG from aligned CO2 molecules by Kanai et al. [6] and by Vozzi et al. [7].

Attosecond pulses can be produced by using the process of high-order harmonic generation in noble gases [1]. Few-cycle driving pulses allow one to confine the emission process to a Single optical cycle so that single X-ray sub-femtosecond pulses can be generated [2],[3]. In the case of multicycle driving pulses, trains of attosecond pulses are produced [1],[4],[5]. Recently, it has been demonstrated that the use of driving pulses with stable carrier-envelope phase (CEP) allows one to control the spectral distribution of the soft X-ray radiation generated by few-cycle infrared (IR) pulses [6]. From both a fundamental and a technical point of view, it is essential to understand the CEP effects on the phase of the harmonics and on the electric-field evolution of the attosecond pulses. © 2007 Springer-Verlag New York.

Introduction. This paper is devoted to the development of the technique for the estimation of a distribution of mechanical stress on the strained specimen based on terahertz absorption data.This hypothesis is based on the relation between density of the stressed material and absorption of THz radiation. The THz technology is strongly demand in a large variety of elds, ranging from the fundamental sciences such as biochemical spectroscopy, astronomy, and condensed-matter physics to practical applications such as high-capacity communication, medicine, agriculture, and security [1, 2]. General part. The deformation of an object is generally accompanied by the change of its density. However, the density depends on a stress. The polymeric foam was chosen for the study of its mechanical properties and absorption ability of the THz radiation. A combination of the dependence of the density and stress with the approximation of the relative absorption-strain plot gives us a relation between absorption of the THz beam and intensity stress tensor. Using the experimental set up developed by NeTHIS the authors estimated the stress distribution near the holes with dierent diameters in stressed polymeric foams. The specimen was xed between two Teon plates and compressed by four screws. Conclusion. As a result of this work authors propose a new way for the estimation of a stress distribution in porous non-metals bodies based on the absorption of the THz beam. The results of the mechanical testing and numerical simulation show the good qualitative agreement and high eciency of the THZ based experimental technique. The THz based methods can be considered as a promising way for the obtaining of the quantitative information about the stress-strain state of the Thz transparent bodies.

In this work we report on a self-referencing technique for the measurement of the effect of the carrier-envelope phase (CEP) of the driving pulses on the phase difference between consecutive harmonics. By using a numerical model based on the nonadiabatic saddle-point method [1] we demonstrate that, in particular experimental conditions, it is possible to directly control the electric field of the attosecond pulses by controlling the electric field of the driving pulses. Harmonic emission has been produced focusing femtosecond light pulses (800-nm central wavelength), into an argon jet, located around the focus of the laser beam to enhance the contribution of the long quantum paths. The CEP of the infrared (IR) pulses has been stabilized as described in [2] and has been varied by introducing in the beam path a glass plate with variable thickness. Upon increasing the intensity of the IR pulses the harmonic peaks broaden and eventually overlap in the spectral region between consecutive odd harmonics, where distinct spectral peaks, whose position is CEP dependent, are formed. This is due to the fact that the temporal variation of the IR pulse intensity produces a harmonic chirp, which broadens the spectrum of each individual harmonic. This effect is more significant in the case of the long quantum paths. If the spectral broadening is larger than the frequency separation between consecutive harmonics, the high-frequency side of the qth-harmonic spectrum, generated on the leading edge of the IR pulse, overlaps the low-frequency side of the spectrum of (q+2)th harmonic, generated on the pulse trailing edge, thus giving rise to the observed interference effect. We have then varied the CEP, ?. Figure 1 shows the portion between 13th and 15th harmonics of nine spectra for different amounts ?z of glass in the beam path, corresponding to different CEPs in the range ?0

In this work, XUV spectra from some alkanes (methane, I/sub p/ = 12.97 eV; propane, I/sub p/ = 11.07 eV; butane, I/sub p/ = 10.63 eV) are measured and compared with those acquired from xenon (I/sub p/ = 12.13 eV). The harmonic emission is obtained focusing femtosecond light pulses on a pulsed gas jet. Two different pulse durations are considered, namely 20 fs and 6 fs; in both cases the pulse energy could be continuously tuned without modification in temporal and spatial structure of the driving pulses. The experimental results have been compared to numerical simulation based on an atom-like Lewenstein model. Preliminary results reproduce the cutoff wavelength behavior of the investigated molecular species.

In this work we report on the first experimental demonstration of selection of the long electron quantum paths in the process of high-order harmonic generation by phase-stabilized multiple-cycle light pulses. A complete experimental investigation of the role of intensity and carrier-envelope phase of the driving pulses on the spectral characteristics of the long quantum paths is performed. Simulations based on the nonadiabatic saddle-point method and on a complete nonadiabatic three-dimensional model reproduce the main features of the experimental results. The use of phase-stabilized driving pulses allows one to control, on an attosecond temporal scale, the spectral and temporal characteristics associated with the electron quantum paths involved in the harmonic generation process.

For the first time, we observe unambiguous signature of broadband (50 eV) XUV harmonic radiation temporally confined down to an isolated attosecond pulse by applying polarization gating to phase-stabilized-few-cycle laser pulses.

We examine the role of the electronic orbital of a molecule in the process of high-order harmonic generation. To do this, measurements of high-order harmonic generation in molecules (O-2, N-2, H-2, CO2 and small alkanes) and atoms (Xe, Ar and Kr) have been carried out with intense few-optical-cycle pulses. Comparison between molecules and their 'companion' atoms (i.e. those with nearly equal ionization potential) was made. The molecule response in the high-field regime is treated by adopting an 'atom-like' model for the forms of the molecular orbitals employing the Lewenstein et al. model, properly modified in order to account for the nonlinear dipole moment of a randomly oriented molecule ensemble. The calculation procedure is described here in detail. There is good agreement between the spectra calculated using this model and the harmonic cut-off position and the shape of the measured spectra. The cut-offs of O-2 and CO2 extend far beyond the cut-offs of Xe and Kr respectively, in contrast with N-2 and H-2 which exhibit cutoffs very close to that of Ar. This behaviour is well explained by adopting the atom-like approximation. A series of alkanes has also been investigated and again the spectra are well matched by this simple model.

By using a self-referencing technique, we have experimentally measured the influence of the carrier-envelope phase of femtosecond light pulses on the phase of the electric field of the radiation produced by high-order harmonic generation. We show that, in particular experimental conditions, the temporal evolution of the electric field of the attosecond pulses, is directly controlled by the carrier-envelope phase of the driving pulses.

High-order harmonic generation from molecules (${\mathrm{O}}_{2}$, ${\mathrm{N}}_{2}$, ${\mathrm{H}}_{2}$, and $\mathrm{C}{\mathrm{O}}_{2}$) and atoms (Xe, Ar, and Kr) has been studied in the few optical cycle domain. Two laser peak intensities; $2\ifmmode\times\else\texttimes\fi{}{10}^{14}$ and $6\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, were compared. At the lower intensity spectra were approximately the same for molecules and atoms with the same ionization potential, at higher laser intensity the cutoff of ${\mathrm{O}}_{2}$ and $\mathrm{C}{\mathrm{O}}_{2}$ extends far beyond the cutoff of Xe and Kr, respectively, in contrast with ${\mathrm{N}}_{2}$ and ${\mathrm{H}}_{2}$ which exhibit cutoffs very close to that of Ar. This behavior is well explained by adopting an atomlike approximation for the molecule response in the high-field regime and employing the Lewenstein's model, properly modified in order to account for the nonlinear dipole moment of a randomly oriented molecule ensemble.

We experimentally investigate the process of intramolecular quantum interference in high-order harmonic generation in impulsively aligned CO2 molecules. The recombination interference effect is clearly seen through the order dependence of the harmonic yield in an aligned sample. The experimental results can be well modeled assuming that the effective de Broglie wavelength of the returning electron wave is not significantly altered by the Coulomb field of the molecular ion. We demonstrate that such interference effects can be effectively controlled by changing the ellipticity of the driving laser field.

Nous demontrons que la generation de second harmonique au voisinage de la condition d'accord de phase dans un cristal de BBO de 2 mm d'epaisseur permet la correction du front de l'onde d'une impulsion laser intense ayant traversee un morceau de verre de silice de 4 cm d'epaisseur. Cette demonstration est effectuee en enregistrant la deformation du front d'onde avec un analyseur de surface d'onde.

Systematic measurements of the dielectric properties of KDP, KNbO3, KTP and KTA non-linear crystals in the Terahertz (THz) spectral range are presented. The index of refraction and the absorption coefficients are measured between 0.1 and 1.5 THz for different crystallographic orientations. The data are deduced from an experimental set-up based on standard Terahertz time domain spectroscopy system at room temperature. These data, key parameters for the optimization of non-linear THz generation by optical rectification as well as electro-optic detection, are analysed in term of non-linear capabilities. We finally review different methods making possible to generate THz wave in these crystals and compare their characteristics.

Summary form only given. Some years ago the potential of increasing the magnitude of /spl chi//sup (3)/ Kerr effect with a combination of cascaded /spl chi//sup (3)/ second order effects (CSOE) mainly due to second harmonic generation (SHG) was discovered. It has recently been shown that the combined processes of optical rectification and linear electro-optical effect lead to a similar phenomenon. Hereafter we evidence in a ZnTe cubic crystal, the temporal behavior and evaluate the amplitudes of both third-order and CSOE contributions to effective nonlinear index of refraction n/sub 2/.

A model is proposed to account for Kerr-like nonlinearity induced by femtosecond pulses via terahertz generation and electro-optical effect. This phenomenon, so far overlooked, is evidenced in a zinc blende single crystal with a heterodyne optical Kerr effect setup. The spectral evolution of this phenomenon as well as its noninstantaneous response character are reported. Its competition with a third-order optical Kerr effect is demonstrated.

Summary form only given. We analysed, for the first time, the temporal evolution of the nonlinear phase shift of a femtosecond pulse due to cascaded optical rectification (i.e., THz generation) and electro-optic effect. Our experimental results obtained in 43 m zinc blende single crystals agree with our theoretical analysis. The nonlinear phase shift is maximum at /spl Delta/k=0.

The possibility of using commercial classical near-infrared (similar to 30 THz) focal-plane micro-bolometer arrays in the sub-terahertz frequency range is studied. Real-time room-temperature pixilated detector imaging of a 0.5 THz (lambda = 600 mu m) spatial beam pro. le emitted from a backward-wave oscillator is presented. Images, with a signal-to-noise ratio of 5 are demonstrated, leading to an estimate of the noise equivalent power of 11 nW . Hz(-1/2).

High-order harmonic generation in argon driven by 25-fs-light pulses is investigated from the gaseous to the cluster regime. The harmonic cutoff observed in presence of clusters shows a considerable extension with respect to the gaseous phase. Harmonic spectra are investigated as a function of cluster size, showing the existence of an optimal cluster dimension, which maximizes the harmonic photon yield. (C) 2005 American Institute of Physics.

The wave-front distortion of femtosecond laser pulses recorded with a Shack-Hartmann analyzer makes it possible to retrieve the nonlinear index of refraction of different glasses and the nonlinear phase shift induced during second-harmonic generation in beta-barium borate (BBO) crystal versus the phase mismatch. It is shown that the nonlinear phase shift induced in a 2-mm-thick BBO crystal allows compensation for up to a 2pi breakup-integral induced in a 4-cm fused-silica glass. The stability of the compensation is reported to be from 10 to 100 GW cm(-2).

The second harmonic generation in a thin ss-barium borate crystal is used to measure chi ((2)) cascading phenomena in the spectral domain. The harmonic generation is induced by two pulses produced by spectrally filtering a femtosecond pulse and centered at the wavelength lambda -Delta lambda and lambda +Delta lambda. New spectral components appear in spectral density of both the fundamental and harmonic pulses. High order cascading phenomena are evidenced. In good agreement with theoretical predictions, for large phase mismatch the evolution of the spectra demonstrates the competition between cascaded chi(2) and chi(3) phenomena.

High-order harmonic spectra generated by few-optical-cycle pulses have been acquired on a single-shot basis. We show that the differences in the generated spectra are related to variations in the electron-quantum-paths contributing to the emission process.

Recently the role of the carrier-envelope phase of few-optical-cycle light pulses on high-order harmonic generation has been experimentally observed. By using a three-dimensional nonadiabatic propagation model, we show that the observed carrier-envelope phase dependence of the spectral characteristics of the harmonic radiation is essentially a single-atom effect. Nonadiabatic single-atom response has been calculated in the framework of the saddle-point approximation.

Summary from only given. We demonstrate that spectral interferometry is a well suited technique for measuring the spectral phase shift due to cascading phenomena produced by femtosecond pulses during SHG. The experiment we performed has slightly non collinear beams that are mixed in a 2 mm BBO crystal cut for type, I phase matching.