Nano-plasmonic near field phase matching of attosecond pulses

Nano-structures excited by light can enhance locally the electric field when tuned to plasmonic resonances. This phenomenon can be used to boost non-linear processes such as harmonic generation in crystals or in gases, Raman excitation, and four wave mixing. Here we present a theoretical investigation of the near-field phase matching of attosecond pulses emitted by high-order harmonic generation (HHG) of an atom immersed in a multi-cycle femtosecond infrared laser field and a spatially inhomogeneous plasmonic field. We demonstrate that the spatial inhomogeneity factor of the plasmonic field strongly affects the electron trajectory and recombination time which can be used to control the attosecond emission. For further insight into the plasmonic field effect, we monitor the phase of each quantum path as a function of the inhomogeneity strength. Moreover, we investigate the attosecond emission as a function of near-field phase matching effects. This is achieved by calculating the coherent field superposition of attosecond pulses emitted from various intensities or field inhomogeneities. Finally, far-field and near-field phase matching effects are combined to modulate the harmonic spectral phase towards the emission of a single attosecond pulse

High-order-harmonic generation from dense water microdroplets

We report on high-order-harmonic generation from micrometer-sized liquid water droplets. In pump-probe experiments, the influence of the time delay onto the emission of harmonic radiation is systematically studied. Phase-matching aspects are observed by controlling the focal position and the intensity of the probe pulse. The spatiotemporal dynamics of the droplet during interaction with intense laser pulses are studied by controlling the intensity of the pump pulse. We find transient phase-matching conditions and the expansion dynamics of the droplet to be of major influence on the harmonic yield. © 2013 American Physical Society.DFG/EXC/QUESTDFG/KO 3798/1-

Femtosecond Field‐Driven On‐Chip Unidirectional Electronic Currents in Nonadiabatic Tunneling Regime

Recently, asymmetric plasmonic nanojunctions have shown promise as on-chip electronic devices to convert femtosecond optical pulses to current bursts, with a bandwidth of multi-terahertz scale, although yet at low temperatures and pressures. Such nanoscale devices are of great interest for novel ultrafast electronics and opto-electronic applications. Here, the device is operated in air and at room temperature, revealing the mechanisms of photoemission from plasmonic nanojunctions, and the fundamental limitations on the speed of optical-to-electronic conversion. Inter-cycle interference of coherent electronic wavepackets results in a complex energy electron distribution and birth of multiphoton effects. This energy structure, as well as reshaping of the wavepackets during their propagation from one tip to the other, determine the ultrafast dynamics of the current. It is shown that, up to some level of approximation, the electron flight time is well-determined by the mean ponderomotive velocity in the driving field

Rayonnement harmonique d'ordre élevé (génération d'impulsions attosecondes)

Les travaux présentés dans ce mémoire de thèse sont consacrés à la caractérisation et à l'optimisation des propriétés uniques de la génération d'harmoniques d'ordre élevé dans les gaz: grande brillance, très courte durée (femtoseconde à attoseconde, 1as = 10^-(18)s) et bonne cohérence mutuelle. Dans une première partie, nous nous consacrons à l'exploitation de la loi d'échelle qui consiste à utiliser un laser de forte énergie faiblement focalisé dans un milieu générateur de grande dimension. Pour la première fois,une énergie par impulsion dépassant 1mJ est générée dans la 15èmc harmonique à une longueur d'onde de 53mn. L'efficacité de conversion atteint 4x10^(-5); elle résulte de la combinaison d'une réponse dipolaire intense et d'un bon accord de phase à l'échelle d'un volume étendu grâce à l'autoguidage de l'impulsion laser génératrice. Dans une deuxième partie, nous nous intéressons au profil temporel de l'émission harmonique et à sa structure attoseconde. Nous montrons d'abord la faisabilité d'une sélection spatiale/spectrale des contributions associées aux deux trajectoires électroniques, permettant ainsi la génération de trains réguliers d'impulsions attosecondes. Puis, nous caractérisons ces trains à partir de la mesure des phases relatives des harmoniques. Finalement, nous décrivons une technique originale de confinement temporel de la génération d'harmoniques d'ordre élevé par manipulation de l'ellipticité du laser générateur. Dans une troisième partie, nous nous intéressons aux propriétés de cohérence mutuelle du rayonnement harmonique. Nous démontrons d'abord le contrôle précis de la phase relative d'impulsions harmoniques par interférence de faisceaux multiples dans l'UVX. Cette expérience d'interférométrie fréquentielle à 4 impulsions bloquées en phase et décalées en temps montre une extrême sensibilité du spectre à la phase relative des impulsions à une échelle de temps attoseconde. Ensuite, nous mesurons pour la première fois l'autocorrélation du 1er ordre du rayonnement harmonique, grâce à la génération de deux sources harmoniques mutuellement cohérentes et séparées spatialement. Nous étudions l'influence de la séparation spatiale des sources harmoniques sur les interférogrammes ainsi obtenus. Ces études ouvrent la voie à la spectroscopie par transformée de Fourier dans l'UVX.The work presented in this thesis is dedicated to the characterization and optimisation of the unique properties of high order harmonic generation in a rare gas: high brilliance, short pulse duration (femtosecond to attosecond, 1as = 10^(-18)s) and good mutual coherence. In the first part of this work, we concentrate on the exploitation of a scaling law using a high-energy laser loosely focused inside an extended gaseous medium. For the first time, the generated harmonic energy exceeds the 1mJ level per laser pulse m the 15th harmonic order at a wavelength of 53nm. The conversion efficiency reaches 4x10^(-5), which results from the combination of a strong dipolar response and a good phase matching within a generating volume that is extended by selfguiding of the generating laser pulse. In the second part, our interest is devoted to the temporal profile of the harmonic emission and its attosecond structure. We first demonstrate the feasibility of a spatial/spectral selection of the contributions associated to the two main electronic trajectories, allowing thereby the generation of regular attosecond pulse trains. We then characterize such an attosecond pulse train by the measurement of the relative phases of consecutive harmonics. Finally, we describe an original technique for the temporal confinement of the harmonic emission by manipulating the ellipticity of the generating laser beam. In the third part, our interest is dedicated to the mutual coherence properties of the harmonic emission. We first demonstrate the precise control of the relative phase of the harmonic pulses by multiple beam interference in the XUV. This frequency-domain interferometry using four phase-locked temporally separated pulses shows an extreme sensitivity to the relative phase of the pulses on an attosecond time scale. We then measure for the first time the first order autocorrelation trace of the harmonic beam thanks to the generation of two harmonic sources mutually coherent and spatially separated. We study the influence of the spatial separation between the harmonic sources on the measured interferograms. These studies provide a way towards Fourier transform spectroscopy in the XUV.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Evidence of coherence in strong-field electron photoemission from a semiconductor

Strong-field quantum electronics is emerging as a potential candidate in information processing but still coherence vs decoherence is a primary concern of the concept. Strong-field coherent processes in band gap materials have led during the last decade to the emergence of high harmonic generation in semiconductors, petahertz electronics, or strong-field quantum states. However, the coherent behavior of the sub-optical cycle-driven electrons has never been directly observed. We report here on the experimental evidence of coherent ultrashort emission of hot electrons from a nanostructured semiconductor. Our method uses sub-wavelength electric field enhancement to localize the electron emission within a nanometer-scale spot. We found similarities with the electron emission from metallic nanotips in the strong-field regime, a topic that has opened a vast domain of applications during the last decade. The electron spectra display both odd and even harmonic orders of the driving femtosecond laser frequency, a signature of the coherent nature of the electron emission and their attosecond timing. Our findings complete our knowledge of phenomena governing coherent strong-field processes in semiconductors and open perspectives for the generation of future quantum devices operating in the strong-field regime

Evidence of coherence in strong-field electron photoemission from a semiconductor

Strong-field quantum electronics is emerging as a potential candidate in information processing but still coherence vs decoherence is a primary concern of the concept. Strong-field coherent processes in band gap materials have led during the last decade to the emergence of high harmonic generation in semiconductors, petahertz electronics, or strong-field quantum states. However, the coherent behavior of the sub-optical cycle-driven electrons has never been directly observed. We report here on the experimental evidence of coherent ultrashort emission of hot electrons from a nanostructured semiconductor. Our method uses sub-wavelength electric field enhancement to localize the electron emission within a nanometer-scale spot. We found similarities with the electron emission from metallic nanotips in the strong-field regime, a topic that has opened a vast domain of applications during the last decade. The electron spectra display both odd and even harmonic orders of the driving femtosecond laser frequency, a signature of the coherent nature of the electron emission and their attosecond timing. Our findings complete our knowledge of phenomena governing coherent strong-field processes in semiconductors and open perspectives for the generation of future quantum devices operating in the strong-field regime

Nano-antenna-assisted harmonic generation

Pfullmann N, Waltermann C, Kovacev M, et al. Nano-antenna-assisted harmonic generation. Applied Physics B. 2013;113(1):75-79.We report on low-order harmonic generation utilising the plasmonic field enhancement in arrays of rod-type gold optical antennae. Furthermore, we examine their suitability to support high-order harmonic generation (HHG). The low-order harmonics are used as a tool to investigate the nonlinear properties of the antennae. Particular attention is paid to the thermal properties, which become significant at the peak intensities necessary for HHG. A theoretical model explains the experimental findings and enables future improvements. In experiments we observe up to the fifth harmonic order and measure a field enhancement sufficient to support high-order harmonic generation. Moreover, we find a damage threshold for the antennae

X-ray Dose Rate and Spectral Measurements during Ultrafast Laser Machining Using a Calibrated (High-Sensitivity) Novel X-ray Detector

Ultrashort pulse laser machining is subject to increase the processing speeds by scaling average power and pulse repetition rate, accompanied with higher dose rates of X-ray emission generated during laser–matter interaction. In particular, the X-ray energy range below 10 keV is rarely studied in a quantitative approach. We present measurements with a novel calibrated X-ray detector in the detection range of 2–20 keV and show the dependence of X-ray radiation dose rates and the spectral emissions for different laser parameters from frequently used metals, alloys, and ceramics for ultrafast laser machining. Our investigations include the dose rate dependence on various laser parameters available in ultrafast laser laboratories as well as on industrial laser systems. The measured X-ray dose rates for high repetition rate lasers with different materials definitely exceed the legal limitations in the absence of radiation shielding