Subfemtosecond light pulses can be obtained by superposing several high harmonics of an intense laser pulse. Provided that the harmonics are emitted simultaneously, increasing their number should result in shorter pulses. However, we found that the high harmonics were not synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. We showed that the synchronization could be improved considerably by controlling the underlying ultrafast electron dynamics, to provide pulses of 130 attoseconds in duration. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter.

We study high-order harmonic generation at a high pumping energy using a long focal length lens. We identify different saturation regimes of the harmonic emission, revealing the interplay between phase matching, absorption, and laser defocusing. In the optimal conditions, high conversion efficiencies are obtained, resulting in an increase of at least one order of magnitude of the harmonic energies compared to previously reported values. In xenon, microjoule energies are reached, opening new perspectives for the applications of this ultrashort coherent radiation.

The generation of attosecond pulses by superposition of high harmonics relies on their synchronization in the emission. Our experiments in the low-order, plateau, and cutoff regions of the spectrum reveal different regimes in the electron dynamics determining the synchronization quality. The shortest pulses are obtained by combining a spectral filtering of harmonics from the end of the plateau and the cutoff, and a far-field spatial filtering that selects a single electron quantum path contribution to the emission. This method applies to isolated pulses as well as pulse trains. DOI: 10.1103/PhysRevLett.93.163901 PACS numbers: 42.65.Ky, 32.80.Wr, 42.65.Re The strongly nonlinear interaction taking place when an intense infrared (IR) laser pulse is focused into a rare gas jet results in the coherent emission of extreme ultraviolet (XUV) light [1] with a characteristic spectrum showing a plateau and a sharp cutoff at high energy [2]. This process offers the unique opportunity of generating attosecond pulses, as recently demonstrated by two techniques. First, starting from a few-cycle laser pulse, the highest energy photons are only emitted at the maximum of the laser envelope. The cutoff is then continuous and by spectrally selecting it, one can obtain a single pulse of 250 as duration [3,4]. Second, using a multicycle IR pulse provides a discrete spectrum containing only odd harmonics of the laser frequency. Selecting many harmonics in the plateau results in emission of XUV bursts every half laser period, forming an attosecond pulse train (APT) whose wagons can be as short as 130 as [5,6]. In both cases, the condition for the production of short pulses is a near-linear spectral phase of the XUV radiation. For a discrete spectrum, this means that harmonics must be phase-locked, i.e., synchronized. The phaselocking of harmonics is closely related to the electronic dynamics in the generation process. In the semiclassical ''three-step'' model [7,8], part of the electron wave packet first tunnels out of the atomic potential lowered by the laser field; it is then driven by the strong IR field in the continuum; finally, it can recombine with the parent ion, emitting a XUV photon whose energy is given by the electron return kinetic energy. The recombination times of the different electron trajectories determine the emission times of the different XUV frequencies, and thus their synchronization.Recent studies have stressed two main sources of asynchronism that raise important questions for the reliable generation of shorter pulses. First, each harmonic is associated to mainly two (''short'' and ''long'') electron trajectories that have the same return energy but very different return times. The single-atom response thus consists of at least two bursts per half laser cycle, which blurs the attosecond structure. Fortunately, the short trajectory contribution can be macroscopically selected by adjusting carefully the phase matching conditions [9]: when the generating laser is focused slightly before the gas jet, the macros...

We report on the generation of extreme ultraviolet radiation utilizing the plasmonic field enhancement in arrays of bow-tie gold optical antennae. Furthermore, their suitability to support high-order harmonic generation is examined by means of finite-difference time-domain calculations and experiments. Particular emphasis is paid to the thermal properties, which become significant at the employed peak intensities. A damage threshold depending on the antenna length is predicted and confirmed by our experimental findings. Moreover, the gas density in the vicinity of the antennae is characterized experimentally to determine the number of atoms contributing to the measured radiation, which is almost an order of magnitude larger than previously reported.

Extreme ultraviolet fourier-transform spectroscopy with high order harmonicsKovacev, M; Fomichev, SV; Priori, E; Mairesse, Y; Merdji, H; Monchicourt, P; Breger, P; Norin, Johan; Persson, Anders; Lhuillier, A; Wahlström, Claes-Göran; Carre, B; Salieres, P General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. We demonstrate a new scheme for extreme ultraviolet (xuv) Fourier-transform spectroscopy based on the generation of two phase-locked high-harmonic beams. It allows us to measure for the first time interferograms at wavelengths as short as 90 nm, and open the perspective of performing high-resolution Fourier-transform absorption spectroscopy in the xuv. Our measurements also demonstrate that a precise control of the relative phase of harmonic pulses can be obtained with an accuracy on an attosecond time scale, of importance for future xuv pump-xuv probe attosecond spectroscopy. Extreme Ultraviolet Fourier-Transform Spectroscopy with High Order Harmonics

The absolute timing of the high-harmonic attosecond pulse train with respect to the generating IR pump cycle has been measured for the first time. The attosecond pulses occur 190+/-20 as after each pump field maxima (twice per optical cycle), in agreement with the "short" quantum path of the quasiclassical model of harmonic generation.

International audiencePlasmonic dimer nanoantennas can significantly boost the electric field strength in the gap region, allowing for a modification of the feed gap geometry by femtosecond laser illumination. Using resonant bowtie antennas to enhance the electric field of a low-fluence femtosecond oscillator, here we experimentally demonstrate highly localized reshaping of the antennas, resulting in a self-optimization of the antenna shape. From high-resolution scanning electron micrographs and two-dimensional energy dispersive x-ray maps, we analyze the near-field enhanced subwavelength ablation at the nanotips and the resulting deposition of ablated materials in the feed gap. The dominant ablation mechanism is attributed to the nonthermal transient unbonding of atoms and electrostatic acceleration of ions. This process is driven by surface plasmon enhanced electron emission, with subsequent acceleration in the vacuum. This ablation is impeded in the presence of an ambient gas. A maximum of sixfold enhancement of the third-harmonic yield is observed during the reshaping process

We report on low-order harmonic generation utilising the plasmonic field enhancement in arrays of rodtype 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.