High-harmonic spectroscopy provides a unique insight into the electronic structure of atoms and molecules 1-5. Although attosecond science holds the promise of accessing the timescale of electron-electron interactions, until now, their signature has not been seen in high-harmonic spectroscopy. We have recorded high-harmonic spectra of atoms to beyond 160 eV, using a new, almost ideal laser source with a wavelength of 1.8 µm and a pulse duration of less than two optical cycles. We show that we can relate these spectra to differential photoionization cross-sections measured with synchrotron sources. In addition, we show that the highharmonic spectra contain features due to collective multielectron effects involving inner-shell electrons, in particular the giant resonance in xenon. We develop a new theoretical model based on the strong-field approximation and show that it is in agreement with the experimental observations. Measuring and understanding the electronic structure and correlated dynamics of matter on its natural timescale represents the main thrust of ultrafast laser science. Electron correlations affect essential properties of complex systems ranging from configuration interactions in molecules to cooperative phenomena in solids, such as superconductivity. Our knowledge of the electronic structure of matter originates from several decades of research on photoionization and photoelectron spectroscopy 6-8 , mainly driven by the development of synchrotron-based sources. Recent advances in strong-field physics have opened an alternative approach to probing both the electronic structure 1,9 and the dynamics 10-12 of molecules using table-top laser sources. These new methods rely on the recollision of an electron, removed from the molecule by a strong laser field, with its parent ion 13 , as illustrated in Fig. 1a. The electronic structure of the molecule is encoded in the emitted high-harmonic spectrum through the amplitude and phase of the photorecombination matrix elements 4,11,14,15. We use high-harmonic spectroscopy to investigate a new class of collective electronic dynamics-induced and probed by the recombining electron. The kinetic energy of the returning electron is usually much larger than the difference between electronic energy levels of the parent ion. Consequently, inelastic scattering followed by recombination is energetically possible, as illustrated in Fig. 1b. Using the xenon atom as an example, we demonstrate that such processes indeed occur and that they can locally enhance the efficiency of high-harmonic generation (HHG) by more than one order of magnitude. We show that such a seemingly complex pathway contributes significantly to the phase-matched process. This observation uncovers a new

A Lower Tunnel Among the peculiarities inherent in quantum mechanics is the ability of particles to tunnel through barriers that they lack the energy to surmount classically, as happens during radioactive decay. Strong laser fields can liberate electrons in this way from atoms and molecules. Akagi et al. (p. 1364 ) elegantly confirm that tunneling is not limited to the highest-energy electrons in a system by mapping the energy and momentum of both the ejected electron and positive ion produced when an intense laser pulse impinges on hydrogen chloride. When the molecule adopts specific orientations relative to the laser field, tunneling occurs from lower-lying states, as well as the highest-energy occupied orbital. This raises the possibility of tunneling microscopy capable of imaging the electronic structure of single molecules.

Wavelength scaling of high harmonic generation efficiencyUsing longer wavelength laser drivers for high harmonic generation is desirable because the highest extreme ultraviolet frequency scales as the square of the wavelength. Recent numerical studies predict that high harmonic efficiency falls dramatically with increasing wavelength, with a very unfavorable Àð5À6Þ scaling. We performed an experimental study of the high harmonic yield over a wavelength range of 800-1850 nm. A thin gas jet was employed to minimize phase matching effects, and the laser intensity and focal spot size were kept constant as the wavelength was changed. Ion yield was simultaneously measured so that the total number of emitting atoms was known. We found that the scaling at constant laser intensity is À6:3AE1:1 in Xe and À6:5AE1:1 in Kr over the wavelength range of 800-1850 nm, somewhat worse than the theoretical predictions.

Attosecond extreme-ultraviolet pulses 1 have a complex space-time structure 2 . However, at present, there is no method to observe this intricate detail; all measurements of the duration of attosecond pulses are, to some extent, spatially averaged 1,3-5 . A technique for determining the full space-time structure would enable a detailed study of the highly nonlinear processes that generate these pulses as a function of intensity without averaging 6,7 . Here, we introduce and demonstrate an all-optical method to measure the space-time characteristics of an isolated attosecond pulse. Our measurements show that intensity-dependent phase and quantum-path interference both play a key role in determining the pulse structure. In the generating medium, the attosecond pulse is strongly modulated in space and time. Propagation modifies but does not erase this modulation. Quantum-path interference of the single-atom response, previously obscured by spatial and temporal averaging, may enable measuring the laser-field-driven ion dynamics with sub-cycle resolution.Fully defining an attosecond pulse requires knowledge of its phase variation both temporally and spatially. Until now, temporal 1,3-5,8,9 and spatial 10-12 measurements are achieved only separately. As the temporal characterization methods (known as RABBIT; ref. 1 and CRAB;9,13) rely on the photoelectric effect, they average over the spatial profile of a pulse, mixing the contribution from the different emitters of the extremeultraviolet (XUV) source at a secondary target. On the other hand, spatial measurements of XUV emission have been achieved using small apertures 10,11 or two foci 12 . Although temporal information remains available in principle, neither method seems compatible with RABBIT or CRAB. Therefore, space-time measurements of attosecond pulses have never been made.To solve the space-time problem, we turn to an in situ technique. The in situ method is a unique method of measurement that is feasible only for highly nonlinear processes [14][15][16] . It relies on the fact that adding a single photon to an already highly nonlinear process only weakly perturbs the process 17,18 . Yet, it can modify the spatial and spectral pattern of a beam. The in situ method has been considered in attosecond pulse metrology to determine only the temporal profile of the average attosecond pulse within attosecond pulse trains 14 . For that measurement, a weak second-harmonic beam co-propagates with the fundamental beam to break the symmetry between adjacent attosecond pulses, thereby allowing an even-order harmonic signal. Temporal information was encoded in the even-order harmonic signal as a function of the phase delay between the fundamental and second-harmonic laser pulses 14 .For our spatially encoded in situ measurement, we produce XUV radiation using the fundamental laser pulse with a time-dependent

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Abstract:We demonstrate a polarization-managed 8-dimensional modulation format that is time domain coded to reduce inter-channel nonlinearity. Simulation results show a 2.33 dB improvement in maximum net system margin (NSM) relative to polarization multiplexed (PM)-BPSK, and a 1.0 dB improvement relative to time interleaved return to zero (RZ)-PM-BPSK, for a five channel fill propagating on 20x80 km spans of 90% compensated ELEAF. In contrast to the other modulations considered, the new 8-dimentional (8D) format has negligible sensitivity to the polarization states of the neighboring channels. Laboratory results from High-density WDM (HD-WDM) propagation experiments on a 5000 km dispersionmanaged link show a 1 dB improvement in net system margin relative to PM-BPSK.

By using the novel approach for pulse compression that combines spectral broadening in hollow-core fiber (HCF) with linear propagation in fused silica (FS), we generate 1.6 cycle 0.24 mJ laser pulses at 1.8 m wavelength with a repetition rate of 1 kHz. These pulses are obtained with a white light seeded optical parametric amplifier (OPA) and shown to be passively carrier envelope phase (CEP) stable. Krausz, "X-ray pulses approaching the attosecond frontier," Science 291(5510), 1923-1927 (2001). ©2011 Optical Society of AmericaTaïeb, B. Carré, H. G. Muller, P. Agostini, and P. Salières, "Attosecond synchronization of high-harmonic soft xrays," Science 302(5650), 1540-1543 (2003).Velotta, S. Stagira, S. De Silvestri, and M. Nisoli, "Isolated single-cycle attosecond pulses," Science 314(5798), 443-446 (2006). 10. E. Goulielmakis, M. Schultze, M. Hofstetter, V. S. Yakovlev, J. Gagnon, M. Uiberacker, A. L. Aquila, E. M.Gullikson, D. T. Attwood, R. Kienberger, F. Krausz, and U. Kleineberg, "Single-cycle nonlinear optics," Science 320(5883), 1614-1617 (2008). 11. P. B. Corkum, "Plasma perspective on strong field multiphoton ionization," Phys. Rev. Lett. 71(13), 1994-1997 (1993). 12. G. Tempea, M. Geissler, M. Schnürer, and T. Brabec, "Self-phase-matched high harmonic generation," Phys.Rev. Lett. 84(19), 4329-4332 (2000). 13. V. S. Yakovlev, M. Ivanov, and F. Krausz, "Enhanced phase-matching for generation of soft X-ray harmonics and attosecond pulses in atomic gases," Opt. "Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating," Rev.