We determine relative photoemission time delays between valence electrons in different noble gas atoms (Ar, Ne and He) in an energy range between 31 and 37 eV. The atoms are ionized by an attosecond pulse train synchronized with an infrared laser field and the delays are measured using an interferometric technique. We compare our results with calculations using the random phase approximation with exchange and multi-configurational Hartree-Fock. We also investigate the influence of the different ionization angular channels.

Nonlinear optical methods are becoming ubiquitous in many areas of modern photonics. They are, however, often limited to a certain range of input parameters, such as pulse energy and average power, since restrictions arise from, for example, parasitic nonlinear effects, damage problems and geometrical considerations. Here, we show that many nonlinear optics phenomena in gaseous media are scale-invariant if spatial coordinates, gas density and laser pulse energy are scaled appropriately. We develop a general scaling model for (3+1)-dimensional wave equations, demonstrating the invariant scaling of nonlinear pulse propagation in gases. Our model is numerically applied to high-order harmonic generation and filamentation as well as experimentally verified using the example of pulse post-compression via filamentation. Our results provide a simple recipe for up-or downscaling of nonlinear processes in gases with numerous applications in many areas of science.Nonlinear interactions of intense short laser pulses with gaseous media form the basis behind a wealth of interesting phenomena such as multiphoton ionization [1] and plasma formation [2], spectral broadening (which can be used for pulse compression [3][4][5]), harmonic generation and wave-mixing [6], as well as the creation of attosecond pulses [7] and the formation of electron or ion beams [8]. Advances in femtosecond laser technology constantly yield shorter pulses, higher pulse energies, and higher repetition rates [9][10][11]. However, to fully explore this newly available parameter regime, which gives access to e.g. faster time scales and higher intensities, is often challenging because of damage problems, additional (unwanted) nonlinear effects, or geometrical restrictions. We illustrate this challenge for two important applications of nonlinear optics, filamentation in gases used e.g. for laser pulse compression, and high-order harmonic generation (HHG) providing the basis for attosecond science.The propagation of an intense short laser pulse in a transparent medium induces nonlinear effects caused e.g. by the intensity dependence of the refractive index. When self-focusing due to the Kerr effect balances defocussing caused by diffraction and plasma generation, a filament can be created. In addition, self-phase modulation and self compression may take place in the filament, resulting, possibly after further compression, in ultrashort pulses close to the fundamental limit of a single cycle [12]. Forming a filament requires a certain power, known as the critical power for self-focusing [13,14]. At slightly higher power, limitations arise and multiple filaments are created [15]. Different attempts were suggested to increase the output energy [12,[16][17][18][19][20]. However, pulse compression using filaments (or similarly hollow fibers) is still limited to pulse energies of typically a few mJ [21,22], which is approximately two to three orders of magnitude below the maximum pulse energies available from today's femtosecond laser sources. To scale up pulse po...

High-order harmonic generation by few-cycle 800 nm laser pulses in neon gas in the presence of a strong terahertz (THz) field is investigated numerically with propagation effects taken into account. Our calculations show that the combination of THz fields with up to 12 fs laser pulses can be an effective gating technique to generate single attosecond pulses. We show that in the presence of the strong THz field only a single attosecond burst can be phase matched, whereas radiation emitted during other half-cycles disappears during propagation. The cutoff is extended and a wide supercontinuum appears in the near-field spectra, extending the available spectral width for isolated attosecond pulse generation from 23 to 93 eV. We demonstrate that phase matching effects are responsible for the generation of isolated attosecond pulses, even in conditions when single atom response yields an attosecond pulse train.

High-order harmonic generation in the presence of a chirped THz pulse is investigated numerically with a complete 3D nonadiabatic model. The assisting THz pulse illuminates the high-order harmonic generation gas cell laterally inducing quasi-phase-matching. We demonstrate that it is possible to compensate the phase mismatch during propagation and extend the macroscopic cutoff of a propagated strong IR pulse to the single-dipole cutoff. We obtain 2 orders of magnitude increase in the harmonic efficiency of cutoff harmonics (≈170  eV) using a THz pulse of constant wavelength, and a further factor of 3 enhancement when a chirped THz pulse is used.

Empirical evidence is given for a significant difference in the collective trend of the share prices during the stock index rising and falling periods. Data on the Dow Jones Industrial Average and its stock components are studied between 1991 and 2008. Pearson-type correlations are computed between the stocks and averaged over stock-pairs and time. The results indicate a general trend: whenever the stock index is falling the stock prices are changing in a more correlated manner than in case the stock index is ascending. A thorough statistical analysis of the data shows that the observed difference is significant, suggesting a constant-fear factor among stockholders. PACS numbers:The world is once again experiencing a major financial-economic crisis, the worst since the crash of Oct. 1929 that initiated the great depression of the 1930s. Many citizens are concerned for obvious reasons; we are facing global recession; banks and financial institutions go bankrupt; companies struggle to get credit and many are forced to reduce their workforce or even go out of business. Interest rates are increasing while private savings invested in the stock market evaporate. Large parts of our contemporary societies are deeply affected by the new financial reality.The current financial crisis is one particular dramatic example of collective effects in stock markets [1][2][3]6]; during crises nearly all stocks drop in value simultaneously. Fortunately, such extreme situations are relatively rare. What is less known, however, is that during more normal "non-critical" periods, collective effects do still represent characteristics of stock markets that in particular influence their short time behavior. One such effect will be addressed in this publication, where our aim is to present empirical evidence for an asymmetry in stock-stock correlations conditioned by the size and direction of market moves. In particular, we will present empirical results showing that when the Dow Jones Industrial Average (DJIA) index ("the market") is dropping, then there exists a significantly stronger stock-stock correlation than during times of a raising market. Our results indicate that such enhanced (conditional) stock-stock correlations are not only relevant during times of dramatic market crashes, but instead represents features of markets during more "normal" periods.Distribution of returns is traditionally used as one of the proxies for the performance of stocks and markets over a certain time history [1][2][3]. In the economics, finance and econometrics literature the problem of market sentiment and investor confidence is usually addressed by the use of various indicators. These indicators are either derived from objective market data [4], or obtained by conducting questionnaire-based surveys among professional and individual investors [5]. In the present study we consider thus the first approach, since we believe that the market data (prices and returns) are more objective proxies than questionnaire-inferred data.The basic quantity of interest is...