Nonsequential double ionization of Ar by 45 fs laser pulses (800 nm) at 4-7 10 13 W=cm 2 was explored in fully differential measurements. Well below the field-modified recollision threshold we enter the multiphoton regime. Strongly correlated back-to-back emission of the electrons along the polarization direction is observed to dominate in striking contrast to all previous data. No effect of Coulomb repulsion can be found, the predicted cutoff in the sum-energy spectra of two emitted electrons is confirmed, and the potential importance of multiple recollisions is discussed. DOI: 10.1103/PhysRevLett.101.053001 PACS numbers: 32.80.Rm, 31.90.+s, 32.80.Fb, 32.80.Wr Within the past three decades extensive studies on the interaction of intense laser fields with atoms and molecules have resulted in a profound understanding of various strong-field phenomena. Prominent examples are abovethreshold ionization (ATI) [1] or high-order harmonic generation [2], both essentially treatable within the single active electron (SAE) approximation. Correlated fewelectron processes, on the other hand, most important, for example, in nonsequential double (multiple) ionization (NSDI) (for a recent review see, e.g., [3]) have, until the present day, resisted any comprehensive modeling.Recently, however, a breakthrough was achieved at high intensities ( PW=cm 2 ) [4]. The application of many-particle imaging techniques [reaction microscopes and cold target recoil ion momentum spectroscopy (COLTRIMS)] [5] has allowed recording (multi)differential data on strong-field few-electron reactions and sophisticated calculations (see, e.g., [6] and references therein) have advanced their theoretical interpretation. As a result, a commonly accepted though simple picture has emerged, characterizing NSDI. Here an electron first tunnels into the field, is then accelerated, and finally thrown back onto its parent ion by the oscillating laser field. During ''recollision'' n-fold ionization might occur either in a direct e; ne -like encounter or indirectly via recollision-induced excitation of the ion plus subsequent field ionization (RESI) [7]. Signatures of the former are that both electrons are exclusively emitted into the same hemisphere along the polarization direction leading to ''double-hump'' shaped parallel momentum distributions of the ions (compensating the electron momenta). For RESI instead, according to the present understanding, the electrons can be emitted either parallel or back to back, thus filling the valley in between the double hump for the ions. Beyond the well-accepted simple scenario, however, major questions about the correlated electron emission are still far from being understood and are extensively investigated [6,8] because recollision is at the very heart of attoscience, molecular tomography, or imaging [9].At low intensities one intriguing though still widely unexplored question did arise early on within the above picture [10]. What happens when the energy of the recolliding electron of up to 3:17U P is not sufficient t...

In kinematically complete studies we explore double ionization (DI) of Ne and Ar in the threshold regime (I>3x10{13} W/cm{2}) for 800 nm, 45 fs pulses. The basic differences are found in the two-electron momentum distributions-"correlation" (CO) for Ne and "anticorrelation" (ACO) for Ar-that can be partially explained theoretically within a 3D classical model including tunneling. Transverse electron momentum spectra provide insight into "Coulomb focusing" and point to correlated nonclassical dynamics. Finally, DI threshold intensities, CO as well as ACO regimes are predicted for both targets.

We quantitatively predict the observed continuum-like spectral broadening in a 90-mm weakly birefringent all-normal dispersion-flattened photonic crystal fiber pumped by 1041-nm 229-fs 76-MHz pulses from a solid-state Yb:KYW laser. The well-characterized continuum pulses span a bandwidth of up to 300 nm around the laser wavelength, allowing high spectral power density pulse shaping useful for various coherent control applications. We also identify the nonlinear polarization effect that limits the bandwidth of these continuum pulses, and therefore report the path toward a series of attractive alternative broadband coherent optical sources.

We demonstrate a passively mode-locked femtosecond Yb:KLu(WO(4))(2) thin-disk laser oscillator. Chirped-pulse operation in the positive dispersion regime as well as solitary operation have been realized, and the laser performance of both configurations are compared. In the solitary mode-locking regime the output power exceeds 25 W in a diffraction-limited beam, and pulse durations as short as 440 fs at a 34.7 MHz repetition rate have been generated. For the first time we present a chirped-pulse operation of a thin-disk oscillator that yields a maximum average output power of 9.5 W with a Fourier limit of 450 fs.

The predicted spectral phase of a fiber continuum pulsed source rigorously quantified by the scalar generalized nonlinear Schrödinger equation is found to be in excellent agreement with that measured by multiphoton intra-pulse interference phase scan (MIIPS) with background subtraction. This cross-validation confirms the absolute pulse measurement by MIIPS and the transform-limited compression of the fiber continuum pulses by the pulse shaper performing the MIIPS measurement, and permits the subsequent coherent control on the fiber continuum pulses by this pulse shaper. The combination of the fiber continuum source with the MIIPS-integrated pulse shaper produces compressed transform-limited 9.6 fs (FWHM) pulses or arbitrarily shaped pulses at a central wavelength of 1020 nm, an average power over 100 mW, and a repetition rate of 76 MHz. In comparison to the 229-fs pump laser pulses that generate the fiber continuum, the compressed pulses reflect a compression ratio of 24.