We present the first detailed demonstrations of octave-spanning SC generation in all-normal dispersion photonic crystal fibers (ANDi PCF) in the visible and near-infrared spectral regions. The resulting spectral profiles are extremely flat without significant fine structure and with excellent stability and coherence properties. The key benefit of SC generation in ANDi PCF is the conservation of a single ultrashort pulse in the time domain with smooth and recompressible phase distribution. For the first time we confirm the exceptional temporal properties of the generated SC pulses experimentally and demonstrate their applicability in ultrafast transient absorption spectroscopy. The experimental results are in excellent agreement with numerical simulations, which are used to illustrate the SC generation dynamics by self-phase modulation and optical wave breaking. To our knowledge, we present the broadest spectra generated in the normal dispersion regime of an optical fiber.

Quantum tunneling is a ubiquitous phenomenon in nature and crucial for many technological applications. It allows quantum particles to reach regions in space which are energetically not accessible according to classical mechanics. In this "tunneling region," the particle density is known to decay exponentially. This behavior is universal across all energy scales from nuclear physics to chemistry and solid state systems. Although typically only a small fraction of a particle wavefunction extends into the tunneling region, we present here an extreme quantum system: a gigantic molecule consisting of two helium atoms, with an 80% probability that its two nuclei will be found in this classical forbidden region. This circumstance allows us to directly image the exponentially decaying density of a tunneling particle, which we achieved for over two orders of magnitude. Imaging a tunneling particle shows one of the few features of our world that is truly universal: the probability to find one of the constituents of bound matter far away is never zero but decreases exponentially. The results were obtained by Coulomb explosion imaging using a free electron laser and furthermore yielded He 2 's binding energy of 151.9 ± 13.3 neV, which is in agreement with most recent calculations.clusters | helium dimer | wavefunction | tunneling A ttractive forces allow particles to condense into stable bound systems such as molecules or nuclei with a ground state and (in most cases) energetically excited bound states, as shown in Fig. 1. Classical particles situated in such a binding potential oscillate back and forth between two turning points. The regions beyond these points are inaccessible for a classical particle due to a lack of energy. Quantum particles, however, can penetrate into the potential barrier by a phenomenon known as "tunneling." Tunneling is omnipresent in nature and occurs on all energy scales from megaelectron volts in nuclear physics to electron volts in molecules and solids and to nanoelectron volts in optical lattices. For bound matter, the fraction of the probability density distribution in this classically forbidden region is usually small. For shallow short-range potentials, this situation can change dramatically: upon decreasing the potential depth, excited states are expelled one after the other as they become unbound (transition from A to B in Fig. 1). A further decrease of the potential depth effects the ground state as well, as more and more of its wavefunction expands into the tunneling region ( Fig. 1 C and D). Consequently, at the threshold (i.e., in the limit of vanishing binding energy), the size of the quantum system expands to infinity. For short-range potentials, this expansion is accompanied by the fact that the system becomes less "classical" and more quantumlike. Systems existing near that threshold (and therefore being dominated by the tunneling part of their wavefunction) are called "quantum halo states" (1). These states are known, for example, from nuclear physics where 11 Be and 11 Li form ...

We report on non-sequential double ionization of Ar by a laser pulse consisting of two counter rotating circularly polarized fields (390 nm and 780 nm). The double ionization probability depends strongly on the relative intensity of the two fields and shows a "knee"-like structure as function of intensity. We conclude that double ionization is driven by a beam of nearly monoenergetic recolliding electrons, which can be controlled in intensity and energy by the field parameters. The electron momentum distributions show the recolliding electron as well as a second electron which escapes from an intermediate excited state of Ar + .Strong laser fields efficiently lead to the ejection of electrons from atoms and molecules. In the continuum the electron wave packet is driven by the laser field and its trajectory can be controlled by tailoring the time evolution of the electric field vector of the laser pulse on a sub cycle basis. A laser pulse composed of two harmonic colors offers already a significant amount of control parameters, such as polarization, relative intensity and phase between the two fields. This allows shaping the light field and thus to steer the electron motion in the continuum or in a bond [1][2][3][4][5][6][7][8][9]. Particularly versatile and in addition well controllable waveforms are generated by counter rotating circular two-color fields (CRTC) shown in Fig. 1 panels (c), (f), (i). These waveforms have spawned recent activities because, unlike circularly polarized light consisting of only a single color, CRTC fields can initially drive electrons away from the atom they have escaped from but later drive them back -often on triangularly shaped trajectories to re-encounter their parent ion. The recapture of these electrons gives rise to the emission of circularly polarized higher harmonic light as predicted in pioneering work by Becker and coworkers [10] and confirmed by recent experimental studies [11]. The recollision in such fields has also been identified by high energetic electrons [12] as well as by characteristic structures in the electron momentum distribution at very low energies where the electrons are Coulomb focused [13].In the present work we experimentally show that CRTC fields also lead to efficient double ionization mediated by the re-colliding electron as predicted by recent classical ensemble calculations [14]. Studying the probability of double ionization and in particular the three dimensional momentum distribution of the emitted electrons gives unprecedented insight into the recollision dy- * Electronic address: doerner@atom.uni-frankfurt.de namics occurring in these two-color laser fields. In particular they support that CRTC fields can be used to create a nearly monoenergetic electron beam for attosecond time-resolved studies. Additionally, very recent theoretical work [15] building on [16] predicts that these recolliding electrons can be generated such that they are spin polarized [17].In order to generate two-color fields, we use a 200 µm BBO to double the frequency of a 7...

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As a fundamental property of the electron, the spin plays a decisive role in the electronic structure of matter from solids to molecules and atoms, e.g. causing magnetism. Yet, despite its importance, the spin dynamics of electrons released during the interaction of atoms with strong ultrashort laser pulses has remained unexplored. Here we report on the experimental detection of electron spin polarization by strong-field ionization of Xenon atoms and support our results by theoretical analysis. We found up to 30% spin polarization changing its sign with electron energy. This work opens the new dimension of spin to strong-field physics. It paves the way to production of subfemtosecond spin polarized electron pulses with applications ranging from probing magnetic properties of matter at ultrafast time scales to testing chiral molecular systems with subfemtosecond temporal and sub-Ångström spatial resolution. Main TextShort laser pulses provide an electric field which can be strong enough to suppress the binding potential of an atom or molecule and lead to field ionization. Electrons passing over the barrier, or tunneling just under it and emerging from the atom are subsequently driven by the laser field. So far, nearly all works exploring the electronic behavior after ionization have solely used the binding energy of the electron and the shape of the barrier as defining properties, omitting the other fundamental property of the electron -its spin. This is even more surprising as a few pioneering theoretical works have indicated the importance of the spin of the outgoing electron in strong field ionization 1,2 . From the fundamental quantum standpoint, the spin of the liberated electron should not be ignored since, first, there is correlation between this electron and the ion left behind, and, second, ionization is known to trigger spin-orbit dynamics in the ion 3 . Rare gas atoms with their

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Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.

Ultrafast supercontinuum generation in gas-filled waveguides is an enabling technology for many intriguing applications ranging from attosecond metrology towards biophotonics, with the amount of spectral broadening crucially depending on the pulse dispersion of the propagating mode. In this study, we show that structural resonances in a gas-filled antiresonant hollow core optical fiber provide an additional degree of freedom in dispersion engineering, which enables the generation of more than three octaves of broadband light that ranges from deep UV wavelengths to near infrared. Our observation relies on the introduction of a geometric-induced resonance in the spectral vicinity of the ultrafast pump laser, outperforming gas dispersion and yielding a unique dispersion profile independent of core size, which is highly relevant for scaling input powers. Using a krypton-filled fiber, we observe spectral broadening from 200 nm to 1.7 μm at an output energy of ∼ 23 μJ within a single optical mode across the entire spectral bandwidth. Simulations show that the frequency generation results from an accelerated fission process of soliton-like waveforms in a non-adiabatic dispersion regime associated with the emission of multiple phase-matched Cherenkov radiations on both sides of the resonance. This effect, along with the dispersion tuning and scaling capabilities of the fiber geometry, enables coherent ultra-broadband and high-energy sources, which range from the UV to the mid‐infrared spectral range.