No abstract

Currently, the shortest laser pulses that can be generated in the visible spectrum consist of fewer than two optical cycles (measured at the full-width at half-maximum of the pulse's envelope). The time variation of the electric field in such a pulse depends on the phase of the carrier frequency with respect to the envelope-the absolute phase. Because intense laser-matter interactions generally depend on the electric field of the pulse, the absolute phase is important for a number of nonlinear processes. But clear evidence of absolute-phase effects has yet to be detected experimentally, largely because of the difficulty of stabilizing the absolute phase in powerful laser pulses. Here we use a technique that does not require phase stabilization to demonstrate experimentally the influence of the absolute phase of a short laser pulse on the emission of photoelectrons. Atoms are ionized by a short laser pulse, and the photoelectrons are recorded with two opposing detectors in a plane perpendicular to the laser beam. We detect an anticorrelation in the shot-to-shot analysis of the electron yield.

R a n t experimental investigations of the high-order above-threshold ionization peaks near the onset of the plateau have exhibited anomalous angular dishibutions of the emitted p h o h l e c " with pronounced side lobes surrounding emission in the direction of the laser elechic field. It is shown thar the existence and andar position of these side lobes are consequences of the classical kinematics of elect" in laser fields.

Over the past decade intense laser fields with a single-cycle duration and even shorter, subcycle multicolour field transients have been generated and applied to drive attosecond phenomena in strong-field physics. Because of their extensive bandwidth, single-cycle fields cannot be emitted or amplified by laser sources directly and, as a rule, are produced by external pulse compression—a combination of nonlinear optical spectral broadening followed up by dispersion compensation. Here we demonstrate a simple robust driver for high-field applications based on this Kagome fibre approach that ensures pulse self-compression down to the ultimate single-cycle limit and provides phase-controlled pulses with up to a 100 μJ energy level, depending on the filling gas, pressure and the waveguide length.

Abstract. We use correlated electron-ion momentum measurement to investigate laser induced non-sequential double ionization of Ar and Ne. Light intensities are chosen in a regime at and below the threshold where, within the rescattering model, electron impact ionization of the singly charged ion core is expected to become energetically forbidden. Yet, we find Ar ++ ion momentum distributions and an electron-electron momentum correlation indicative of direct impact ionization. Within the quasistatic model this may be understood by assuming that the electric field of the light wave reduces the ionization potential of the singly charged ion core at the instant of scattering. The width of the projection of the ion momentum distribution onto an axis perpendicular to the light beam polarization vector is found to scale with the square root of the peak electric field strength in the light pulse. A scaling like this is not expected from the phase space available after electron impact ionization. It may indicate that the electric field at the instant of scattering is usually different from zero and determines the transverse momentum distribution. A comparison of our experimental results with several theoretical results is given.

The strong-field approximation, amended so as to allow for rescattering, is used to calculate high-order above-threshold ionization (ATI) spectra. The single-active-electron binding potential is modelled by a zero-range potential. The emphasis is on enhancements of groups of ATI peaks that occur for sharply defined laser intensities. The enhancements are traced to multiphoton resonance with the ponderomotively upshifted continuum threshold. Good agreement both with experimental data and with numerical simulations using the three-dimensional time-dependent Schrödinger equation for an optimized one-electron binding potential is observed. The physical reason for the close agreement of the results of the two, apparently so different, models is discussed. For quantitative agreement with the experimentally observed positions of the resonances, an 'effective' continuum threshold has to be introduced. The effects of focal averaging are evaluated and discussed. Resonant enhancement of highorder harmonic generation is also considered.

Multiphoton resonances with ponderomotively upshifted Rydberg states are believed to have dramatic effects on the plateau of high-order above-threshold ionization. For short pulses and intense laser fields, individual Rydberg states lose their physical significance. Under these conditions, on the basis of experimental and theoretical investigation of the dependence of the photoelectron spectra on the laser intensity, we propose that the observed effects can be largely understood in terms of the channel closings that are characteristic of a short-range model potential.Above-threshold ionization ͑ATI͒ is photoionization of atoms in strong laser fields ͓1͔; for a review see Ref. ͓2͔. In the present paper we investigate a phenomenon that has recently caught significant interest: strongly intensity-dependent enhancements of groups of ATI peaks that have been attributed to resonance with ponderomotively upshifted bound states close to the ionization limit ͑Rydberg states͒ ͓3-5͔. We describe experiments performed with the highest electron count rates and the shortest laser pulses ͑50 fs͒ used in this kind of work. With such intense and short pulses, the excited bound states of the atoms under investigation are broadened so dramatically that their significance is put into question. The features of the spectra presented here are studied in a Keldyshtype theoretical approach that does not incorporate any excited bound states, and explained by channel closings.We start by reviewing the key points of the present understanding of ATI. For fixed laser intensity, electrons are emitted with energieswith E 0 the ground-state energy of the atom, the laser frequency, and U p the ponderomotive energy, i.e., the cycleaveraged kinetic energy of a free electron in the presence of the laser field. The vector p n specifies the electron's momentum at the detector, i.e., its drift momentum. The envelope of the electron's spectrum exhibits various features that are independent of the atomic species: ͑i͒ ionization such that the ionized electron never re-encounters the ion yields a maximal classical energy of 2U p ͓6͔; ͑ii͒ enhanced ionization occurs for intensities where a ponderomotively upshifted Rydberg state is multiphoton resonant with the ground state ͑Freeman resonance ͓7͔͒; ͑iii͒ an ionized electron that does re-encounter the ion and elastically rescatters may acquire a maximal classical energy of 10U p ͓8,9͔; ͑iv͒ different electronic ''quantum trajectories'' may interfere, for a fixed laser intensity, constructively or destructively, and under auspicious circumstances these interferences remain visible in experimental spectra ͓10,11͔.In the past few years, new data ͓3,4͔ gave evidence of some resonance-like mechanism that is able to enhance the yields of groups of ATI peaks within the lower two thirds of the plateau by up to one order of magnitude at certain precisely defined intensities. These ''resonances'' are well reproduced by simulations using the single-active-electron approximation ͓12-14͔. In fact, the simulations suggest...

In this Letter we demonstrate a method for real-time determination of the carrier-envelope phase of each and every single ultrashort laser pulse at kilohertz repetition rates. The technique expands upon the recent work of Wittmann and incorporates a stereographic above-threshold laser-induced ionization measurement and electronics optimized to produce a signal corresponding to the carrier-envelope phase within microseconds of the laser interaction, thereby facilitating data-tagging and feedback applications. We achieve a precision of 113 mrad (6.5°) over the entire 2π range.