F. Grasbon

Absolute-phase phenomena in photoionization with few-cycle laser pulses.

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 pulses 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.

Laser-induced non-sequential double ionization investigated at and below the threshold for electron impact ionization

We use correlated electron–ion momentum measurements 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 Ar2+ 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.

Dispersion control in a 100-kHz-repetition-rate 35-fs Ti:sapphire regenerative amplifier system

Presents a 100-kHz femtosecond amplifier system delivering pulses with a duration of 35 fs and an energy of 7 /spl mu/J. The system does not include a stretcher, since the large amount of dispersion accumulated during the amplification process is sufficient to prevent self-focusing. Compensation in approximately all orders is achieved through a combination of a prism compressor, chirped mirrors, and a liquid-crystal modulator, allowing the amplified pulses to be shortened to nearly the bandwidth limit.

Femtosecond interferometric autocorrelations in the presence of pulse-front distortions

The tilt of the pulse front caused by misalignment in stretcher-compressor devices which are used in chirped pulse amplification should be carefully considered in the design of femtosecond laser systems. We present a convenient procedure for online measurement and minimization of the tilt in a grating stretcher/compressor setup. In addition, we present a theoretical model for the autocorrelation signal in the presence of pulse front distortion. The influence of the pulse front tilt to the autocorrelation function is numerically simulated and compared with the case for pulses with fourth order chirp.

Interference effects in above-threshold ionization

Abstract We discuss four recent photoionization experiments with atoms and molecules in intense laser fields. So far, photoionization under these conditions was considered to be amenable to classical arguments. In contrast with this, the effects described here are due to quantum interference and, accordingly, have to be described by a quantum-mechanical theory.

Evidence of absolute-phase phenomena in above-threshold ionization with few-cycle laser pulses

First experimental evidence for an effect of the absolute phase is presented. Atoms are ionized by a short laser pulse and the electrons leaving the ion core are registered with two detectors facing each other on opposite sides of the laser beam. Each laser pulse can thus be characterized by the number of electrons emitted to the left and the right. Accordingly, the laser shots are recorded in a contingency map.

Dispersion control in a 100-kHz repetition rate 35-fs laser system

We present a 100 kHz femtosecond amplifier system delivering pulses with a duration of 35 fs and an energy of 7 μJ at 800 nm. The system does not include a stretcher, since the large amount of dispersion accumulated during the amplification process is sufficient to prevent self-focusing. Compensation in all orders is achieved through a combination of a special prism compressor, chirped mirrors, and a liquid-crystal modulator.

Auf Feynmans Quantenpfaden: Atome im intensiven Laserfeld

Heute kann man Laserpulse erzeugen, deren Feldstarken diejenigen ubertreffen, welche die Atome zusammen halten. In den letzten Jahren gelangen mithilfe des Feynmanschen Pfadintegrals grose Fortschritte im Verstandnis der Effekte, die durch die Wechselwirkung intensiver Laserpulse mit Atomen entstehen. Die begriffliche Nahe des Pfadintegralformalismus zu klassischen Trajektorien erlaubt in vielen Fallen sogar ein intuitives Verstandnis des physikalischen Mechanismus.

Quantum effects in above threshold ionization

We report on the relevance of quantum effects in above threshold ionization for low and high energy photoelectrons. High repetition rate lasers are used to study energy resolved electron yields as a function of ellipticity as well as the angular distribution of the photoelectrons. In comparison with theory it is possible to identify quantum effects.

Nonlinear interaction of ring dark solitary waves with coaxial dark beams

Theoretical and experimental results on optical ring dark solitary waves are presented, emphasizing the interplay between initial dark beam contrast, phase-shift magnitude, background-beam intensity and saturation of the nonlinearity. An enhanced RDSW transverse dynamics originating in the interaction with a second coaxial dark beam is analyzed theoretically in a good agreement with the experiment. The results may open the way to construct a parallel all-optical three-position radial switch.