Fabrizio Lindner

Attosecond Double-Slit Experiment

A new scheme for a double-slit experiment in the time domain is presented. Phase-stabilized few-cycle laser pulses open one to two windows (slits) of attosecond duration for photoionization. Fringes in the angle-resolved energy spectrum of varying visibility depending on the degree of which-way information are measured. A situation in which one and the same electron encounters a single and a double slit at the same time is observed. The investigation of the fringes makes possible interferometry on the attosecond time scale. From the number of visible fringes, for example, one derives that the slits are extended over about 500 as.

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.

Non-sequential double ionization in a few-cycle laser pulse: the influence of the carrier–envelope phase

Non-sequential double ionization (NSDI) using phase-stabilised few-cycle laser pulses is investigated. We report differential measurements of Ar ++ ion momentum distributions. They show a strong dependence on the carrier-envelope (CE) phase. Via control over the CE phase one is able to direct the NSDI dynamics. Data analysis through a classical model calculation reveals that the influence of the optical phase enters via: (1) the cycle-dependent electric field ionization rate, (2) the electron recollision time and (3) the accessible phase space for inelastic collisions. Our model indicates that the combination of these effects allows a look into single-cycle dynamics already for few-cycle pulses.

Generation and Measurement of Intense Phase-Controlled Few-Cycle Laser Pulses

Intense, ultrashort waveforms of light can be produced with a predetermined electromagnetic field. These waveforms are essential in many applications of extreme nonlinear optics, most prominently in laser-driven sources of high-energy attosecond radiation. Field reproducibility in each laser shot requires a stable carrier-envelope phase. We analyze different schemes of phase-stable amplification and identify constraints limiting the precision. We describe a phase-stabilized system based on a 20 fs multipass Ti:sapphire amplifier supplemented with a fiber compression stage for producing few-cycle pulses. The amplifier introduces only a slow phase drift and, therefore, can be seeded by a standard phase-stabilized oscillator. The phase stability of the 5 fs, 400 µJ pulses is verified by high harmonic generation, in which different carrier-envelope phases produce distinctly different XUV spectra. The carrier-envelope phase (with a±π ambiguity) is calibrated from a series of spectra. The calibration allows full characterization of the electric field. The estimated precision of the phase control is better than π/5, which reduces the timing jitter between the driving laser pulse and the XUV bursts to ∼250 as and enables the generation of stable, isolated attosecond pulses. We demonstrate a more robust phase measurement based on the detection of electron emission from photoionized atoms in opposite directions. This method determines the carrier-envelope phase with the π/10 accuracy and without inversion ambiguity. Using the photoionization technique, we demonstrate the Gouy effect for focused few-cycle pulses. This result is of critical importance for any phase-dependent strong-field applications of ultrashort laser pulses.

Spatiotemporal determination of the absolute phase of few-cycle laser pulses

We determined the carrier-envelope (“absolute”) phase of linearly polarized few-cycle laser pulses by measuring the energy-resolved asymmetry of electron emission from noble gases. The Gouy phase shift in the laser focus is also measured, providing the first full spatiotemporal depiction of the electric field in the whole focal region.

Measurement of the Absolute Phase of few-Cycle Laser Pulses

Recent developments of laser technology led to the generation of laser pulses with a duration (measured at full-width half maximum) of less than 5fs [1]. At a typical wavelength of 750nm, an optical cycle has a duration of 2.5fs; therefore, the pulses consist of only very few optical cycles (few-cycle pulses). Since the pulse envelope varies almost as fast as the electromagnetic field itself, the shape of the laser’s field will significantly depend on the phase of the carrier frequency with respect to the envelope, the so-called absolute phase.

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.