H. G. Muller

Parametric generation in β-barium borate of intense femtosecond pulses near 800 nm

We use amplified colliding-pulse, mode-locked dye-laser pulses of 150-μJ energy to pump a two-stage, single-path parametric converter of β-barium borate (β-BBO). The wavelength-noncritical phase-matching condition in β-BBO at 620 nm permits generation of 200-fs signal pulses with at least the bandwidth of the pump. At 830 nm the overall energy conversion efficiency is 17%. This is an efficient, tunable way to produce near-IR pulses as an alternative to continuum generation. The signal is used for amplification in IR dye and titanium-sapphire and for continuum generation. The input/output characteristics of the parametric generator are presented. Additionally, we observe off-axis parametric generation followed by sum-frequency generation for pump pulse intensities larger than 5 GW/cm2.

Resonant multiphoton ionisation of xenon with high-intensity femtosecond pulses

The authors report on the intensity and polarisation dependences of electron energy spectra from multiphoton ionisation of xenon with 120 fs pulses at 615 nm. The spectra show a large number of narrow structures assigned to resonances induced by the AC Stark shift. The structures are suppressed with circular polarisation. The maximum shift is observed for the 6s state which is up-shifted by 1.6 eV. The assignments are based on a lowest-order perturbative calculation of the AC Stark shifts.

Generation of intense sub-picosecond pulses in the mid-infrared

Abstract We describe the use of difference-frequency generation, by mixing two intense femtosecond pulses in a 5-mm long BBO crystal, to generate high power 2.5 μm pulses with a duration of less than 0.5 ps. The pulses are generated by mixing the output of an amplified CPM laser with an amplified part of a continuum. The generated pulses have an energy of 4 μJ which amounts to a photon-conversion efficiency of 5 percent.

A systematic study of AC Stark shifts in xenon at super-high laser intensities

Experimentally the authors determine the energy levels of the 4f and 5f states in xenon as a function of laser intensity. In short-pulse (400 fs) photoelectron spectra they observe an enhancement of the seven-photon ionization yield due to resonances at the six-photon level. Two prominent peaks are attributed to the 4f and 5f states. Since the peaks are observed over a large range of wavelengths (555 nm to 650 nm), the authors follow the energy levels of these states from low to high intensity. The AC Stark shifts of these states are well described by a shift equal to the ponderomotive shift, up to the highest intensities studied (7*1013 W cm-2).

Control of atomic ionization by two-color few-cycle pulses

We have studied the ionization of Rydberg atoms by few-cycle radio-frequency pulses and used two-color fields to control the ionization dynamics. We show that the number of times that electrons are emitted during a pulse can be limited and that the duration of the electron emission can be shortened. These results, once they are transposed to the optical domain, may inspire new strategies for the production of single attosecond pulses.

Intensity-induced chirped Rydberg wavepackets: a new way of creating short light pulses

The authors report calculations on the time evolution of a Rydberg wavepacket excited with a short intense laser pulse. The wavepacket is chirped and compressed due to the AC Stark shift of the Rydberg states. This effect can be used to shorten optical pulses ranging from the IR to the UV regime. For a practical case, reduction of the duration of femtosecond pulses by a factor of six is predicted.

Return of an electronic wavefunction to the core

Picosecond laser pulses are used to excite xenon atoms to Rydberg states and photoionise in a three-plus-one-photon ionisation process. When high Rydberg levels are excited, an electronic wavepacket is created. This wavepacket does not return to the core during the laser pulse: the direct four-photon ionisation is not enhanced. When lower Rydberg levels are excited the electronic wavefunction does return to the core: the ionisation is enhanced. The authors observe the transition from enhanced to non-enhanced ionisation.

Similarities between atomic wave packets and optical pulses

Abstract We consider the similarities between short optical pulses and atomic electron wave packets. The atomic potential acts in a similar way on the electron wave function as an optical cavity does on electromagnetic waves. It is shown that effects well known in optics, such as creation of pulsed light, dispersion, and shortening of optical pulses, can also occur for matter waves.

Absence of resonances in femtosecond photoionisation spectra

Xenon was ionised by 100 fs UV pulses. The electron spectra showed a decreasing energy of the photoelectrons with increasing light intensity. No resonances were observed in the electron spectrum. The absence of resonances is understood in terms of the ratio of the pulse duration and the characteristic time (2 pi n3 in au) of the intermediate state. From the observed spectra it appears that the four-photon ionisation of xenon at 309 nm is not saturated at an intensity of 1.5*1013 W cm-2.

Time domain effects in photoionisation of Rydberg atoms

The authors report wavelength spectra of the photoelectron yield resulting from three-photon ionisation and four-photon excess-photon ionisation (EPI/ATI) of xenon in a magnetic field. The spectra are scanned over a wavelength region of 1 nm to 307 nm (three-photon threshold for ionisation of Xe). The EPI behaviour is studied while they scan through the threshold. They conclude from the measurements that the phenomena which are typical for EPI do not only take place above the threshold, but also below it as long as the ionisation pulse is short compared with the time period that elapses before the wavefunction of the intermediate state is relocalised around the core. This time period is at least one classical revolution, but is increased in this experiment by the application of a magnetic field.