Florian Schapper

Spatio-temporal characterization of few-cycle pulses obtained by filamentation

Intense sub-5-fs pulses were generated by filamentation in a noble gas and subsequent chirped-mirror pulse compression. The transversal spatial dependence of the temporal pulse profile was investigated by spatial selection of parts of the output beam. Selecting the central core of the beam is required for obtaining the shortest possible pulses. Higher energy efficiency is only obtained at the expense of pulse contrast since towards the outer parts of the beam the energy is spread into satellite structures leading to a double-pulse profile on the very off-axis part of the beam. Depending on the requirements for a particular application, a trade-off between the pulse duration and the pulse energy has to be done. The energy of the sub-5-fs pulses produced was sufficient for the generation of high order harmonics in Argon. In addition, full simulation is performed in space and time on pulse propagation through filamentation that explains the double-pulse structure observed as part of a conical emission enhanced by the plasma defocusing.

Spectral signature of short attosecond pulse trains.

We report experimental measurements of high-order harmonic spectra generated in Ar using a carrier-envelope-offset (CEO) stabilized 12 fs, 800 nm laser field and a fraction (less than 10%) of its second harmonic. Additional spectral peaks are observed between the harmonic peaks, which are due to interferences between multiple pulses in the train. The position of these peaks varies with the CEO and their number is directly related to the number of pulses in the train. An analytical model, as well as numerical simulations, support our interpretation.

Spatial fingerprint of quantum path interferences in high order harmonic generation

We have spatially and spectrally resolved the high order harmonic emission from an argon gas target. Under proper phase matching conditions we were able to observe for the first time the spatial fine structure originating from the interference of the two shortest quantum paths in the harmonic beam. The structure can be explained by the intensity-dependent harmonic phase of the contributions from the two paths. The spatially and spectrally resolved measurements are consistent with previous spatially integrated results. Our measurement method represents a new tool to clearly distinguish between different interference effects and to potentially observe higher order trajectories in the future with improved detection sensitivity. Here, we demonstrate additional experimental evidence that the observed interference pattern is only due to quantum-path interferences and cannot be explained by a phase modulation effect. Our experimental results are fully supported by simulations using the strong field approximation and including propagation.

Ionization effects on spectral signatures of quantum-path interference in high-harmonic generation.

The interference between the emission originating from the short and long electron quantum paths is intrinsic to the high harmonic generation process. We investigate the universal properties of these quantum-path interferences in various generation media and discuss how ionization effects influence the observed interference structures. Our comparison of quantum-path interferences observed in xenon, argon, and neon demonstrates that our experimental tools are generally applicable and should also allow investigating more complex systems such as molecules or clusters.

First High Harmonic Generation (HHG) in a Photonic Crystal Fiber (PCF)

We generate the 7th-13th harmonic of ≈800 nm by propagating 30-fs pulses at >1014W/cm2through a xenon-filled hollow-core PCF. The extremely low HHG threshold of 0.4 µJ would be achievable by multimegahertz solid-state lasers.

Two-quantum-path interferences in high order harmonic generation

We have investigated intensity dependent high-harmonic generation, when short and long trajectories are both visible in the generated signal. We have measured a plateau-harmonic spectral broadening and yield modulations consistent with quantum-paths interferences calculations.

Electron Wavepacket Interference Observed by Attosecond Transient Absorption Spectroscopy

Attosecond time-resolved transient absorption spectroscopy is performed in a dense helium target by superimposing an attosecond pulse train (APT) with a moderately strong infrared field. We observe rapid oscillations of the absorption of the individual harmonics as a function of time-delay between the APT and IR field even for harmonic energies well below the ionization threshold. The phase dependence of these modulations on atto-chirp and IR intensity yields direct evidence for the interference of transiently bound electronic wavepackets as the underlying mechanism.

Direct optical observation of attosecond electron wavepacket interference

Attosecond transient absorption spectroscopy is used to investigate electron dynamics in the vicinity of the first ionization threshold of helium. Rapid oscillations of the absorption of individual harmonics are observed when delaying an attosecond pulse train (APT) with respect to a superimposed, moderately strong infrared laser field. The phase relation between the absorption modulation of individual harmonics gives direct evidence for the interference of transiently bound electronic wavepackets as the underlying mechanism. Furthermore, the understanding of these dynamics opens a route towards attosecond XUV pulse shaping.

Harmonic Continua by Chirp Assisted Polarization Gating

We demonstrate a new scheme to generate harmonic continua starting with 12 fs laser pulses based on polarization gating. The effectiveness of the gating method is confirmed by an SFA calculation.

High harmonic generation (HHG) in a Kagome-type hollow-core photonic crystal fiber (HC-PCF)

Hollow-core photonic crystal fibers (HC-PCF) offer a combination of long effective interaction length and small mode areas which has resulted in several major breakthroughs in low-power nonlinear optics (see e.g. [1–3]). Recently, it was shown numerically that such fibers offer unique properties in terms of efficiency and low threshold for high harmonic generation (HHG) [4]. In addition to the substantial decrease in pump threshold, the efficiency is expected to be significantly enhanced by guiding and improved phase-matching using optimized dispersion engineering in PCFs. Traditionally, HHG has been carried out using complex femtosecond laser amplifier systems operating at low repetition rates in the kHz-regime with pulse energies of more than 100 µJ and an unguided interaction using a gas-jet target. In recent years, however, there has been considerable interest in the development of HHG sources operating in the VUV/XUV spectral region with multi-megahertz repetition rates, which can substantially increase the signal-to-noise ratio for numerous applications.