Attosecond pulse walk-off in high-order harmonic generation

Abstract

We study the influence of the generation conditions on the group delay of attosecond pulses in high-order harmonic generation in gases. The group delay relative to the fundamental field is found to decrease with increasing gas pressure in the generation cell, reflecting a temporal walk-off due to the dispersive properties of the nonlinear medium. This effect is well reproduced using an on-axis phase-matching model of high-order harmonic generation in an absorbing gas. The nonlinear interaction between a focused high-power laser beam and a gas target generates broadband light bursts emitted once every half cycle of the driving field [1,2]. In the spectral domain, the half-cycle periodicity and symmetry properties of the target gas result in peaks centered at the odd-order harmonics of the driving pulse carrier frequency. The large bandwidth combined with the coherence of the process, inherited from the generating field, yields pulses with a time duration on the attosecond scale [3,4]. The intrinsic synchronization of the attosecond pulses with the generating field [5] allows for pump-probe experiments on an ultrafast time scale. By cross correlating the attosecond pulse train (APT) with a weak copy of the generating pulse in a detection gas, while monitoring the generated photoelectron spectrum, the phase difference between consecutive harmonic orders can be extracted [5]. Combined with a measurement of the relative spectral amplitudes, this information allows for a reconstruction of the average time structure of the pulses in the train. This is the well-known RABITT scheme (reconstruction of attosecond beating by interference of two-photon transition) for characterization of APTs.This technique has recently gained a lot of interest since the comparison of RABITT measurements in different systems, for example, different atomic shells, allows for the determination of their relative photoionization delays [6]. These experiments demand high interferometric stability, on the order of tens of attoseconds, and require control of the timing of the attosecond pulses relative to the fundamental field. In some early implementations of the RABITT scheme [5,7,8], the probe pulse and the generation pulse were both propagated through the generation medium, making it possible to encode the phase relation between the generating and the probing field into the recorded electron spectrogram, but at the same time perturbing the regularity of the pulse train.In this work, we perform RABITT measurements using an actively stabilized interferometer. We present an experimental study of the group delay of the attosecond pulses relative to the generating field as a function of the gas density in the generation cell. Our results show that the detected pulse train advances by almost 200 as, relative to the fundamental field, as the pressure is increased by a factor of three. This observation is interpreted as a temporal walk-off of the attosecond pulses due to dispersion in the medium and simulated using an on-axis phase-matching model.The e...

Imaging Localized Surface Plasmons by Femtosecond to Attosecond Time‐Resolved Photoelectron Emission Microscopy – “ATTO‐PEEM”

“…For the ATTO-PEEM experiments the laser pulses are sent into an actively stabilized Mach-Zehnder interferometer [32], as illustrated in Figure 10.13. In one arm of the interferometer, attosecond XUV pulse trains are produced via high-order harmonic generation by focusing the laser in a pulsed Ar gas cell.…”

Attosecond beamline with actively stabilized and spatially separated beam paths

“…[7][8][9][10] Another successful approach consists in splitting the IR pulse before HHG and recombining XUV and IR pulses after the generation. 2,[11][12][13][14][15][16] This configuration offers a high a) Electronic mail: hwoerner@ethz.ch.…”

Macroscopic study and control of high-order harmonic and attosecond pulse generation in noble gases

“…The results to be presented here focus on the possibilities to increase the HHG cutoff, isolate single attosecond pulses from a pulse train while also minimizing its duration, increase harmonic yield at high photon energies, and in general to understand macroscopic processes in HHG. These are based on our theoretical papers about THz assisted attosecond pulse generation [2,3,4], quasi-phase matching of HOH radiation and its study for the special case of perpendicularly propagating IR and THz fields [5,6,7], optimization of attosecond pulse generation in light-field synthesizers [10], and an experimental study of attosecond GD dependence on generation gas pressure [8].…”

“…From the dependence of harmonic intensities on the background pressure, the generation pressure can be approximated using for example Equation II.38, (assuming a linear relation between the two pressure values). And, in fact, a similar method has been used in the theoretical model presented in [8] to interpret the results.…”

“…After reasonable stability had been achieved, we measured the variation in the group delay of attosecond pulses by changing the gas pressure in the generation cell. I assisted the research group at both preparation and measurement stages of these experiments, contributed to the analysis of the measured data, and assisted in the preparation of manuscript [8]. I also participated in experiments of photo-ionization delay measurements, summarized in a recently submitted paper [9].…”