Abstract
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.
Highlights
High order harmonic generation (HHG), a coherent up-conversion process, has been widely studied ever since its first experimental observations [1,2,3]
By recording spatially resolved spectra of high order harmonics generated in argon, we found fingerprints of quantum path interference between the two shortest quantum paths
Our measurements represent the first observation of quantum path interferences (QPI) inside the harmonic beam
Summary
High order harmonic generation (HHG), a coherent up-conversion process, has been widely studied ever since its first experimental observations [1,2,3]. This recombination is accompanied by the emission of a high-energy photon This semi-classical model predicts that, in the plateau region, different electron trajectories lead to the same harmonic photon energy. This was further confirmed and expanded by quantum mechanical calculations [6] that identified the different quantum paths contributing to the emission of a given harmonic order. It was predicted that the contributions from these different paths should interfere in the high harmonic emission with the phases accumulated during the corresponding electron trajectories These quantum path interferences (QPI) could be controlled by varying the laser parameters such as its wavelength [7] or intensity [8]. A theoretical model [8,11] based on the strong-field approximation (SFA) and including propagation reproduces the experimental data very well and aids its interpretation
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