Abstract
One key parameter in the high-order harmonic generation (HHG) phenomenon is the exact frequency of the generated harmonic field. Its deviation from perfect harmonics of the laser frequency can be explained by considering (i) the single-atom laser-matter interaction and (ii) the spectral changes of the driving laser. In this work, we perform an experimental and theoretical study of the causes that generate spectral changes in the HHG radiation. We measured the driving-laser spectral shift after HHG in a long medium by using a correction factor to take into account the multiple possible HHG initiation distances along the laser path. We separate out the contribution of laser spectral shift from the resultant high-harmonic spectral shift in order to elucidate the microscopic effect of spectral shift in HHG. Therefore, in some cases we are able to identify the dominant electron trajectory from the experimental data. Our investigations lead to valuable conclusions about the atomic dipole phase contribution to a high-harmonic spectral shift. We demonstrate that the significant contribution of a long electron path leads to a high-harmonic shift, which differs from that expected from the driving laser. Moreover, we assess the origin of the high-order harmonics spectral broadening and provide an explanation for the narrowest high-harmonic spectral width in our experiment.
Highlights
Development of coherent light sources in the extreme ultraviolet (XUV) spectral domain is of very high importance, because it allows researchers to carry out wavelength-limited imaging, observation and potentially control of various physical, biological and chemical phenomena at their natural spatial, nanometric, and temporal, sub-femtosecond, scales
We can distinguish the wavelength shift coming directly from the driving IR laser pulse from the one produced by the atomic dipole phase
We have found the contribution to the resultant high harmonic spectral shift due to atomic dipole phase in case of high-order harmonic generation (HHG) in argon is negligible
Summary
Development of coherent light sources in the extreme ultraviolet (XUV) spectral domain is of very high importance, because it allows researchers to carry out wavelength-limited imaging, observation and potentially control of various physical, biological and chemical phenomena at their natural spatial, nanometric, and temporal, sub-femtosecond, scales. The highly nonlinear interaction of intense femtosecond laser pulses with matter leads to the generation of harmonics of the driving laser light up to very high orders This phenomenon, so-called high-order harmonic generation (HHG), became the flagship source of short wavelength coherent radiation [1]. Besides its high degree of coherence, HHG radiation exhibits collimated beams [2] with a Gaussian-like transverse energy distribution and a nearly diffraction-limited wavefront [3], allowing for efficient focusing of the light onto samples. These properties facilitate the analysis of the source features and allow engineering of their temporal and spatial characteristics. The polarization state of the emitted radiation can be controlled to a high degree [4] (for a recent review see e.g. [5])
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