| 1. |
- Arnold, Cord L., et al.
(författare)
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Spatiotemporal coupling of attosecond pulses
- 2019
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Ingår i: 2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019. - 9781728104690 ; Part F140-CLEO_Europe 2019
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Konferensbidrag (refereegranskat)abstract
- Attosecond pulses in the extreme ultraviolet (XUV) spectral range are today routinely generated via high-order harmonic generation (HHG), when intense ultrashort laser pulses are focused into a gaseous generation medium. The effect is most easily understood in a semi-classical picture [1]. An electron can tunnel ionize from the distorted atomic potential, pick up kinetic energy in the laser field, potentially return to its parent ion and recombine. The excess energy is emitted as XUV photon. The process repeats for every half-cycle of the driving field, resulting in a train of attosecond pulses and in the frequency domain in the well-known, odd-order comb of harmonics. Two main families of electron trajectories leading to the same photon energy can be distinguished into 'short' and 'long', according to their time of travel in the continuum. Due to the complicated nature of the HHG process, attosecond pulses usually cannot be separated into their temporal and spatial profiles, but instead have strong chromatic aberration and are spatio-temporally coupled [2-4].
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| 2. |
- Du, Mengqi, et al.
(författare)
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High-resolution wavefront sensing and aberration analysis of multi-spectral extreme ultraviolet beams
- 2023
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Ingår i: Optica. - : Optica Publishing Group. - 2334-2536. ; 10:2, s. 255-263
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Tidskriftsartikel (refereegranskat)abstract
- Coherent multi-spectral extreme ultraviolet beams have great potential for providing high spatial and temporal resolution for microscopy and spectroscopy applications. But due to the limitations of short-wavelength optics and the broad bandwidth, it remains a challenge to perform quantitative, high-resolution beam characterization. Here we present a wavefront sensing solution based on multiplexed ptychography, with which we show spectrally resolved, high-resolution beam reconstructions. Furthermore, using these high-fidelity quantitative wavefront measurements, we investigate aberration transfer mechanisms in the high-harmonic-generation process, where we present and explain harmonic-order-dependent astigmatism inheritance from the fundamental wavefront. This ptychographic wavefront sensing concept thus enables detailed studies of the high-harmonic-generation process, such as spatiotemporal effects in attosecond pulse formation.
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| 3. |
- Hoflund, Maria, et al.
(författare)
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Focusing Properties of High-Order Harmonics
- 2021
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Ingår i: Ultrafast Science. - : American Association for the Advancement of Science (AAAS). - 2765-8791. ; 2021
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Tidskriftsartikel (refereegranskat)abstract
- Many applications of the extreme ultraviolet (XUV) radiation obtained by high-order harmonic generation (HHG) in gases require a small focus area in order to enable attosecond pulses to reach a high intensity. Here, high-order harmonics generated in Ar with a multiterawatt laser system in a loose focusing geometry are focused to a few micrometers using two toroidal mirrors in a Wolter configuration with a high demagnification factor. Using a knife-edge measurement technique, we determine the position and size of the XUV foci as a function of harmonic order. We show that the focus properties vary with harmonic order and the generation conditions. Simulations, based on a classical description of the harmonic dipole phase and assuming that the individual harmonics can be described as Gaussian beams, reproduce the experimental behavior. We discuss how the generation geometry affects the intensity and duration of the focused attosecond pulses
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| 4. |
- Plach, Marius, et al.
(författare)
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Spatial Aberrations in High-Order Harmonic Generation
- 2024
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Ingår i: Ultrafast Science. - 2097-0331. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the spatial characteristics of high-order harmonic radiation generated in argon and observe cross-like patterns in the far field. An analytical model describing harmonics from an astigmatic driving beam reveals that these patterns result from the order and generation position-dependent divergence of harmonics. Even small amounts of driving field astigmatism may result in cross-like patterns, coming from the superposition of individual harmonics with spatial profiles elongated in different directions. By correcting the aberrations using a deformable mirror, we show that fine-tuning the driving wavefront is essential for optimal spatial quality of the harmonics.
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| 5. |
- Wikmark, Hampus, et al.
(författare)
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Spatiotemporal coupling of attosecond pulses
- 2019
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424. ; 116:11, s. 4779-4787
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Tidskriftsartikel (refereegranskat)abstract
- The shortest light pulses produced to date are of the order of a few tens of attoseconds, with central frequencies in the extreme UV range and bandwidths exceeding tens of electronvolts. They are often produced as a train of pulses separated by half the driving laser period, leading in the frequency domain to a spectrum of high, odd-order harmonics. As light pulses become shorter and more spectrally wide, the widely used approximation consisting of writing the optical waveform as a product of temporal and spatial amplitudes does not apply anymore. Here, we investigate the interplay of temporal and spatial properties of attosecond pulses. We show that the divergence and focus position of the generated harmonics often strongly depend on their frequency, leading to strong chromatic aberrations of the broadband attosecond pulses. Our argument uses a simple analytical model based on Gaussian optics, numerical propagation calculations, and experimental harmonic divergence measurements. This effect needs to be considered for future applications requiring highquality focusing while retaining the broadband/ultrashort characteristics of the radiation.
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