| 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. |
- Carlström, Stefanos, et al.
(författare)
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Spatially and spectrally resolved quantum path interference with chirped driving pulses
- 2016
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18:12
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Tidskriftsartikel (refereegranskat)abstract
- We measure spectrally and spatially resolved high-order harmonics generated in argon using chirped multi-cycle laser pulses. Using a stable, high-repetition rate laser we observe detailed interference structures in the far-field. The structures are of two kinds; off-axis interference from the long trajectory only and on-axis interference including the short and long trajectories. The former is readily visible in the far-field spectrum, modulating both the spectral and spatial profile. To access the latter, we vary the chirp of the fundamental, imparting different phases on the different trajectories, thereby changing their relative phase. Using this method together with an analytical model, we are able to explain the on-axis behaviour and access the dipole phase parameters for the short (${\alpha }_{{\rm{s}}}$) and long (${\alpha }_{{\rm{l}}}$) trajectories. The extracted results compare very well with phase parameters calculated by solving the time-dependent Schrödinger equation. Going beyond the analytical model, we are also able to successfully reproduce the off-axis interference structure.
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| 3. |
- Gaarde, Mette, et al.
(författare)
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High-order tunable sum and difference frequency mixing in the xuv region
- 1996
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 29:5
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of tunable high-order sum and difference frequency mixing in the 7-70 eV range, in xenon, argon and neon. We mix the light from a terawatt, subpicosecond titanium-sapphire laser and that from a tunable optical parametric generator. The radiation generated through mixing processes involves absorption or emission of one or Mo photons from the weak tunable source. The relative strengths of the sum and difference frequency processes depend strongly on the wavelength range of the generated radiation. The sum frequency processes are stronger at low photon energies in xenon, whereas the difference frequency processes dominate at high photon energies, in neon, being almost comparable to harmonic generation.
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| 4. |
- Gaarde, Mette, et al.
(författare)
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Strong species dependence of high order photoelectron production in alkali metal atoms
- 2000
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Ingår i: Physical Review Letters. - 1079-7114. ; 84:13, s. 2822-2825
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical and experimental study of the production of very high order photoelectrons from alkali metal atoms interacting with intense, midinfrared radiation. The strength of this process shows an unexpectedly strong species dependence. We find that this dependence can be explained via the difference in the cross section for electron-ion scattering from the different atoms. This allows us to directly relate the high energy portion of the photoelectron spectrum to specific features of the electron-ion potential.
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| 5. |
- Guo, Chen, et al.
(författare)
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Phase control of attosecond pulses in a train
- 2018
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 51:3
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Tidskriftsartikel (refereegranskat)abstract
- Ultrafast processes in matter can be captured and even controlled by using sequences of few-cycle optical pulses, which need to be well characterized, both in amplitude and phase. The same degree of control has not yet been achieved for few-cycle extreme ultraviolet pulses generated by high-order harmonic generation (HHG) in gases, with duration in the attosecond range. Here, we show that by varying the spectral phase and carrier-envelope phase (CEP) of a high-repetition rate laser, using dispersion in glass, we achieve a high degree of control of the relative phase and CEP between consecutive attosecond pulses. The experimental results are supported by a detailed theoretical analysis based upon the semi-classical three-step model for HHG.
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| 6. |
- Neoričić, Lana, et al.
(författare)
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4D spatio-temporal electric field characterization of ultrashort light pulses undergoing filamentation
- 2022
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Ingår i: Optics Express. - 1094-4087. ; 30:15, s. 27938-27950
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Tidskriftsartikel (refereegranskat)abstract
- We present an experimental method capable of capturing the complete spatiotemporal dynamics of filamenting ultrashort laser pulses. By employing spatially resolved Fourier transform spectrometry in combination with the capability to terminate the filament at any length, we can follow the nonlinear dynamics in four dimensions, i.e. the transverse domain, time and filament length. Our method thus not only enables the full characterization of the filamentation process throughout its evolution, but also allows to identify and select laser pulses with desired parameters.
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| 7. |
- Sheehy, B, et al.
(författare)
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High harmonic generation at long wavelengths
- 1999
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Ingår i: Physical Review Letters. - 1079-7114. ; 83:25, s. 5270-5273
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Tidskriftsartikel (refereegranskat)abstract
- High harmonic radiation spectra up to 19th order in alkali metal vapors excited by an intense, picosecond mid-infrared (3 -4 mu m) laser are reported and compared to theory. The strong-field dynamics in the alkali metal atoms exhibit significant differences from all previously studied systems due to the strong coupling between their ground and first excited states.
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| 8. |
- Varju, Katalin, et al.
(författare)
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Frequency chirp of harmonic and attosecond pulses
- 2005
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Ingår i: Journal of Modern Optics. - : Informa UK Limited. - 0950-0340 .- 1362-3044. ; 52:2-3, s. 379-394
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Tidskriftsartikel (refereegranskat)abstract
- We study the phase of the atomic polarization in the process of high-order harmonic generation. Its dependence on the laser intensity and the harmonic order induce a frequency variation in time (chirp) respectively of the harmonic pulses and attosecond pulses. We review the recent experimental results on the temporal characterization of the harmonic emission and show that measurements performed using very different techniques (like XFROG and RABITT), probing the phase in different parameter spaces, can be connected through the mixed phase derivatives, demonstrating the common underlying physics
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| 9. |
- 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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