| 1. |
- Altucci, C, et al.
(författare)
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Frequency-resolved time-gated high-order harmonics
- 1998
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Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 58:5, s. 3934-3941
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Tidskriftsartikel (refereegranskat)abstract
- We report measurements of high-order harmonic spectra obtained with a 800-nm 150-fs loser pulse with a time-varying degree of ellipticity. The modulation of the polarization in time is achieved by using birefringent optics and self-phase modulation in a glass plate. We can create one or two temporal gates of a few femtoseconds width, during which the polarization is linear and harmonic emission is efficient. The harmonic spectra observed experimentally demonstrate that harmonics generated with linear polarization are frequency chirped. The values measured experimentally are consistent with theoretical predictions based on the strong field approximation. [S1050-2947(98)05511-5].
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| 2. |
- 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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| 3. |
- Balcou, P, et al.
(författare)
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High-order-harmonic generation: towards laser-induced phase-matching control and relativistic effects
- 2002
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 74:6, s. 509-515
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Tidskriftsartikel (refereegranskat)abstract
- We present a review of some recent results on high-order-harmonic generation, aiming at optimizing the photon flux to allow for future applications in extreme-ultra-violet non-linear optics. We first present new schemes to control phase matching of high harmonics in gases, by using the effect of the spatially varying atomic phase displayed by the high harmonics. An enhancement by a factor of 50 is observed in neon in conditions for which the gradient of the atomic dispersion balances the electronic dispersion. A new scheme to manipulate the laser field was demonstrated, and shown to improve phase matching. We then turn to high-harmonic generation by solid targets, and show that high harmonics generated by an intense 30-fs laser pulse remain collimated even at the threshold of the relativistic regime.
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| 4. |
- Balcou, P, et al.
(författare)
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Quantum-path analysis and phase matching of high-order harmonic generation and high-order frequency mixing processes in strong laser fields
- 1999
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 32:12, s. 2973-2989
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Tidskriftsartikel (refereegranskat)abstract
- We study phase-matching conditions for high-order harmonic generation as well as high-order sum- and difference-frequency mixing processes in strong laser fields, using a graphical approach described in Balcou et al (1997 Phys. Rev. A 55 3204-10). This method is based on the analysis of the different quantum paths that contribute, with different phase properties, to the single-atom response. We propose a simple numerical method to disentangle the quantum paths contributing to the generation process. We present graphical maps of the phase matching around the laser focus, which allow one to predict the geometries that optimize the conversion efficiency of the process considered. The method is applied to the study of sum- and difference-frequency mixing processes. The qualitative predictions of the graphical phase-matching approach are confirmed by numerical propagation calculations.
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| 5. |
- Biegert, Jens, et al.
(författare)
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Control of the frequency chirp rate of high harmonic pulses
- 2004
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Ingår i: Springer Series in Chemical Physics. - 0172-6218. ; 79, s. 204-206
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Tidskriftsartikel (refereegranskat)abstract
- We measured the frequency chirp rate of harmonics 13 to 23 in argon by cross-correlation with a short femtosecond probe pulse. We directly measured the negative chirp due to the atomic dipole phase and demonstrated that an additional chirp on the pump pulse was transferred to the qth harmonic as q times the fundamental chirp.
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| 6. |
- 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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| 7. |
- Delfin, C, et al.
(författare)
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Influence of the medium length on high-order harmonic generation
- 1999
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Ingår i: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 32:22, s. 5397-5409
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Tidskriftsartikel (refereegranskat)abstract
- We study high-order harmonic generation using a 110 fs Ti:sapphire laser loosely focused into a variable-length gas cell filled with neon or argon at 5 mbar pressure. The harmonic intensity is recorded as a function of the medium length, varying between 2 and 21 mm. Several cases are examined, the 17th and the 29th harmonic in argon, and the 29th and 51st harmonic in neon, at the same intensity 4 x 10(14) W cm(-2). We find that the length which maximizes the harmonic yield varies from 10 mm to more than 20 mm. We discuss the different effects affecting the photon yield of the high-order harmonics.
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| 8. |
- Erny, Christian, et al.
(författare)
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Metrology of high-order harmonics for free-electron laser seeding
- 2011
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 13
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Tidskriftsartikel (refereegranskat)abstract
- We examine the characteristics of high-order harmonics generated with 800 nm, 25 mJ, 160 fs laser pulses in an Ar gas cell with the objective of seeding a free electron laser. We measure the energy per pulse and per harmonic, the energy jitter, the divergence and the position stability of the harmonic beam. We perform ab initio numerical simulations based on integration of the time-dependent Schrodinger equation and of the wave equation within the slowly varying envelope approximation. The results reproduce the experimental measurements to better than a factor of two. The interaction of a frequency comb of harmonic fields with an electron bunch in an undulator is examined with a simple model consisting of calculating the energy modulation owing to the seed-electron interaction. The model indicates that the undulator acts as a spectral filter selecting a given harmonic.
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| 9. |
- Gaarde, Mette, et al.
(författare)
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Determination of Radiative Lifetimes of Excited-states In Neutral Gold Using Time-resolved Vacuum-ultraviolet Laser Spectroscopy
- 1994
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Ingår i: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 50:1, s. 209-213
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Tidskriftsartikel (refereegranskat)abstract
- Natural radiative lifetimes of the states in the highly perturbed 5d10np 2P sequence in neutral gold, Au I, for n = 6-9, as well as of four of the perturbing 5d(9)6s6p 2P states, have been measured. This was done by direct excitation from the ground state with short-pulse vacuum-ultraviolet laser light and time-resolved detection of the laser-induced fluorescence. We found the lifetimes to be tau(6p P-2(1/2))=6.2(2) ns, tau(6p P-2(3/2))=4.7(2) ns, tau(7p P-2(1/2))=6.4(7) ns, tau(7p P-2(3/2))=5.3(7) ns, tau(8p P-2(1/2))=63(3) ns, tau(8p P-2(3/2))=24(2) ns, tau(9p P-2(1/2))=28(2) ns, and tau(9p P-2(3/2))=75(4) ns. For the perturbing states (two states of each term have been observed) we found tau(P-2(1/2), 60983.05 cm-1)=17.5(10) ns, tau(P-2(1/2), 62540.65 cm-1)=6.7(7) ns, tau(P-2(3/2), 58826.30 cm-1)=2.1(15) ns, and tau(P-2(3/2), 63005.62 cm-1)=2.7(15) ns. Furthermore, we have investigated the hyperfine structure of the 6p P-2(3/2) State.
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| 10. |
- 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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