Influence of atomic density in high-order harmonic generation

Abstract

Influence of atomic density in high-order harmonic generationAltucci, C; Starczewski, Tomas; Mevel, E; Wahlström, Claes-Göran; Carre, B; Lhuillier, A Link to publication Citation for published version (APA): Altucci, C., Starczewski, T., Mevel, E., Wahlström, C-G., Carre, B., & L'Huillier, A. (1996). Influence of atomic density in high-order harmonic generation. Optical Society of America. Journal B: Optical Physics, 13(1), 148-156. DOI: 10.1364/JOSAB.13.000148General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. We have investigated how high-order harmonics generated in rare gases depend on the atomic density. The peak and the profile of the atomic density in the interaction region were measured as a function of the backing pressure and the distance from the nozzle by a differential interferometry technique. The conversion efficiency for the harmonics in the plateau was found to increase approximately quadratically over the entire range of peak pressures investigated (3 -80 mbar). The intensity of the harmonics in the cutoff region, in contrast, increased only until an optimum peak pressure was reached, beyond which it decreased. This optimum peak pressure was found to be dependent on both the laser intensity and the process order. To understand this effect, we have performed extensive propagation calculations of both the fundamental and the harmonic fields, using ionization rates and dipole moments from a tunnel ionization model. We obtained good agreement with the experimental results. The observed effect is attributed to ionization-induced defocusing of the fundamental laser beam, which reduces the peak intensity obtained in the medium and shortens the extent of the plateau.

The influence of plasma defocusing in high harmonic generation

“…6 show optimal HHG efficiency at a few hundreds mbar for gas jets and a few tens mbar for Ne cells. These are typical values appearing in experiment [12,28]. To further verify our model, we simulate the cases studied experimentally in Ref [22].…”

“…This approach especially improves the low HHG efficiency of helium [9]. For low atomic density, the efficiency is enhanced quadratically with pressure [12], but plasma defocusing starts to become important as the free electron density increases. A previous study has shown that plasma defocusing in neon at 80mbar can significantly reduce the peak intensity of an 800nm driver pulse and diverges the driver pulse energy within a few hundred microns [12].…”

“…This effect not only decreases the cutoff photon energy but also shortens the interaction length for HHG. Although many HHG works have emphasized the importance of plasma defocusing and attributed many experimental phenomena to it [12][13][14][15][16], a quantitative discussion about the influence of plasma defocusing in HHG efficiency is missing.…”

Influence of the medium length on high-order harmonic generation

“…Lompré et al pointed out the importance of using a loose focusing geometry [11]. Altucci et al studied the influence of the pressure on the harmonic yield, showing the influence of defocusing due to free electrons at high pressure [12]. Recently, several groups have studied harmonic generation using ultra-short laser pulses focused into a hollow fibre [13][14][15], or forming a filament [16], reporting relatively high conversion efficiencies.…”

Spectral control of high-harmonic generation via drive laser pulse shaping in a wide-diameter capillary

“…This observation is consistent with what was reported in [23] by Wang et al, and can be understood as due to the pressure dependence of phase matching and reabsorption of harmonics. A second effect can be identified in the spectra, a wavelength blue-shift (tuning) of all harmonic orders with increasing pressure [24]. In the figure we have used the center frequency (determined from a Gaussian least-square fit to each harmonic) of the harmonics at lowest pressure (2 mbar) as reference, and these are indicated as vertical dashed lines.…”