Comparison and real-time monitoring of high-order harmonic generation in different sources

J.-P. Brichta, M. C. H. Wong, J. B. Bertrand, H.-C. Bandulet, D. M. Rayner, and V. R. Bhardwaj

Phys. Rev. A 79, 033404 – Published 11 March 2009

Abstract

We compare the generation of coherent extreme ultraviolet (xuv) photons in argon from three different high-order harmonic sources: a semi-infinite gas cell, a finite gas cell, and a pulsed valve. We demonstrate photoionization of the background gas by the coherent xuv photons as an alternative to xuv spectroscopy for real-time and in-line monitoring of the high-order harmonics. Using this technique for measuring photocurrent, we show that the gas cells produce 50–100-fold brighter harmonics than the pulsed valve, with an estimated conversion efficiency of $10^{- 4}$ . The spectral distribution of the harmonics produced in the gas cells peaks at the 27th harmonic compared to the 19th harmonic in the pulsed valve. We attribute this difference to the interplay between phase matching and absorption.

Received 8 October 2008

DOI:https://doi.org/10.1103/PhysRevA.79.033404

Authors & Affiliations

J.-P. Brichta¹, M. C. H. Wong¹, J. B. Bertrand^1,2, H.-C. Bandulet³, D. M. Rayner², and V. R. Bhardwaj^1,*

¹Department of Physics, University of Ottawa, 150 Louis-Pasteur, Ottawa, Ontario, Canada K1N 6N5
²National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario, Canada K1A 0R6
³Institut National de la Recherche Scientifique–Centre Énergie, Matériaux et Télécommunications, 1650 boulevard Lionel-Boulet, Varennes, Québec, Canada J3X 1S2

^*ravi.bhardwaj@uottawa.ca

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Vol. 79, Iss. 3 — March 2009

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Images

Figure 1
(Color online) (a) Experimental setup for generation of xuv photons. SIGC, semi-infinite gas cell; FGC, finite gas cell; DG, dispersive grating. The grating sits on a motorized stage and must be removed from the beam to measure photocurrent with the parallel plate detector; MCP, microchannel plates with phosphor screen; PPD, parallel-plate detector. (b) Semi-infinite gas cell. GI, gas input; PH1, pinhole $(100 - 250 μ m)$ ; PH2, 1 cm pinhole. (c) Finite gas cell. GI, gas input; PH, $4 \times 600 μ m$ pinholes.Reuse & Permissions
Figure 2
(Color online) Total photocurrent as a function of laser intensity for different gas cell geometries. The photocurrent was recorded under optimum pressure conditions which were 43 Torr for the semi-infinite gas cell (black squares), 7 Torr for the finite gas cell (red circles), and 3.5 Torr for the pulsed valve (blue triangles). The inset graph shows the intensity calibration performed with the pulsed valve and a fast ionization gauge (see text). It indicates a saturation energy of $560 μ J$ , which for argon corresponds to the saturation intensity of $2.37 \times 10^{14} W {cm}^{- 2}$ .Reuse & Permissions
Figure 3
(Color online) Pressure dependence of total photocurrent (top panel) and individual harmonics (bottom panel) measured using the x-ray spectrometer for [(a) and (d)] the semi-infinite gas cell, [(b) and (e)] the finite gas cell, and [(c) and (f)] the pulsed valve. The laser intensity was $3 \times 10^{14} W {cm}^{- 2}$ for the gas cells and $6 \times 10^{14} W {cm}^{- 2}$ for the pulsed valve. Error bars are a result of shot-to-shot variation in the recorded photocurrent. In the bottom panel, the harmonics shown are the 15th (black squares), 19th (red circles), and 23rd (blue triangles).Reuse & Permissions
Figure 4
(Color online) Diameter of harmonic beam at parallel plate detector measured for the semi-infinite gas cell (black squares), finite gas cell (red circles), and pulsed valve (blue triangles). The fit line plots the relationship $D_{0} / \sqrt{q}$ , where $D_{0}$ is 12 mm in this case.Reuse & Permissions
Figure 5
(Color online) Observed high harmonic spectrum for different gas sources. The laser intensity is $3 \times 10^{14} W {cm}^{- 2}$ for the semi-infinite gas cell (black squares) and the finite gas cell (red circles). The pulsed valve data (blue triangles) were recorded with a laser intensity of $6 \times 10^{14} W {cm}^{- 2}$ .Reuse & Permissions
Figure 6
(Color online) Theoretical harmonic signal for the SIGC (black squares), FGC (red circles), and PV (blue triangles) calculated using Eq. (5). The ionization fraction is $η = 0.08$ for the SIGC and FGC, and $η = 0.42$ for the PV, corresponding to laser intensities of $I = 4 \times 10^{14} W {cm}^{- 2}$ and $I = 7 \times 10^{14} W {cm}^{- 2}$ , respectively.Reuse & Permissions

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