41.8-nm Xe 8+ laser driven in a plasma waveguide A. Butler, A. J. Gonsalves, C. M. McKenna, D. J. Spence, and S. M. Hooker * Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom S. Sebban, ² T. Mocek, and I. Betttaibi Laboratoire d’Optique Appliquée, ENSTA/Ecole Polytechnique, CNRS UMR 7639, F-91761 Palaiseau cedex, France B. Cros ‡ LPGP, UMR 8578, CNRS, Université Paris XI, Bâtiment 210, 91405 Orsay, France (Received 19 April 2004; published 30 August 2004) An experimental demonstration of an optical field ionization short-wavelength laser driven in a gas-filled capillary-discharge waveguide is described in detail. Guiding of high-intensity laser pulses has previously been demonstrated with this type of waveguide for capillary discharges in hydrogen. For the present experiments xenon gas was mixed with the hydrogen, and strong lasing on the 4d 9 5d-4d 9 5p transition in Xe 8+ at 41.8 nm was observed. Under optimum conditions the short-wavelength laser output achieved with the waveguide was found to be greater than that from a Xe gas cell. Measurements of the transmission of the pump laser pulses through the waveguide show that the short-wavelength laser signal was greatest under conditions for which the pump laser pulses were well guided. Simulations of the propagation of the pump laser radiation are presented for a range of initial plasma conditions, and these indicate that the laser-plasma interaction length achieved was greatly increased compared to that which can be achieved in a gas cell. DOI: 10.1103/PhysRevA.70.023821 PACS number(s): 42.55.Vc, 52.38.Hb I. INTRODUCTION In order to create a population inversion on an extreme- ultraviolet (xuv) or soft-x-ray laser transition, a large pump power density must be supplied to overcome the high rate of spontaneous decay of the upper laser level. Ever since the first demonstrations [1,2] of lasing at short wavelengths, con- siderable effort has been devoted to reducing the size and increasing the repetition rate of the visible pump lasers em- ployed. Recently progress towards these goals has been made by using picosecond or femtosecond pump laser pulses of only moderate energy [3–6]. Short-pulse lasers of this type are compact and able to operate at high repetition rates, making them attractive pump sources for short-wavelength lasers. An alternative technique, in which fast electrical dis- charges are used to provide the pump power, has been suc- cessfully employed [7] to pump xuv lasers operating at 46 nm or longer, but it has proved difficult to scale to shorter wavelengths. Traveling-wave excitation is often necessary when driving short-wavelength lasers owing to the brief duration of the gain. For this geometry the pump laser energy required is minimized by employing longitudinal pumping. However, with this configuration diffraction and refraction of the pump beam limit the gain length to only a few millimeters, and as a consequence the small-signal gain coefficient must be very high if lasing is to be achieved. Furthemore, the energy ex- traction and transverse coherence of the short-wavelength beam are likely to be poor. A direct way to increase the interaction length would be to employ a larger focal spot, but this would greatly increase the pump laser pulse energy re- quired. A more favorable approach is to guide the pump laser pulse over long lengths. Several techniques for guiding high-intensity laser radia- tion have been investigated, including guiding in hollow cap- illaries [8,9], relativistic channeling [10], and several types of plasma waveguide [11–15]. However, few of these have been used to increase the gain length of short-wavelength lasers. Plasma channels formed by discharge or laser ablation of a LiF capillary have been used to demonstrate [16] recom- bination lasing on the n =2 → 1 transition at 13.5 nm in Li 2+ . In that work lasing was achieved with 248 nm pump radia- tion for 5-mm-long laser-ablated capillaries and with 1053 nm pumping for 14-mm-long discharge-ablated capil- laries. Janulewicz et al. have investigated [17] collisionally excited short-wavelength lasers driven within a plasma chan- nel formed by a Z-pinch discharge. Those authors employed 1 ps pump pulses from a Nd:glass laser to achieve transient gain on the 3p-3s transition at 60.8 nm in S 6+ . However, in that work the spot radius of the pump laser was very large, approximately 170 m, so that the capillary length 30 mm was significantly less than the Rayleigh range of the pump beam. While these early results show the promise of driving short-wavelength lasers within a plasma channel, evidence for generation of gain over the full length of the waveguide has not yet been presented. In this paper we present in detail the results of the first experimental demonstration of a short-wavelength laser driven within a gas-filled capillary-discharge waveguide. This is also the first time that a collisionally excited optical field ionization laser has been driven in any waveguide. The *Electronic address: simon.hooker@physics.ox.ac.uk ² Electronic address: sebban@ensta.fr ‡ Electronic address: brigitte.cros@pgp.u-psud.fr PHYSICAL REVIEW A 70, 023821 (2004) 1050-2947/2004/70(2)/023821(10)/$22.50 ©2004 The American Physical Society 70 023821-1