DOI: 10.1007/s00340-004-1456-z Appl. Phys. B 78, 863–867 (2004) Lasers and Optics Applied Physics B h.t. kim 1, ✉ i.j. kim 1 v. tosa 2 y.s. lee 1 c.h. nam 1 High brightness harmonic generation at 13 nm using self-guided and chirped femtosecond laser pulses 1 Dept. of Physics, KAIST, Yuseong-gu, Daejeon 305-701, Korea 2 NIRDIMT, Donath 69-103, Cluj Napoca, Romania Received: 13 November 2003/ Revised version: 19 January 2004 Published online: 23 March 2004 • © Springer-Verlag 2004 ABSTRACT We have optimized the brightness of high-order har- monics from a long neon gas jet using self-guided and chirped laser pulses. The self-guided and chirped laser pulses effec- tively reduced the ionization effects in space and time, produc- ing bright high-order harmonics with narrow bandwidth. The brightness of the 61st harmonic was about 10 15 W/cm 2 /s rad with a bandwidth of 0.7 ˚ A. PACS 42.65.Ky; 42.65.Wi; 32.80.-t; 52.38.-r 1 Introduction Atoms driven by intense femtosecond laser pulses emit high-order harmonics of laser frequency induced by the periodic modulation of the electron motion. Recent progress in high-order harmonic generation (HHG) has opened the way to generate extreme ultraviolet (EUV)/soft X-ray sources with the unique properties of excellent spatial coherence [1, 2], ultrashort pulse duration [3], and wavelength tunability [4]. Generation of bright harmonics has been in- tensively pursued for applications in nonlinear optics and ultrafast spectroscopy in EUV/soft X-ray regime [5–7]. In particular, bright harmonics at the 13-nm region can be easily applied to the at-wavelength metrology of EUV-lithography optics due to its good coherence and the availability of high- reflectivity Mo : Si mirror at this wavelength. High-order harmonics can be efficiently generated by an intense femtosecond laser pulse propagating through a par- tially ionized gas medium. When a high intensity laser pulse drives a high-density gas medium, the ionization of the gas strongly influences the high-order harmonic generation pro- cess. In particular, the ionized medium distorts the spatial and temporal profiles of laser pulses, which must be ap- propriately controlled to ensure an efficient harmonic gen- eration. In this report, we describe a method to control the ionization effects on high-order harmonic generation using self-guided and chirped laser pulses. Bright harmonics with narrow bandwidth at the 13-nm region were obtained when ✉ Fax: +82-42/869-2510, E-mail: htkim@kaist.ac.kr a negatively chirped femtosecond laser pulse was propagated through a long neon gas jet placed before the laser focus. 2 Laser pulse propagation through a long neon gas jet The propagation of intense femtosecond laser pulses through an ionizing gas medium is affected by the spatio-temporal modification of the refractive index. The spa- tial effect of ionization is the well known plasma defocusing caused by the high electron concentration in the central part of the medium [8]. Since the plasma defocusing makes the laser beam quickly diverge, it shortens the effective harmonic generation length, thereby reducing the harmonic conver- sion efficiency. The other important effect is the self-phase- modulation (SPM) of the propagating laser pulse induced by the rapid change of refractive index in time during ioniza- tion. The harmonic spectrum becomes complex due to the SPM induced chirping of the laser pulse [9]. Therefore, these two effects of ionization in space and time must be properly controlled for bright harmonic generation with narrow band- width. The plasma defocusing effect significantly reduces the laser intensity and shortens the effective harmonic generation length. This problem can be overcome by creating a condition for guided propagation of the laser pulse. When a gas medium is positioned before the laser focus, the central part of laser beam is refracted outwards due to the higher concentration of free electrons, while the less-affected outer beam continues to converge. A proper selection of the target position can re- distribute the spatial profile of the laser beam, giving rise to a flattop intensity profile. The flattop laser profile also gen- erates a flattened electron distribution, leading to a flattened refractive index profile that increases steeply at the boundary of the flattop region. This field configuration is particularly fa- vorable to the harmonic generation process because it ensures a uniform phase and amplitude of the laser field. To study the self-guiding and profile flattening of the laser beam, we captured visible plasma image and laser beam pro- file at the exit of the gas medium with charge-coupled device (CCD) detectors. For the experiment, laser pulses of 27-fs du- ration and wavelength centered at 827 nm were focused, using a spherical mirror ( f = 1.2m), onto a long neon gas jet with a 9-mm slit nozzle. The peak gas density was about 40 Torr. The measured beam waist at the laser focus was 72 μ m in full