Appl. Phys. B 64, 323 — 330 (1997) Harmonic generation in an ionized gas medium with a 100-femtosecond, high repetition rate laser source at intermediate intensities C. de Lisio, C. Altucci, R. Bruzzese, F. De Filippo, S. Solimeno, M. Bellini, P. Foggi Dipartimento di Scienze Fisiche, Universita´ di Napoli ‘‘Federico II’’ and Istituto Nazionale di Fisica della Materia (INFM), Padiglione 20, Mostra d’ Oltremare, 80125 — Napoli, Italy (Fax: #39-81/239-508, E-mail: delisio@na.infn.it) European Laboratory for Nonlinear Spectroscopy (LENS), Largo E. Fermi, 2, 50125-Firenze, Italy Received: 29 March 1996/Revised version: 25 July 1996 Abstract. We report the realization of a vacuum-ultra- violet radiation source based on high-order harmonic generation in noble-gas samples, operating at high repeti- tion rate. In particular, we observed up to the 13th har- monic ("61 nm) of the fundamental frequency of a short pulse, high repetition rate titanium—sapphire laser after its interaction with a Xe gas jet. The effects of the propagation of the fundamental and harmonic beams through an ionized medium are studied by analysing the spectral profile of the 9th and 7th harmonics. Finally, we report a study of the dependence of the harmonic conver- sion efficiency on relative position of the focus and the gas target. PACS: 42.65.Ky; 32.80.Rm In recent years, high-order harmonic generation in noble gases has become one of the most promising ways for realizing coherent radiation sources in the VUV—XUV region [1]. It is well known that by irradiating a noble gas sample of finite extension with a very intense laser pulse, odd harmonics of the fundamental laser frequency are generated with considerable efficiency [2, 3]. For instance, harmonics as high as the 143rd [4] and wavelengths as short as 7 nm [5] have been observed. Some interesting characteristics of the harmonic radiation are temporal and spatial coherence, low divergence, short time dura- tion, which result in an extremely high brightness [typi- cally 10—10 photons/(A s s mrad) in the 20—200 nm VUV spectral region]. This is several orders of magnitude higher than the typical brightness of a synchrotron radi- ation source in the same spectral region. So far, the shortest wavelengths observed have been obtained with laser sources delivering pulses of high energy (E  K0.5 J) and short duration (   K100 fs). These perfor- mances can be obtained only with table-top-terawatt laser sources operating at very low repetition rate (typically 10 Hz) compared with that of a synchrotron ( K100 MHz). In spite of the extremely high peak power available with table-top-terawatt lasers, most of the results published so far in the field of harmonic generation, have been obtained at intensities between 10 and 10 W/cm, thus employing a weak focusing geometry. There are at least two reasons for this. First, it is well known that the laser intensity dependence of the photon yield of a given harmonic lying in the plateau region follows a power law with a quite high exponent (typically between 5 and 7) until saturation due to ionization occurs [6]. Thus, increasing the laser intensity well above the saturation intensity does not result in a strong enhancement in the harmonic photon yield and only a minor increase in the plateau extent can be observed. This means that only a small fraction of the laser pulse energy is efficiently used for harmonic generation. As an example, the 15th harmonic of a 250 fs Tita- nium—Sapphire (Ti:S) laser generated in xenon saturates at I"I  +1.510 W cm, which corresponds (in the experimental conditions of [6]) to a laser pulse energy of a few mJ, i.e. a very small fraction of the maximum pulse energy the laser can deliver. According to [6], the 15th harmonic photon yield remains almost constant by in- creasing the laser intensity by a factor of 5 above the saturation intensity. Secondly, due to the coherence of the harmonic genera- tion process, the harmonic photon yield depends quadrati- cally on the number of atoms involved in the generation process, and, hence, in a weak focussing geometry, on the confocal parameter b"2w  /  [7, 8], where w  is the laser beam radius in the focal region and  the laser wavelength. Thus, an alternative and interesting way to consider- ably increase the average flux of harmonic photons is to use moderate pulse energies (corresponding approxim- ately to the saturation intensity), but at much higher repetition rates. After suitable focusing, intensities in ex- cess of 10 W/cm are now available also with commer- cial lasers operating at high repetition rate (1 kHz) and with low energy pulses (1 mJ). The above intensity is sufficient for efficient generation of harmonics lying in the