Laser and Particle Beams



On the effects of strong ionization in medium-order harmonic generation


M.  BELLINI  a1 a2 c1, C.  CORSI  a2 and M.C.  GAMBINO  a3
a1 Istituto Nazionale di Ottica Applicata, Largo E. Fermi, 6, I-50125 Florence, Italy
a2 European Laboratory for Non Linear Spectroscopy (LENS) and INFM, Largo E. Fermi, 2, I-50125 Florence, Italy
a3 Department of Physics, University of Florence, Largo E. Fermi, 2, I-50125 Florence, Italy

Abstract

Strong ionization of the gas medium has significant effects in the process of medium-order harmonic generation. The combined effect of neutral atom depletion and defocusing of the pump beam due to the intensity-dependent density of free electrons, significantly modifies the conversion characteristics and efficiency. For moderate harmonic orders, the yield is optimized for well-defined values of the pump laser intensity that do not depend on the order or on the focusing geometry, but only on the ionization potential of the gas. In particular focusing conditions, the ionization-induced defocusing can effectively guide the pump beam along channels of optimum intensity, thus enhancing the overall conversion efficiency. We demonstrate that a very simple model is able to reproduce all our experimental results in a surprisingly good way.

(Received November 6 2001)
(Accepted December 9 2001)


Key Words: Beam propagation; High-order harmonic generation; Strong-field ionization; Ultrashort laser pulses.

Correspondence:
c1 Address correspondence and reprint requests to: M. Bellini, Istituto Nazionale di Ottica Applicata, Largo E. Fermi, 6, I-50125 Florence, Italy. E-mail: bellini@ino.it