Influence of the medium length on high-order harmonic generation

We study high-order harmonic generation using a 110 fs Ti:sapphire laser lo osely focused into a variable-length gas cell filled with neon or argon at 5 mbar pressure. The harmonic intensity is recorded as a function of the me dium length, varying between 2 and 21 mm. Several cases are examined, the 1 7th and the 29th harmonic in argon, and the 29th and 51st harmonic in neon, at the same intensity 4 x 10(14) W cm(-2). We find that the length which m aximizes the harmonic yield varies from 10 mm to more than 20 mm. We discus s the different effects affecting the photon yield of the high-order harmon ics.