Phase-matching high-order harmonics in a semi-infinite gas cell geometry

Here we report a detailed study of the generation conditions considering the phase-matching in a low ionization potential atom (Xenon) and a high one (Helium). We investigate standard high-harmonic phase-matching parameters like the focusing position, pressure dependence and the intensity dependence in order to optimize the harmonic yield in this gas cell geometry. Harmonics are generated by a fundamental driving field with 30 fs pulse duration, 3 kHz repetition rate and up to 2 mJ of pulse energy. An intensity modulation of the harmonics as a function of the focus position is observed which indicates different phase-matching regions. The generation of intense attosecond pulses with a regular temporal structure depends critically on different quantum path contributions and their correlation by phase-matching the harmonic emission process.