Phase-matched high-order harmonic generation by guided intense femtosecond pulses

We describe the phase-matched high-order harmonic generation of femtosecond Ti:sapphire laser pulses a with two different guiding methods. Generation efficiency of the high-order harmonic was improved by phase-matched propagation in the guiding channel. More than 100-fold enhancement around the 25th harmonic (32 nm) was obtained with a 3-cm-long Ar-filled hollow fiber. The harmonics around the 49th harmonic (16 nm) were also enhanced by two orders of magnitude compared to those in the plateau with a 7-mm-long, self-guided pulse in Ne. High-harmonic conversion efficiency of 10^-6 was obtained by phase-matched propagation in Ne, producing >nJ harmonics in the cutoff region around the 49th harmonic. In both experiments, the harmonics near the cutoff region were preferentially enhanced. The results are well explained by considering both the intrinsic phase based on the single-atom response and the macroscopic phase matching in the high-intensity interaction region. The flat intensity profile achieved in the guided structure is considered to clearly manifest the intrinsic phase behavior of the harmonics