Phase matching effects in high-order harmonics generated by 1.5-µm few-cycle pulses

We investigated phase matching effects on high-order harmonics (HH) generated by a few-cycle parametric source in the near-IR. This source is characterized by 1.5 mum central wavelength, almost transform limited pulse duration of about 20 fs and pulse energy up to 1.2 mJ. The harmonic structure is resolved only on a portion of the spectrum, due to the finite instrumental resolution. We observed a significant dependence of the spectral features and cutoff extension on phase-matching condition in harmonics generated in argon. These features are well reproduced by HH calculation, taking into account macroscopic effects. For HH calculation we use the model which solves the propagation equation for the laser and for the harmonic fields making use of strong field approximation for the single dipole response. The spatial structure of the generated harmonic field, as well as their spectral/temporal characteristics were obtained and will be discussed in connection to the phase-matching generation conditions.