Interplay between absorption, dispersion and refraction in high-order harmonic generation

We report a detailed experimental and theoretical study on high-order harmonic generation of a femtosecond Ti-sapphire laser focused at an intensity of around 1015 W cm−2 onto a high-pressure (50–210 mbar) neon gas cell of variable length (1–3 mm). Using thorough three-dimensional simulations, we discuss the interplay between the different factors influencing the harmonic-generation efficiency, i.e. phase matching determined by the electronic and atomic dispersions, re-absorption of the harmonics by the medium and refraction of the generating laser beam. Generically, we find that, in our generation conditions, the emission yield of harmonics from the plateau region of the spectrum is absorption limited, whereas the emission from harmonics in the cut-off is strongly reduced due to both electron dispersion and ionization-induced refraction of the laser beam. A good agreement between the numerical results and the experimental data is obtained for the harmonic yield dependence on the various generation parameters (gas pressure, medium length and laser intensity).