INFLUENCE OF ATOMIC DENSITY IN HIGH-ORDER HARMONIC-GENERATION

We have investigated how high-order harmonics generated in rare gases depend on the atomic density. The peak and the profile of the atomic d ensity in the interaction region were measured as a function of the ba cking pressure and the distance from the nozzle by a differential inte rferometry technique, The conversion efficiency for the harmonics in t he plateau was found to increase approximately quadratically over the entire range of peak pressures investigated (3-80 mbar). The intensity of the harmonics in the cutoff region, in contrast, increased only un til an optimum peak pressure was reached, beyond which it decreased. T his optimum peak pressure was found to be dependent on both the laser intensity and the process order. To understand this effect, we have pe rformed extensive propagation calculations of both the fundamental and the harmonic fields, using ionization rates and dipole moments from a tunnel ionization model. We obtained good agreement with the experime ntal results. The observed effect is attributed to ionization-induced defocusing of the fundamental laser beam, which reduces the peak inten sity obtained in the medium and shortens the extent of the plateau. (C ) 1996 Optical Society of America