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We numerically simulate the propagation of high-intensity laser pulses in helium to investigate the role of nonlinear effects in gas-cell high-harmonics experiments. An aperture located before the focusing lens is also included in the simulation. Numerical results for the radial fluence profile as a function of axial position, as well as for the spectral shift and ionization levels, agree with experimental observations. The simulations confirm that a significant Kerr effect is not required to generate the observed double focus in the fluence. The beam simulation also permits an investigation of high-harmonic phase matching. Most of the harmonic energy is seen to come from the forward portion of the laser pulse, whereas the latter portion gives rise to the incidental double laser focusing. Good phase matching for the harmonics arises in large measure from a balance between the linear phase delay of the neutral atoms and the Gouy shift, which is elongated and nearly linearized when the aperture is partially closed on the beam. (c) 2008 Optical Society of America.

We concede the point raised by Tosa and Nam regarding the interpretation of our recent measurements of intense laser pulse dynamics. Although the measured fluence shows a double focus that is suggestive of filamentation, pulse-propagation simulations show no resurgence in intensity at the second beam waist. An interplay between the generation of plasma and natural diffraction of the apertured beam gives rise to this unusual behavior without requiring a sizable n(2).

We show how to determine the transmittance of short focal length lenses (f approximate to 19 mm and f approximate to 25 mm, in this case) with a combined uncertainty of 3 parts in 104 or better by measuring transmittances of lens pairs of a set of three or more lenses with the same nominal focal length. Uncertainties are minimized by optimizing the radiometric design of the setup and the measurement procedure. The technique is particularly useful in systems where the detector acceptance angle limits the beam geometry to relatively collimated beams.