Abstract
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.
Highlights
We recently published experimental measurements of laser beam evolution in our highharmonic generation setup [1, 2]
We find that good phase matching occurs for the front of the laser pulse but not the back where ionization and nonlinear effects take place
Simulations demonstrate that the interplay between the focusing geometry of the apertured beam combined with refraction from plasma generation is responsible for the double focus observed in our experiments
Summary
We recently published experimental measurements of laser beam evolution in our highharmonic generation setup [1, 2]. The measurements showed the beam fluence narrowing to two separate foci, separated by a distance of about 7 cm. This behavior occurs with peak pulse power an order of magnitude smaller than expected for Kerr self-focusing in our helium-filled cell. Tosa and Nam [3] pointed out that Kerr self-focusing is not necessary to explain the double focusing in our experimental arrangement, based on simulations of laser propagation. We find that good phase matching occurs for the front of the laser pulse but not the back where ionization and nonlinear effects take place. Because the phase matching varies within the pulse as it travels, it is helpful to view it dynamically in the form of a movie
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