Abstract
Numerous applications of high harmonic generation (HHG), such as attosecond pulse synthesis, depend on the ability to increase the electron recollision energy, which is a quadratic function of the driver wavelength. High-energy infrared pulses obtained from an optical parametric amplifier (OPA) are thus attractive for driving the HHG process, thereby offering the opportunity to yield shorter attosecond pulses. However, the increase in driver wavelength is often outweighed by the poor spatial quality of the OPA source. In this paper, we demonstrate that HHG using OPA signal pulses is significantly improved by spatial filtering in a hollow-core fibre prior to focusing in the gas target in comparison with the unfiltered case. Ion yield measurements in combination with beam profile monitoring in the far field enabled control over the interaction volume. For similar interaction volumes, we observe that with less than half the energy per pulse, the HHG yield can increase by one order of magnitude with spatial filtering. The comparison between the harmonic yields in argon and krypton, and their respective dependence on the peak laser intensity, provide experimental evidence that strongly suggests that the enhancement is due to improved phase matching.
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