Abstract
The macroscopic build-up of the high-order harmonic signal depends on the free electron density in the generation medium. The free electrons affect the harmonic yield and spectral shape through modifying the refractive index and the phase matching conditions. These dependences allow studying the He(+) ion channel formation in a He gas jet. The evolution of an ion channel created by an ultrashort laser pulse via optical field ionization was monitored using the harmonic signal generated by a collinear propagating second laser pulse. From the measured high harmonic signal as function of the delay we are able to gain information about the free electron density. Under our experimental condition, the ion channel has been fully formed 300 fs after the first laser pulse, resulting in an enhancement of harmonic yield of the second laser pulse by two orders of magnitude.
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