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Laser-induced plasma influence onto intrapulse four-wave mixing under femtosecond filamentation in air

We experimentally investigate four-wave mixing under femtosecond filamentation in air of a pre-collimated/loosely focused beam with a single pulse. We show that its angular-frequency spectrum depends on preliminary focusing conditions and attribute this to the plasma density change. A simple model of the phase matched four-wave mixing gives a good agreement with the experimental results only if the plasma dispersion is taken into account.