Amplification of femtosecond pulses at 126 nm in optical field-induced plasma filamentation in Ar

We have observed an optical gain at the wavelength of 126 nm in an Ar excimer (Ar2 *) amplifier by utilizing a femtosecond vacuum ultraviolet (VUV) seed beam tuned at 126 nm. The maximum optical gain value of 1.1 cm-1 with a spatial distribution in the optical-field-induced ionization (OFI) Ar plasma was observed. The plasma diagnosis revealed that the plasma contraction near the plasma amplifier axis together with the plasma expansion was a key issue to observe such a high optical gain value inside the Ar plasma filament. The center axis of the contracted plasma amplifier showed the high electron density more than 1018 cm-3 even after 100 ns from the plasma production of Ar at 1 MPa. Our OFI plasma/excimer kinetics code reproduced the temporal progress of the optical gain distribution as well as the maximum gain value.