We have observed the optical amplification of Ar2* at 126 nm, pumped by optical-field-induced ionization (OFI) created by an infrared high-intensity laser. We evaluate the plasma density of OFI Ar plasma as a gain media by using laser interferometer. A second harmonics (532nm, 4 ns) of Nd:YAG laser were used to probe the temporal evolution of the OFI Ar plasma densities.
