Lasing of atomic argon induced by filamentation of ultraviolet femtosecond laser pulses at 250 nm has been experimentally studied. Emission on the \(4p \to 4s\) transitions (763.5, 801.5, and 842.5 nm) is observed to propagate both in the forward direction, along the laser pulse, and in the backward direction. An energy threshold of ~90 μJ is identified, above which the emission intensity increases sharply. The forward lasing energy exceeds the backward one by nearly an order of magnitude. The spatial profile of the emission consists of concentric rings with an angular divergence below 10 mrad. Based on estimates of the gain length, the gain coefficient of the medium is approximately 0.1 cm–1.
