Three-photon femtosecond pumped backwards lasing in argon.

We demonstrate backwards lasing in atomic argon directly excited via a three-photon pumping in air mixtures with argon mole fractions down to 10%. We achieve well collimated, narrowband coherent emission at 1327nm by using both broadband femtosecond excitation and narrow linewidth picosecond excitation in the vicinity of 261nm. This approach shows promise for standoff trace detection in the atmosphere.