Observation and origin of multi-THz quantum beats in superfluorescent emission from argon

We report observation of multi-THz quantum beats in the amplitude of superfluorescence in pure argon gas, using a time delayed 780-nm probe beam that interrogates the relevant population densities. The superfluorescence is induced by three-photon absorption of an ultrashort 260.5-nm laser pulse that creates coherent and coupled populations in multiple closely spaced $3d$ and $5s$ Ar levels. Argon pressure and pump laser focusing parameters are seen to modify the frequency of the quantum beats, which is attributed to the absorption of the self-generated, nonresonant third harmonic of the pump from Ar near the zero of the real part of the overlapping linear susceptibilities of the closely spaced $3d$ and $5s$ excited states. The self-absorption of the phase-matched THG photons creates a coherent population of atoms which acts like an atomic excited state ($3\ensuremath{\omega}$-induced state). We have also observed a new two-step superfluorescent cascade that results from the population redistribution enabled by the 780-nm probe pulse between the $3{d}^{\ensuremath{'}}{[5/2]}_{3}$ and the $3\ensuremath{\omega}$-induced state and gives rise to a previously unknown transition that emits on 1587 nm.