Nonperturbative effects in attosecond four-wave-mixing spectra

We study the nonlinear optical response of argon to a four-wave-mixing pulse sequence consisting of an extreme-ultraviolet pulse, a collinear IR dressing pulse, and a delayed noncollinear IR probe pulse. Sequential absorption of an extreme ultraviolet photon and an IR photon from the collinear beams excites the 3s⁻¹4pbright state and the 3s⁻¹3d/5s dark states. The subsequent absorption of an IR photon from the noncollinear beam then leads to an angled extreme ultraviolet emission, whose variation with delay encodes the coupling between autoionizing states, the lifetimes of dark states, and nonperturbative effects. Both our measurements and abinitio simulations of the angled four-wave-mixing signal reveal a double-peak structure in the delay dependence, in good agreement with each other. We attribute the central minimum between the two peaks to rapid Rabi cycling, driven by the collinear IR dressing pulse, between the dark states and the 3s⁻¹4p resonance, which leads to destructive interference in the final transition amplitude.