We demonstrate a transient absorption scheme that uses a fixed-spectrum attosecond pulse train in conjunction with a tunable probe laser to access a wide range of nonlinear light-atom interactions. We exhibit control over the time-dependent Autler–Townes splitting of the 1s4p absorption line in helium, and study its evolution from a resonant doublet to a light-induced sideband with changing probe wavelength. The non-commensurate probe also allows for the background-free study of two-infrared-photon emission processes in a collinear geometry. Using this capability, we observe two different emission pathways with non-trivial delay dependencies, one prompt and the other delayed. We identify the nonlinear processes underlying these emissions by comparing the experimental results to calculations based on the time-dependent Schrödinger equation.
Publication Source (Journal or Book title)
Harkema, N., Bækhøj, J., Liao, C., Gaarde, M., Schafer, K., & Sandhu, A. (2018). Controlling attosecond transient absorption with tunable, non-commensurate light fields. Optics Letters, 43 (14), 3357-3360. https://doi.org/10.1364/OL.43.003357