Characteristic fringe patterns of spectral interference in dynamic Rabi sidebands are used as a means to investigate decoherence phenomena in a highly nonequilibrium underdense microplasma formed in atmospheric-pressure oxygen gas in the wake of a strong-field (∼10${}^{13}$ W/cm${}^2$) femtosecond pump pulse. The same transient processes in the nonequilibrium plasma work both to create the effective two-level systems producing coherent sidebands and to destroy the coherence and smear the sideband spectral structure; the observed spectral fringe pattern is a result of the trade-off of these two effects. The sidebands generated on a moderately intense (∼10${}^{10}$ W/cm${}^2$) picosecond laser pulse reveal the decoherence rate as a function of the pump-pulse intensity and the pump-probe delay time. The rate increases with the pump laser intensity and decreases with the delay with corresponding decoherence times in the range of 750 fs to 3 ps, in good agreement with theory revealing that electron scattering dominates the dynamics for the subnanosecond relaxation processes.

• Received 31 December 2011
• Corrected 25 February 2013

DOI:https://doi.org/10.1103/PhysRevA.87.023419