Dynamic Rabi sidebands in laser-generated microplasmas: Tunability and control

Abstract

Broadband, coherent radiation in the optical-frequency range is generated using microplasma channels in atmospheric gases in a pump-probe experiment. A microplasma medium is created in a gas by a focused intense femtosecond pump pulse. A picosecond probe pulse then interacts with this microplasma channel, producing broad, coherent sidebands that are associated with luminescence lines and are redshifted and blueshifted with respect to the laser carrier frequency. These sidebands originate from the induced Rabi oscillations between pairs of excited states that are coupled by the probe pulse. Thus the sideband radiation intensity tracks the microplasma evolution. The sidebands arise from broad and tunable Rabi shifts corresponding to varying values of the electric-field magnitude in the probe pulse. The \ensuremath{\sim}10${}^{10}$ W cm${}^{\ensuremath{-}2}$ probe beam creates a maximum sideband shift of g90 meV from the carrier frequency, resulting in an effective bandwidth of 200 meV. The sidebands can be tuned and controlled by the intensity and temporal profile of the probe pulse. The fact that the coherence is observed in a microplasma demonstrates that Rabi cycling is possible at high temperature with moderately high laser intensities as long as transitions close to the driving frequency ($\ensuremath{\Delta}$\ensuremath{\sim}2% ${\ensuremath{\omega}}_{c}$) are available. Plasma excitation combined with Rabi-shifting measurements also serves as a means to simultaneously extract quantitative ratios for the transition-dipole moments between multiple sets of highly excited states with transitions in the optical regime.