Tunable Broadband Optical Generation via Giant Rabi Shifting in Micro-Plasmas

Abstract

Broadband, coherent radiation in the optical frequency range is generated using micro-plasma channels in atmospheric gases in a pump-probe experiment. A micro-plasma medium is created in a gas by a focused intense femtosecond pump pulse. A picosecond probe pulse then interacts with this micro-plasma channel, producing broad, coherent sidebands that are associated with luminescence lines and are red-and blue-shifted 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. These excited states become populated in the process of plasma cooling. Thus, the sideband radiation intensity tracks the micro-plasma evolution. The sidebands incorporate Rabi shifts corresponding to varying value of the electric field magnitude in the probe pulse: this makes them broad and malleable to tuning. The intensity of the probe beam ~ 10 10 W cm -2 , creates a maximum sideband shift of > 90 meV from the carrier frequency, resulting in an effective bandwidth of 200 meV. The sidebands may be effectively controlled by the intensity and temporal profile of the probe pulse. The giant Rabi shift is both tunable and coherent over a wide range of frequencies and over a wide range of atomic transitions. The fact that the coherence is observed in a micro plasma demonstrates that Rabi cycling is possible at high temperature with moderately high laser intensities (10 10 W cm -2 ) as long as transitions close to the driving frequency (' ~ 2% Z c ) are available.