Population inversion by near-resonant sequential optical pulses

Nikolay V. Vitanov

M. M. T. Loy

Liang Xu, Chaohui Zhou, Guangyu Fan +5 more

We investigate the pump wavelength dependence of population inversion in a nitrogen ion when nitrogen gas is irradiated by a femtosecond laser pulse in the wavelength range from ultraviolet to near-infrared. It is found that the population inversions corresponding to the transitions at 391 and 428 nm are very sensitive to the laser wavelength, with extreme values near 400 and 1000 nm. For the ultraviolet pump, the population inversions for 391 and 428 nm transitions occur at different wavelengths: a maximum inversion for the first corresponds to a minimum for the second. This is attributed to the near-resonance single-photon transition combined with the Raman process. A significantly stronger inversion is observed for the near-infrared laser, with the maximum depending on the laser intensity, ascribed to the ac Stark effect. The Floquet theory confirms this conclusion and reveals that the three-photon transition assisted by the strong laser field leads to this enhanced population inversion. This study provides the optimal conditions for air lasing based on ionic transitions of nitrogen molecules.

A. Turrin

Georgy A. Kazakov, Thorsten Schumm

Recently, several theoretical proposals addressed the generation of an active optical frequency standard based on atomic ensembles trapped in an optical lattice potential inside an optical resonator. Using atoms with a narrow linewidth transition and population inversion together with a ``bad'' cavity allows us to realize the super-radiant photon emission regime. These schemes reduce the influence of mechanical or thermal vibrations of the cavity mirrors on the emitted optical frequency, overcoming current limitation in passive optical standards. The coherence time of the emitted light is ultimately limited by the lifetime of the atoms in the optical lattice potential. Therefore these schemes would produce one light pulse per atomic ensemble without a phase relation between pulses. Here we study how phase coherence between pulses can be maintained by using several inverted atomic ensembles, introduced into the cavity sequentially by means of a transport mechanism. We simulate the light emission process using the Heisenberg-Langevin approach and study the frequency noise of the intracavity field.

D. Grischkowsky, M. M. T. Loy

We show that, by virtue of the linewidth reduction provided by counter-propagating beams, it is possible to use the optical Stark shift to sweep a two-photon resonance through the sum frequency of the applied light while simultaneously satisfying the conditions required for optical adiabatic rapid passage. Consequently, the application of a near-resonant pulse of light can result in complete population inversion of a two-photon transition.