Two-level system in a strong amplitude-modulated field

A. E. Kaplan

ZIGMANTAS JANKAUSKAS, VYGAUDAS KVEDARAS, V. Zaveckas

Interaction between a two-level system and strong nonmonochromatic electromagnetic field has been the focus of attention of the laser technique and fiber optics for a long time. The influence of the modulated optical signal on the active medium, when the amplitude and phase (or frequency) vary arbitrary in time, is important in many problems, such as interaction of ultrashort pulses with the resonant amplifier, change of the line shape, the emission perturbation theory, self-focusing of light, etc. In this article, a new computational procedure is proposed for a two-level system in the strong harmonically modulated optical field. Expressions for polarization and population difference of a quantum system have been obtained in the form of the Fourier series. The characteristic frequency has been determined numerically. It depends not only upon the intensity of the carrier wave and the detuning magnitude but also upon the depth and frequency of modulation. The computational technique is applicable both for amplitude and phase modulation. Ill. 6, bibl. 3 (in English; summaries in English, Russian and Lithuanian).

I. V. Lebedev

I. V. Lebedev

Michael Z. Smirnov

The parametric resonances and the quasi-level structure of an atom strongly driven by a near-resonant amplitude-modulated optical field are studied theoretically. An approximate analytical solution of the density-matrix equations of motion is obtained for two-level and three-level atoms. Based on this solution the quasi-level shifts are analyzed, and the conditions for the quasi-levels to cross each other are derived. Expressions are given for the probe-wave absorption/gain spectrum near the parametric resonance and far from it. It is pointed out that the parametric resonances appear when the pumping field sidebands are tuned to resonance with some transitions among the quasi-levels.

P. Thomann

The fluorescent intensity of two-level atoms irradiated by a near-resonant amplitude-modulated light wave is calculated in the Bloch model. The analogy with magnetic resonance in a strong oscillating field making an arbitrary angle with the static field is pointed out. Experimental evidence of the predicted parametric and multiple quantum resonances has been obtained with an atomic beam of sodium. Optical pumping was used to prepare the sodium atoms as two-level systems.