| This paper mainly deals with the steady-state population of a moving ladder-type three-level atom (in the case of nearly equispaced levels ) driven by a superposition of a monochromatic laser wave with a broad-band squeezed vacuum. From the Hamiltonian of the atomic system, making use of Born-Markoff approximation, we derive the master equation for the atomic system and the atomic Bloch equation and obtain the seady-state solutions of the optical Bloch equation using numerical calculation. Then we discuss, respectively for the case of an ordinary vacuum and the case of a broadband squeezed vacuum, the dependence of the steady-state population Sii on parameters such as detuning A, Rabi frequencies e1,e2, phase f , squeezing photon number N and spontaneous decay rates, γ1,γ2 etc. in different cases by using graphical method. We also analyze and compare qualitatively one-photon and two-photon processes of the atomic system and discuss quantitatively the Doppler-shift caused by the motion of the atom.By analyzing and comparing the one-photon and two-photon processes occurring for various cases, it is found that for a squeezed vacuum, only when the carrier frequency of the squeezed vacuum equals the frequency of the driving field and the phase matching condition is fulfilled, will there be a strong two-photon resonant absorption from the squeezed vacuum (quite different from absorption of photons from a classical field). Otherwise, the two-photon absorption rate will be greatly diminished.Considering the influence of the motion of the atom on the steady-state populations of the atomic levels, we get the conclusions as follows: a situation of two-photon Doppler shifted resonance occurs near zero two-photon detuning, a situation of one-photon Doppler shifted resonance occurs near zero one-photon detuning, and two-photon Doppler shifted resonance does not occur near zero two-photon detuning when the moving atom is driven by two counter propagating laser beams with the same frequency... |
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