Recoil-Induced Resonances in Pump-Probe Spectroscopy
When a pump field and probe field simultaneously drive an electronic-state atomic transition, the probe field absorption spectrum can consist of an absorption peak centered near δ=−Δ and an amplification peak centered near δ′=Δ (δ′ is the frequency difference between the probe and pump fields and Δ is the pump field detuning from the atomic transition frequency) (see Fig. 1). This type of spectrum is seen in the limit that |Δ| »χ»72, where χ is the Rabi frequency associated with the pump field and γ 2 is the homogeneous width associated with the atomic transition. For atoms cooled below the recoil limit of laser cooling, the qualitative nature of the probe absorption spectrum can undergo a dramatic change. Provided that the recoil splitting ω k is larger than the homogeneous decay rate (as might occur in the case of a forbidden transition), the absorption and amplification features each split into an absorption — amplification doublet. In addition, new structure is found in the probe absorption spectrum near δ′=0; this structure consists of two absorption - amplification doublets. Both doublets can be resolved if ω k > γ 2. If ω k < γ 2, one of the doublets can be resolved provided that ω>Г1, where Г1 is some effective ground state width in the problem. A spectrum in which all the recoil-induced structure is resolved is shown in Fig. 2. The positions, widths, and relative weights of all the components are readily predicted using a dressed-atom theory in which quantization of the center-of-mass momentum is included.1 Moreover, an analytical expression for the probe field spectrum can be obtained in a model calculation in which spontaneous decay to the lower level is neglected.1
KeywordsProbe Field Rabi Frequency Atomic Transition Qualitative Nature Laser Cool
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© Springer Science+Business Media New York 1996