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Acta Optica Sinica

Supernarrow resonant effect of coherent excitation atoms using mode-locked laser

He Li

Published Jan 1, 1988• 0 citations
OpenAlex

Abstract

A theoretical scheme of coherent excitation atoms are proposed to explain the essence of supernarrow resonant effects, which are produced by using the mode-locked laser pulse train exciting the atoms. The expressions of the atomic density operator ρA(t) and of the signals of spectra are derived and the relevant ourves are given. The new results for the supernarrow resonant effects are also predicted. The theoretical scheme can be extended to the application for multiphoton processes.

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A theoretical scheme of coherent excitation atoms are proposed to explain the essence of supernarrow resonant effects, which are produced by using the mode-locked laser pulse train exciting the atoms. The expressions of the atomic density operator ρA(t) and of the signals of spectra are derived and the relevant ourves are given. The new results for the supernarrow resonant effects are also predicted. The theoretical scheme can be extended to the application for multiphoton processes.
A quantum theory for pulse-train from the mode-looked laser has been developed. In the theory the author uses superposition model, i. e. the atomic states are coherent after absorbed photons from different modes of a mode-locked laser. So the final atomic state should be coherently added. The resolvent operator and projector operation are used to derive the operator U. Considering the phase relationship between the modes, the light field are described by multi-mode coherent state. This method can be used to describe the resonant multi-photon processes that the pulse train from the mode locked laser interacting with atoms.
A general, analytic theory is presented of the excitation of a two-level atom by a series of short optical laser pulses. The atom is assumed to be isolated (collisionless) with radiative decay from upper to lower state at rate γ. The laser pulses are assumed to have a duration tp such that γtp ≪ 1 (the transient regime), and the spacing between pulses tr is such that tr ≫ tp. In this regime the atomic density matrix is calculated as a function of time during each optical pulse. An analytic expression for the atomic state (Bloch vector) after n optical pulses is also derived. For large n the atomic state converges to a repeating (steady state) cycle of pump excitation followed by radiative decay. The excited-state population in the steady state can be large but only if the spacing between pulses tr is small so that γtr ≲ 0.5. These results indicate that lasers that produce trains of pulses, such as mode-locked conventional lasers and free-electron lasers, can be effective in optical pumping of atoms.
We have studied the two-photon resonant excitation and enhancement effect of a high excitation state 15d in Rb atom by using a synchronously pumped tunable mode-locked dye laser and obtained the frequency up-conversion at 1.065μm→4210 A. The relationship between self-phase matching and output of UV coherent radiation output is given. This paper emphasizes the study of the transient coherent effect and dephasing process of this state. It has been shown experimentally that the maximun polarized time by two-photon resonant excitation is 120 ps and UV coherent radiation could be induced under the action of probing signal until 350ps. However up to now no report has been declared about the study of trasient coherent dephasing in higher excitation states oi atom systems. All experimental set-ups, results, principle explanation are given.
We have studied Doppler-free two-photon excitation of atoms with a train of phase-coherent standing-wave light pulses, originating from the same laser pulse. Quantum interference effects produce narrow spectral fringes which have a physical origin similar to that of Ramsey fringes. Linewidths much less than the Fourier-transform limit of an individual light pulse have been observed for the sodium $3S\ensuremath{-}5S$ transition; and a dramatic enhancement of the resonant signal is possible.