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Physical Review Letters

Coherent Two-Photon Excitation by Multiple Light Pulses

Richard E. Teets, J. N. Eckstein, Theodor W. Hänsch

Published Apr 4, 1977• 177 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.38.760

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Pulse-bandwidth dependence of coherent phase control of resonance-mediated (2+1) three-photon absorption

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arXiv:0709.0601•Physical Review A•Nov 27, 2007•21 citations
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−5S3S\ensuremath{-}5S3S\ensuremath−5S transition; and a dramatic enhancement of the resonant signal is possible.
Using two coherent time-delayed short pulses, we have demonstrated that one can obtain, in the profile of Doppler-free two-photon resonances, interference fringes with a splitting 12T\frac{1}{2T}2T1​ (TTT, delay time between the two pulses) much smaller than the spectral width $\frac{1}{\ensuremath{\tau}}$ of each pulse ($\ensuremath{\tau}$ duration of each pulse). This method should lead to a number of important improvements in the presently available techniques of ultrahigh-resolution laser spectroscopy of atoms and molecules.
We have demonstrated the feasibility of Doppler-free two-photon spectroscopy with a train of picosecond standing-wave light pulses from a synchronously pumped mode-locked cw dye laser. The actively controlled mode spectrum provides a means for accurate measurements of large frequency intervals. From a multipulse spectrum of the sodium $3s\ensuremath{-}4d$ transition we have determined a new value of the 4d4d4d fine-structure splitting, 1028\ifmmode\pm\else\textpm\fi{}0.4 MHz.
We describe the influence of the intensity variation of the light in a standing wave on the lineshape of a Doppler-free multi-photon transition. We show that as long as the light shifts and the saturation are small compared to the Doppler width, a motional narrowing effect occurs and the line remains Lorentzian, the centre being shifted by an amount equal to the mean value of the dynamic Stark effect. We consider also the case of transients. We show that at resonance, it is possible to observe different responses according to the light intensity. In particular, it is possible to observe Rabi oscillations only if the Rabi frequency is larger than the natural width and smaller than the Doppler width of the transition.
We study in detail coherent phase control of femtosecond resonance-mediated (2+1)(2+1)(2+1) three-photon absorption and its dependence on the spectral bandwidth of the excitation pulse. The regime is the weak-field regime of third perturbative order. The corresponding interference mechanism involves a group of three-photon excitation pathways that are on resonance with the intermediate state and a group of three-photon excitation pathways that are near resonant with it. The model system of the study is atomic sodium (Na), for which experimental and numerical-theoretical results are obtained. Prominent among the results is our finding that with simple proper pulse shaping an increase in the excitation bandwidth leads to a corresponding increase in the enhancement of the three-photon absorption over the absorption induced by the (unshaped) transform-limited pulse. For example, here, a 40 nm bandwidth leads to an order-of-magnitude enhancement over the transform-limited absorption.