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Physical Review Letters

Coherent population transfer via the continuum

C. E. Carroll, F. T. Hioe

Published Jun 15, 1992• 118 citations
Publisher

Abstract

We present a remarkable analytic result which suggests that a continuum can be used as an intermediary for a significant transfer of population from one discrete state to another discrete state in a stimulated Raman transition. The population transfer is accomplished by employing two laser pulses that overlap in time, arranged in the so-called counterintuitive order. That a continuum can be used for population transfer is shown here for the first time. It promises to open up more channels for selective coherent excitation of atoms or molecules.

DOI
10.1103/physrevlett.68.3523

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We present a remarkable analytic result which suggests that a continuum can be used as an intermediary for a significant transfer of population from one discrete state to another discrete state in a stimulated Raman transition. The population transfer is accomplished by employing two laser pulses that overlap in time, arranged in the so-called counterintuitive order. That a continuum can be used for population transfer is shown here for the first time. It promises to open up more channels for selective coherent excitation of atoms or molecules.
Two laser pulses that overlap in time have been successfully used for coherent excitation of a molecule or atom to a desired state. An intermediate state that is at or near resonance with the laser frequencies has been used in past experiments and calculations. Here we replace this intermediate state by a continuum of intermediate states. An analytic solution of a simple model suggests that the continuum can be used in such an excitation scheme, provided that the laser pulses are arranged in the so-called counterintuitive order. We use a special case of this solution to relate this work to ionization suppression and coherent population trapping.
We report the first experimental demonstration of coherent population transfer, induced by stimulated Raman adiabatic passage, via continuum states. Population is transferred from the metastable state 2s(1)S(0) to the excited state 4s(1)S(0) in helium atoms in a two-photon process mediated by coherent interaction with the ionization continuum. While incoherent techniques usually do not permit any population transfer in such a process, we show that stimulated Raman adiabatic passage allows significant population transfer to take place also via ultrafast decay channels.
We propose a technique which produces nearly complete ionization of the population of a discrete state coupled to a continuum by a two-photon transition via a lossy intermediate state whose lifetime is much shorter than the interaction duration. We show that using counterintuitively ordered pulses, as in stimulated Raman adiabatic passage (STIRAP), wherein the pulse coupling the intermediate state to the continuum precedes and partly overlaps the pulse coupling the initial and intermediate states, greatly increases the ionization signal and strongly reduces the population loss due to spontaneous emission through the lossy state. For strong spontaneous emission from that state, however, the ionization is never complete because the dark state required for STIRAP does not exist. We demonstrate that this drawback can be eliminated almost completely by creating a laser-induced continuum structure (LICS) by embedding a third discrete state into the continuum with a third control laser. This LICS introduces some coherence into the continuum, which enables a STIRAP-like population transfer into the continuum. A highly accurate analytic description is developed and numerical results are presented for Gaussian pulse shapes.
We study the ir excitation of HeH+{\mathrm{HeH}}^{+}HeH+ using the counterintuitive arrangement of two pulsed lasers with the continuum as the intermediate state. The time-dependent Schr\"odinger equation describing the system is integrated numerically using the split-operator technique. We examine the maximum population transfer as a function of the pulse separation, pulse width, relative pulse strength, two-photon detuning, and relative position of the intermediate continuum state. The largest population transfer found is 63%.
Transfer of molecules or atoms from an initial state to a certain final state is often wanted, and use of a single laser may be impractical. Transfer via an intermediate state of higher energy has been demonstrated, using laser pulses that overlap in time. We have proposed using the continuum in place of the intermediate state, and encouraging results have been computed for a simple model. Our latest calculations show the effects of operating this scheme near the bottom of the continuum or near a resonance having the Fano shape, and the effects of laser-driven continuum-continuum transitions.