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Physical Review A

Evolution of maximal coherence and nonlinear mixing in an ionizing three-level medium

Roberto Buffa, M. Anscombe, J. P. Marangos

Published Mar 10, 2003• 16 citations
Publisher

Abstract

We have carried out a numerical investigation of the influence of photoionization, causing neutral atom depletion and accompanying electron impact dephasing, on a stimulated Raman adiabatic passage (STIRAP) scheme in a dense medium of three-level atoms. The nonlinear mixing from the large (maximal) coherence generated during the population transfer is calculated within the framework of a time-dependent model for a single atom with simplified propagation appropriate to a thin slab of material. For definiteness and consistency of the atomic parameters employed we have treated a scheme in atomic calcium. For generation of intense vacuum ultraviolet fields, laser intensities in the range from 100 ${\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ to 1 ${\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ are required, and these are found to cause significant ionization of the atom. We find that at atomic densities $>{10}^{16}/{\mathrm{cm}}^{3}$ the effects of electron impact dephasing on the coherence evolution is very significant and will dominate over other dephasing mechanisms. In contrast at lower densities $\ensuremath{\sim}{10}^{14}/{\mathrm{cm}}^{3}$ (compatible with a sealed vapor cell) the conversion efficiency is relatively unaffected by electron impact dephasing. We conclude that in general photoionization depletion and electron impact dephasing will be significant in maximal coherence wave mixing and STIRAP population transfer schemes operated at high density.

DOI
10.1103/physreva.67.033801

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Stimulated-Raman-adiabatic-passage (STIRAP) process provides an effective technique to transfer electronpopulation from an initial state (e.g. ground state) to excited final state for both atoms and molecules. Inthis paper, we present the results of the study on electron population transfer in three level system. Wehave analyzed the effects of various conditions on the transfer process, such as the time delay of the twolaser beams, two-photon off-resonance, one-photon off-resonance and the change of relative laser intensity.The numerical result is compared with experiment, and the reasons for the effects are also given.

Chinese Optics Letters•Jan 1, 2003•1 citation
We have carried out a numerical investigation of the influence of photoionization, causing neutral atom depletion and accompanying electron impact dephasing, on a stimulated Raman adiabatic passage (STIRAP) scheme in a dense medium of three-level atoms. The nonlinear mixing from the large (maximal) coherence generated during the population transfer is calculated within the framework of a time-dependent model for a single atom with simplified propagation appropriate to a thin slab of material. For definiteness and consistency of the atomic parameters employed we have treated a scheme in atomic calcium. For generation of intense vacuum ultraviolet fields, laser intensities in the range from 100 kW/cm2{\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}kW/cm2 to 1 MW/cm2{\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}MW/cm2 are required, and these are found to cause significant ionization of the atom. We find that at atomic densities >1016/cm3>{10}^{16}/{\mathrm{cm}}^{3}>1016/cm3 the effects of electron impact dephasing on the coherence evolution is very significant and will dominate over other dephasing mechanisms. In contrast at lower densities \ensuremath∼1014/cm3\ensuremath{\sim}{10}^{14}/{\mathrm{cm}}^{3}\ensuremath∼1014/cm3 (compatible with a sealed vapor cell) the conversion efficiency is relatively unaffected by electron impact dephasing. We conclude that in general photoionization depletion and electron impact dephasing will be significant in maximal coherence wave mixing and STIRAP population transfer schemes operated at high density.
We discuss the possibility of using a three-level ladder atomic system, with a final autoionizing state, for the efficient generation of short radiation pulses in the extreme ultraviolet through a process of sum-frequency mixing in the presence of maximal coherence. The results of numerical calculations are presented, investigating the effects of photoionization, dynamic Stark shift and electron impact dephasing on the conversion efficiency in a single atom model. The results of this investigation can be useful in identifying a real candidate for a possible experiment.
This work explores the effect of phase relaxation on the population transfer efficiency in stimulated Raman adiabatic passage (STIRAP). The study is based on the Liouville equation, which is solved analytically in the adiabatic limit. The transfer efficiency of STIRAP is found to decrease exponentially with the dephasing rate; this effect is stronger for shorter pulse delays and weaker for larger delays, since the transition time is found to be inversely proportional to the pulse delay. Moreover, it is found that the transfer efficiency of STIRAP in the presence of dephasing does not depend on the peak Rabi frequencies at all, as long as they are sufficiently large to enforce adiabatic evolution; hence increasing the field intensity cannot reduce the dephasing losses. It is shown also that for any dephasing rate, the final populations of the initial state and the intermediate state are equal. For strong dephasing all three populations tend to 13\frac{1}{3}31​.
By generalizing the definition of fidelity for the nonlinear system, we investigate the dynamics and adiabaticity of the population transfer for atom-molecule three-level $\ensuremath{\Lambda}$ system on a stimulated Raman adiabatic passage (STIRAP). We find that the adiabatic fidelity for the coherent population trapping state or dark state, as the function of the adiabatic parameter, approaches to unit in a power law. The power exponent, however, is much less than the prediction of the linear adiabatic theorem. We further discuss how to achieve higher adiabatic fidelity for the dark state through optimizing the external parameters of the STIRAP.
The problem of the coherent control of atomic states, preparation of atoms in a stable superposition state, and atomic population transfer has attracted much attention in the last decade. One of the main motivations is associated with the rapid development of the field of quantum computing. One of the key problems of quantum information processing is the construction of given coherent superposition atomic states in a macroscopic volume. Stimulated Raman adiabatic passage (STIRAP) [1] is considered to be the most common method for atomic population transfer in quantum systems. Being analyzed in detail on various quantum systems from both theoretical and experimental points of view, this method is widely adopted for the implementation of tools as optical devices for atomic beams, control of chemical reactions, high-precision magnetometry, atom cooling, etc. Meanwhile, an alternative method for population transfer, based on the intuitive pulse sequence with large one-photon detunings was demonstrated in [2]. In contrast with the STIRAP method, where the population transfer is realized via a dark state, the population transfer for the intuitive pulse sequence is obtained via a bright state. The authors of [2] called the atomic population transfer of this kind b-STIRAP. A detailed theoretical analysis of the b-STIRAP process in media with equal oscillator strength on adjacent transitions was performed in [3]. However, it is well-known that an oscillator strengths ratio different from unity can considerably impact on the coherent processes in Λ-media (see e.g. [4]). In the present work an analytical and numerical analysis of the b-STIRAP process in a medium with unequal oscillator strengths is performed. It is shown that the length of population transfer can be considerably increased by an appropriate choice of the dipole transitions. 1. N.V.Vitanov, B.W.Shore,K.Bergman, Adv. Atom.,Mol, Optical Physics, 46, 55-190, (2001). 2. J.Klein, F.Beil, Th.Halfmann, , Phys.Rev.Lett. 99,113003 (2007) 3. G.G. Grigoryan, G. Nikoghosyan, T. Halfmann, Y.T. Pashayan-Leroy, C. Leroy, S.Guerin, Phys.Rev. A 80, 033402 (2009) 4. G.G.Grigoryan, Y.Pashayan, Propagation of pulses in a three-level medium at exact two-photon resonance. Phys.Rev. A 64, 013816, 2001
A comprehensive study of two simultaneous three wave mixing processes, a second harmonic generation followed by difference frequency generation, under nonlinear stimulated Raman adiabatic passage (STIRAP) is presented. An input pump is up-converted to its second harmonic, which then gets down-converted to a signal and idler pair with frequencies lying very close to the input pump in such a manner that complete conversion from the pump to the signal and idler takes place without exciting the second harmonic under counterintuitive adiabatic passage. This process involves nonlinear STIRAP with a nonlinear dark state similar to atomic population transfers, and we bring an analogy from atomic systems to our nonlinear dynamics to linearize the problem and analytically obtain the adiabaticity condition required for complete conversion. We also show that the nonlinear STIRAP mechanism results in a large bandwidth of about 380 nm with almost complete conversion of the pump to the signal and idler.
Stimulated-Raman-adiabatic-passage (STIRAP) process provides an effective technique to transfer electronpopulation from an initial state (e.g. ground state) to excited final state for both atoms and molecules. Inthis paper, we present the results of the study on electron population transfer in three level system. Wehave analyzed the effects of various conditions on the transfer process, such as the time delay of the twolaser beams, two-photon off-resonance, one-photon off-resonance and the change of relative laser intensity.The numerical result is compared with experiment, and the reasons for the effects are also given.