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The Journal of Physical Chemistry A

Delaying the Decay of a Superposition of Resonance States

Alberto García-Vela

Published Jul 31, 2019• 2 citations
Publisher

Abstract

A weak-field coherent control scheme is applied in order to enhance the decay lifetime of a superposition of overlapping resonance states. The scheme uses a pump laser field consisting of two pulses delayed in time, each of them exciting a different energy at which several resonances of the Ne–Br 2 (B) complex overlap. Simultaneous excitation of these two energies induces interference between the overlapping resonances, which causes an enhancement of the lifetime of the superposition created. By variation of the delay time between the pulses, the mechanism of resonance interference can be controlled and optimized to achieve a maximum lifetime enhancement. The optimal delay time between pulses leading to maximum superposition lifetime can be quantitatively predicted with a simple law. The effect of the interference mechanism on the lifetime enhancement is investigated. It is found that interference induces a transfer of amplitude between the different resonances back and forth, which delays significantly the natural resonance decay, increasing the global lifetime of the superposition. Due to the simplicity of the control scheme, a wide applicability is envisioned.

DOI
10.1021/acs.jpca.9b04892

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A weak-field coherent control scheme is applied in order to enhance the decay lifetime of a superposition of overlapping resonance states. The scheme uses a pump laser field consisting of two pulses delayed in time, each of them exciting a different energy at which several resonances of the Ne–Br 2 (B) complex overlap. Simultaneous excitation of these two energies induces interference between the overlapping resonances, which causes an enhancement of the lifetime of the superposition created. By variation of the delay time between the pulses, the mechanism of resonance interference can be controlled and optimized to achieve a maximum lifetime enhancement. The optimal delay time between pulses leading to maximum superposition lifetime can be quantitatively predicted with a simple law. The effect of the interference mechanism on the lifetime enhancement is investigated. It is found that interference induces a transfer of amplitude between the different resonances back and forth, which delays significantly the natural resonance decay, increasing the global lifetime of the superposition. Due to the simplicity of the control scheme, a wide applicability is envisioned.
The enhancement of the resonance lifetime that occurs upon interference of two overlapping resonances excited coherently by two pulses with delayed time has been investigated as a function of the pulse temporal width and the delay time between the pulses. A general law predicting quantitatively the optimal delay time that maximizes the lifetime enhancement of the two resonances has been established in terms of the pulse width and of the lifetimes of both resonances when they are excited isolatedly. The specific form of the law and all the results found can be closely related to the characteristic features of the mechanism of interference between the overlapping resonances, providing a detailed understanding on how the mechanism operates. The proposed law is envisioned as a useful tool to design experimental strategies to control the resonance lifetime.
It is shown that new possibilities for control of the lifetime of a system in a resonance state emerge when the density of resonances overlapping and interfering with the target resonance increases. When using a control scheme combining two pump laser pulses, it is found that increasing the density of resonance states overlapping with the target one increases the selectivity of the scheme applied, and leads to achieve a remarkably higher degree of control. Lifetime enhancements by factors up to 20 are obtained when this selectivity is applied. The underlying reasons for such strong enhancements are analyzed and explained in the light of the equations of the model applied. Application of this strategy to control and enhance the lifetime of a system in excited states is envisioned.
The lifetime of a resonance state overlapping with a second resonance is shown to be strongly enhanced (by a factor of 3) by combining two pump pulses to simultaneously excite the two resonances. Such an enhancement is produced by interference effects occurring between the two coherently excited overlapping resonances. A high degree of control on the intensity of interference, and thus on the resonance lifetime enhancement, is found to be achieved by varying the delay time between the pulses and their relative intensities.
We consider a duplicated two-level system where each two-level subsystem is driven by a strong resonant femtosecond pulse and a weak resonant femtosecond pulse connects cross transitions. Strong interference effects are induced in such a configuration leading to a modulation of the medium gain as a function of the relative phase between the pulses []. We study here the influence of the temporal overlap between the pulses on the dynamics of the system and the medium gain. We show that the dynamics is dominated by two competing phenomena: Light shifts (LS) induced by the driving pulse that prevails for small delays and free induction decay (FID) that prevails for large delays. LS enhance the medium gain for and reduce it for , whereas FID always leads to a decrease of the medium gain.
We investigate the effect of the spontaneous decay-induced coherence on the control of coherent population transfer in a closed double $\ensuremath{\Lambda}$-type four-level system with the energy separation of the excited doublet comparable to their decay rates by using the stimulated Raman adiabatic passage technique. It is shown that when the two pulse laser fields are tuned to the particular frequency where the condition for quantum interference is satisfied, remarkable enhancement or suppression of population transfer can be realized through the spontaneous emission constructive or destructive quantum interference even with a finite pulse area, which is very favorable to coherent population transfer in quantum systems with weak oscillator strengths.
The manipulation of population transfer and the creation of an arbitrary superposition between two atomic states in the framework of a closed inverse Y-type four-level system driven by a series of pulse pairs and an external control field is reported. It is shown that when the laser fields are tuned to resonance with respective transitions, the non-adiabatic coupling between two degenerate adiabatic states can be reduced tremendously and complete population transfer from the initial state to the target state can be achieved in a piecewise manner. Moreover, an arbitrary superposition between the initial state and target state can be created by suitably tuning the Rabi frequency of the control laser field, inter-pair time delay, pulse number or intra-pair time delay between the pump and Stokes pulses. This scheme combines stimulated Raman adiabatic passage and temporal coherent control, and may be useful for quantum information processing and control of chemical reactions.