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The Journal of Chemical Physics

Experimental control of excitation flow produced by delayed pulses in a ladder of molecular levels

Ruth Garcia-Fernandez, Bruce W. Shore, Klaas Bergmann, Aigars Ekers, Леонід Петрович Яценко

Published Jul 1, 2006• 6 citations

Abstract

We study a method for controlling the flow of excitation through decaying levels in a three-level ladder excitation scheme in Na(2) molecules. Like the stimulated Raman adiabatic passage (STIRAP), this method is based on the control of the evolution of adiabatic states by a suitable delayed interaction of the molecules with two radiation fields. However, unlike STIRAP, which transfers a population between two stable levels g and f via a decaying intermediate level e through the interaction of partially overlapping pulses (usually in a Lambda linkage), here the final level f is not long lived. Therefore, the population reaching level f decays to other levels during the transfer process. Thus, rather than controlling the transfer into level f, we control the flow of the population through this level. In the present implementation a laser P couples a degenerate rovibrational level in the ground electronic state X 1Sigma(g)+, v" = 0, j" = 7 to the intermediate level A 1Sigma(u)+, v' = 10, J' = 8, which in turn is linked to the final level 5 1Sigma(g)+, v = 10, J = 9 by a laser S, from which decay occurs to vibrational levels in the electronic A and X states. As in STIRAP, the maximum excitation flow through level f is observed when the P laser precedes the S laser. We study the influence of the laser parameters and discuss the consequences of the detection geometry on the measured signals. In addition to verifying the control of the flow of population through level f we present a procedure for the quantitative determination of the fraction kappa(f) of molecules initially in the ground level which is driven through the final level f. This calibration method is applicable for any stepwise excitation.

DOI: 10.1063/1.2203629

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We study a method for controlling the flow of excitation through decaying levels in a three-level ladder excitation scheme in Na(2) molecules. Like the stimulated Raman adiabatic passage (STIRAP), this method is based on the control of the evolution of adiabatic states by a suitable delayed interaction of the molecules with two radiation fields. However, unlike STIRAP, which transfers a population between two stable levels g and f via a decaying intermediate level e through the interaction of partially overlapping pulses (usually in a Lambda linkage), here the final level f is not long lived. Therefore, the population reaching level f decays to other levels during the transfer process. Thus, rather than controlling the transfer into level f, we control the flow of the population through this level. In the present implementation a laser P couples a degenerate rovibrational level in the ground electronic state X 1Sigma(g)+, v" = 0, j" = 7 to the intermediate level A 1Sigma(u)+, v' = 10, J' = 8, which in turn is linked to the final level 5 1Sigma(g)+, v = 10, J = 9 by a laser S, from which decay occurs to vibrational levels in the electronic A and X states. As in STIRAP, the maximum excitation flow through level f is observed when the P laser precedes the S laser. We study the influence of the laser parameters and discuss the consequences of the detection geometry on the measured signals. In addition to verifying the control of the flow of population through level f we present a procedure for the quantitative determination of the fraction kappa(f) of molecules initially in the ground level which is driven through the final level f. This calibration method is applicable for any stepwise excitation.

We explore theoretically the rovibrational dynamics of stimulated Raman adiabatic passage (STIRAP) in molecular electronic states including both the rotational and the vibrational degrees of freedom, with the LiH molecule as an example. By using a pure rovibrational state as initial state, we find that, under the condition of two-photon resonance, an efficient rovibrational population transfer can be achieved via STIRAP in molecular electronic states and the desired target state can be selected by adjusting laser-pulse parameters. Besides the interested rovibrational states, however, some unwanted rovibrational states may affect population transfer process, especially while the two-photon detuning is taken into account. By inspecting the evolution process of wave packet, we can easily search these unwanted rovibrational states and study their influences. Moreover, the effect of rotational temperature on the population transfer process is discussed using thermal mixed state as initial state.

The rovibrational dynamics of an extended ladder stimulated Raman adiabatic passage (STIRAP) system through permanent dipole moment transitions is investigated theoretically using the time-dependent quantum-wave-packet method for the ground electronic state of the HF molecule. The calculated results show that nearly 100

%

of the population can be transferred to the target state through (1+2), (1+3), and (2+2) STIRAP schemes. By choosing a suitable excitation pathway, the effects of the background states on the final population of the target state can be removed. For the multiphoton STIRAP process, the one-photon overtone pump scheme is more efficient than the two-photon pump scheme in controlling the population transfer to the target state.

We analyze the influence of three pulsed laser fields interacting with a four-state atomic system for which there occur two degenerate population-trapping dressed states. We present a simple expression for the geometrical phase acquired by such an atom during a stimulated Raman adiabatic passage (STIRAP) process using a delay between Stokes and pump pulses, during which the two Rabi frequencies adiabatically traverse a closed path in the parameter space. We describe techniques that can produce (geometrical) state-vector phases that are independent of the longitudial velocity with which atoms move across a laser beam, and that are insensitive to radiative decay from the intermediate excited state. The geometrical phase can be changed by changing the relative delay of the pulses. The geometrical phase can be measured by observing the population in atomic states. We show that when the pump and Stokes pulses are properly timed, high population transfer from the initial state to the target state is achieved. In the adiabatic limit the robustness of the population transfer is equivalent to that of STIRAP.

We have considered a coherent population transfer to a higher excited singlet state ( S2 ) of molecules with anomalous fluorescence in molecular assemblies (e.g., a dense medium). A direct excitation to S2 requires light in the UV region. Because of this, the transition is conveniently realized by a two-step (two-photon) process: S0→S1→S2 , where transitions S0→S1 and S1→S2 correspond to the optical region. We have shown that efficient stimulated Raman adiabatic passage (STIRAP) in the ladder configuration can be realized in this case, using suitably chirped pulses, to compensate a change of the two-photon transition frequency in time, induced by the pulses themselves, due to near dipole-dipole interactions. We have provided a reduced state formulation of the optical control process. Chirping the “pump” pulse that excites transition S0→S1 is nonequivalent to chirping the “Stokes” pulse that excites transition S1→S2 , with respect to the population of the intermediate state ( S1 ) in the pulse nonadiabatic regime. We have also shown that with suitably chirped pulses, efficient STIRAP still persists even for a rather large decay of the intermediate state.

The rovibrational dynamics of stimulated Raman adiabatic passage (STIRAP) through permanent dipole moment transitions are investigated theoretically using a time-dependent quantum wave packet method for the ground electronic state of an HF molecule. The two basic STIRAP processes, $\ensuremath{\Lambda}$ and ladder systems, are simulated. The calculated results show that nearly

100%

of the population can be transferred to the target state. Besides the interested transitions, the pulses can induce other transitions which affect the dynamics of STIRAP. The final populations of the initial and target states depend on delay time.