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arXiv:1506.00704

Reinvigorating Excited State Coherences in $V$ Level Systems with the use of Trap States

Z. S. Sadeq

Published Jun 1, 2015• 0 citations
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Abstract

We investigate potential schemes to reinvigorate coherences between excited states in a $V$ level system under various excitation conditions. Coherent pulsed, cw and noisy pulsed excitation are considered and the existence of a trap state is shown to allow for the resurgence of excited state coherence in the two pulsed scenarios. With a short trap time (strong decoherence), it is possible to restart excited state coherence in the pulsed scenarios. In the case of cw excitation with relaxation back to the ground state, the coherences between the excited states do not vanish in the long time limit but instead approach a non-zero value.

DOI
10.48550/arxiv.1506.00704

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Reinvigorating Excited State Coherences in VVV Level Systems with the use of Trap States
We investigate potential schemes to reinvigorate coherences between excited states in a VVV level system under various excitation conditions. Coherent pulsed, cw and noisy pulsed excitation are considered and the existence of a trap state is shown to allow for the resurgence of excited state coherence in the two pulsed scenarios. With a short trap time (strong decoherence), it is possible to restart excited state coherence in the pulsed scenarios. In the case of cw excitation with relaxation back to the ground state, the coherences between the excited states do not vanish in the long time limit but instead approach a non-zero value.
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We consider a double two-level system in which each single two-level system is driven by a strong pulse. The light shifts induced by the strong field are probed by a weak pulse that connects the two sub-system in a crossed manner connecting the ground state of one system with the excited state of the other and vice versa. We are interested in coherent control of medium gain for the weak pulse and we demonstrate the conditions on laser and atomic parameters that render this control possible. Both ultra-short and long pulse regimes are discussed
Coherent excitation of an atomic excited state occurs during the propagation of near-resonant light pulses and is responsible for the induced polarization. Simultaneously, incoherent excitation occurs due to the relaxation processes described by the absorption coefficient. Here, the theory for the coherent and incoherent excitation is initially presented in terms of the traditional vector model. While a complete understanding of the two-level system is provided by the vector model, it is shown to be incomplete when the problem of directly monitoring the coherent and incoherent excitation is considered. This is because this latter problem involves more than two levels. For this more complicated multilevel problem, adiabatic states are introduced to gain further understanding. The adiabatic states are the stationary states of the atom in the presence of the near-resonant laser field; they help to explain the intimate connection between the coherent excitation and the two-photon resonance. Experimental measurements of the coherent and incoherent excitation associated with near-resonant pulse propagation in Rb vapor are presented. The double-resonance technique used a relatively strong pulsed dye laser tuned near the $5{S}_{\frac{1}{2}}\ensuremath{\leftrightarrow}5{P}_{\frac{1}{2}}$ transition (7948 \AA{}A) of Rb to produce the coherent and incoherent excitation, and a weak, tunable cw dye laser tuned in the region of the $5{P}_{\frac{1}{2}}\ensuremath{\leftrightarrow}6{D}_{\frac{3}{2}}$ transition (6206 \AA{}A) to monitor this excitation. In agreement with theory, the experimental results demonstrate that coherent excitation is responsible for two-photon absorption, while the incoherent excitation corresponds to one-photon absorption to the 5P125{P}_{\frac{1}{2}}5P21​​ state.
We analyze a model where a closed VVV system is excited by two uncorrelated partially coherent fields. We use a collisionally broadened cw laser, which is a good model for an experimentally realizable partially coherent field, and show that it is possible to generate excited-state coherences even if the two fields are uncorrelated. This transient coherence can be increased if splitting between the excited states is reduced relative to the radiation coherence time ${\ensuremath{\tau}}_{d}$. For small excited-state splitting, one can use this scheme to generate a long-lived coherent response in the system.
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This Letter demonstrates the possibility of generating coherent population trapping in C(60). Similar to a three-level Λ system, C(60) has a forbidden transition between the highest occupied molecular orbital (HOMO) (|a}) and the lowest unoccupied molecular orbital (LUMO) (|c}), but a dipole-allowed transition between HOMO and LUMO+1 (|b}) and between |b} and |c}. We employ two cw laser fields, one coupling and one probe. The strong coupling field is switched on first to resonantly excite the transition between |b} and |c}. After a delay, the probe is switched on; the coherent interaction between the coupling and probe fields traps the population in |a} and |c}. This forms a partially dark state in C(60), analogous to that in atomic vapors. Turning off the coupling field restores C(60)'s absorption. Pulsed lasers work as well. We use two pulses to steer the system into a dark state; when we send in a cw probe field, the electric polarization of C(60) plunges by five times, in comparison with the noncontrol case. This should be detectable experimentally.