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Optics Express

Pump-control approach to enantiospecific state transfer

Бо Лю, Yong Li, Chong Ye, C. P. Sun

Published Jul 17, 2024• 4 citations
Publisher

Abstract

Enantiospecific state transfer of chiral molecules is extremely important because enantiomers coexist in many biologically active compounds and play significantly different physiological, pharmacological, and biological roles. The inherently strong electric-dipole optical approaches based on the cyclic three-level model of chiral molecules have been extensively discussed. But, for the cases of large chiral molecules and/or chiral molecules of low asymmetry, the four-level model with two sub-loops is more realistic to describe the molecules. Based on the four-level model, we propose a pump-control approach to realize the highly efficient enantiospecific state transfer. In our approach, two pump pulses are applied to generate molecular coherence between the ground state and the first excited state of our working model. According to the coherence of the molecules, we adjust the phase and pulse area of the control pulse, then we obtain the highly efficient enantiospecific state transfer in the first excited working state. In addition, we further optimize the fraction of enantiopure samples by adjusting the area of the two pump pulses.

DOI
10.1364/oe.528182

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Enantiospecific state transfer of chiral molecules is extremely important because enantiomers coexist in many biologically active compounds and play significantly different physiological, pharmacological, and biological roles. The inherently strong electric-dipole optical approaches based on the cyclic three-level model of chiral molecules have been extensively discussed. But, for the cases of large chiral molecules and/or chiral molecules of low asymmetry, the four-level model with two sub-loops is more realistic to describe the molecules. Based on the four-level model, we propose a pump-control approach to realize the highly efficient enantiospecific state transfer. In our approach, two pump pulses are applied to generate molecular coherence between the ground state and the first excited state of our working model. According to the coherence of the molecules, we adjust the phase and pulse area of the control pulse, then we obtain the highly efficient enantiospecific state transfer in the first excited working state. In addition, we further optimize the fraction of enantiopure samples by adjusting the area of the two pump pulses.
We study the enantiospecific state transfer for gaseous symmetric-top chiral molecules by constructing a four-level model. This model is formed by coupling the electric dipole transitions among four appropriate rovibrational states with three electromagnetic fields. It includes two cyclic three-level substructures, where the overall phases of the coupling strengths differ by $\ensuremath{\pi}$ with enantiomers and reflect the chirality dependence of the molecules. Based on this four-level model, two dynamic ways are proposed to achieve the approximately perfect enantiospecific state transfer for gaseous symmetric-top chiral molecules when all the molecules are initially in the ground state of the system.
Enantio-conversion with the help of electromagnetic fields is an essential issue due to the chirality dependence of many chemical, biological, and pharmaceutical processes. Here, we propose a method for this issue based on a five-level double-$\mathrm{\ensuremath{\Delta}}$ model of chiral molecules. By utilizing the breaking of left-right symmetry in two $\mathrm{\ensuremath{\Delta}}$-type substructures, we can establish chiral-state-selective excitations with one chiral ground state being excited to an achiral excited state and the other one being undisturbed. In the meanwhile, the achiral excited state will relax to two chiral ground states. The two effects simultaneously acting on chiral mixtures can convert molecules of different chiralities to ones of the same chirality, i.e., enantio-conversion via optical pumping. We numerically show that highly efficient enantio-conversion can be achieved. Our method works in the appearance of decoherences and without the precise control of pulse durations (pulse areas) or pulse shapes. These advantages offer it promising features in promoting the future exploration of enantio-conversion.
Enantiomer-specific pumping of chiral molecules in high-JJJ rotational states
Enantiomer-specific state transfer (ESST) based on electric-dipole transitions has attracted considerable interest in the studies of manipulating molecules with electromagnetic fields. It also holds promise as a powerful tool for enantiomer detection and separation. To date, experimental demonstrations of ESST have focused mainly on molecules in the lowest rotational angular-momentum states, namely, J=0J=0J=0 and J=1J=1J=1. Realistic molecular samples, however, may contain appreciable populations in higher-JJJ rotational states, making it desirable to develop ESST schemes applicable to such states. Nevertheless, one cannot derive such schemes via a simple and straightforward extension of existing low-JJJ schemes, because higher-JJJ manifolds contain more magnetic sub-levels and involve more complicated networks of electric-dipole transitions. Here, we propose an ESST scheme for chiral molecules involving the J=1J=1J=1 and J=2J=2J=2 rotational states. Specifically, our scheme is based on enantiomer-specific pumping driven by three linearly polarized microwave fields and one laser beam. Under this scheme, one enantiomer is pumped into an enantiomer-specific dark state within a sub manifold with J=1,2J=1,2J=1,2, whereas the other enantiomer is dissipatively pumped out of this manifold. Our strategy can be extended to systems involving rotational states with even higher angular momenta.
Enantiomer-specific state transfer (ESST), which involves transferring enantiomers with different handedness of a chiral molecule into different-energy internal states, is a challenging yet significant task. Previous ESST methods are based on dynamic processes and thus require the preparation of initial states and precise control of microwave operation times. We propose a novel ESST approach, called enantiomer-specific pumping (ESP), which is based on a dissipative process and thereby eliminates the need for these two technical requirements. This approach utilizes a special microwave-induced dark state that appears exclusively for the enantiomer with a specific handedness. Specifically, in ESP, the enantiomer lacking the dark state can be pumped out of the subspace of relevant internal states, while the enantiomer with the dark state maintains a finite probability within this subspace, offering high efficiency in ESST. Notably, ESP facilitates enantiodetection without the need for enantiopure samples as reference.
We derive a pulse-area theorem for a single-loop cyclic three-level system without applying the rotating-wave approximation while explicitly taking into account the time delays of control fields. This corresponds to an archetypal model for exploring enantioselective state transfer (ESST) in chiral molecules driven by three linearly polarized microwave pulses. By dividing the closed-loop excitation into two separate stages, we obtain both amplitude and phase conditions of three control fields to generate high fidelity of ESST. As a proof of principle, we apply this pulse-area theorem to cyclohexylmethanol molecules, for which three rotational states are connected by aaa-type, bbb-type, and ccc-type components of the transition dipole moments in both center-frequency-resonant and -detuned conditions. Our results show that two enantiomers with opposite handedness can be transferred to different target states by designing three microwave pulses that satisfy the amplitude and phase conditions at the transition frequencies. The corresponding control schemes are robust against the time delays between the two stages. We suggest that the two control fields used in the second stage should be applied simultaneously for practical applications. This work contributes an alternative pulse-area theorem to the field of quantum control, which has the potential to determine the chirality of enantiomers in a mixture.
Abstract Laser-assisted enantio-conversion is an ambitious issue related to chiral molecules in atomic, molecular, and optical physics. The theoretical laser-distillation method was proposed to realize enantio-conversion based on a four-level double-Δ model of two degenerated chiral ground states and two achiral excited states. Here, we re-investigate and improve the laser-distillation method so that a chiral mixture can be converted to an enantiopure sample of the desired chirality, i.e. complete enantio-conversion, which has not been discussed in previous theoretical works on the laser-distillation method. Since the undesired chirality may be inefficient, or even cause serious side effects in pharmacology, our work provides an important improvement to the original theoretical works on laser-assisted enantio-conversion.