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Physical Review A

Enantiodetection via two-dimensional spectroscopy: Extending the methodology to general experimental conditions

Mao-Rui Cai, Chong Ye, Yong Li, Hui Dong

Published Nov 13, 2023• 0 citations
Publisher

Abstract

Developing effective methods to measure the enantiomeric excess of chiral mixtures is one of the major topics in chiral molecular research but remains challenging. An enantiodetection method via two-dimensional (2D) spectroscopy based on a four-level model containing a cyclic three-level system (CTLS) of chiral molecules was recently proposed and demonstrated, but with a strict condition of one-photon resonance (where three driving fields are exactly resonantly coupled to the three electric dipole transitions) in the CTLS and narrow-band probe-pulse assumption. Here, we extend the 2D spectroscopy method to more general experimental conditions, with three-photon resonance (where the sum of the two smaller frequencies among the three driving fields equals the third one) and a broadband probe pulse. Our method remains effective on enantiodetection with the help of experimental techniques such as the chop-detection method, which is used to eliminate the influence of the other redundant levels that exist in the real system of chiral molecules. Under these more general conditions, the enantiomeric excess of the chiral mixture is estimated by taking an easily available standard sample (usually the racemic mixture) as the reference.

DOI
10.1103/physreva.108.052810

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Developing effective methods to measure the enantiomeric excess of chiral mixtures is one of the major topics in chiral molecular research but remains challenging. An enantiodetection method via two-dimensional (2D) spectroscopy based on a four-level model containing a cyclic three-level system (CTLS) of chiral molecules was recently proposed and demonstrated, but with a strict condition of one-photon resonance (where three driving fields are exactly resonantly coupled to the three electric dipole transitions) in the CTLS and narrow-band probe-pulse assumption. Here, we extend the 2D spectroscopy method to more general experimental conditions, with three-photon resonance (where the sum of the two smaller frequencies among the three driving fields equals the third one) and a broadband probe pulse. Our method remains effective on enantiodetection with the help of experimental techniques such as the chop-detection method, which is used to eliminate the influence of the other redundant levels that exist in the real system of chiral molecules. Under these more general conditions, the enantiomeric excess of the chiral mixture is estimated by taking an easily available standard sample (usually the racemic mixture) as the reference.
Developing effective methods to measure the enantiomeric excess of the chiral mixture is one of the major topics in chiral molecular researches, yet remains challenging. Enantiodetection method via two-dimensional (2D) spectroscopy based on a four level model, containing a cyclic three-level system (CTLS), of chiral molecules was recently proposed and demonstrated, yet with a strict condition of the one-photon resonance (where three driving fields are exactly resonantly coupled to the three electric-dipole transitions, respectively) in the CTLS and narrowband probe pulse assumption. Here, we extend the 2D spectroscopy method to more general experimental conditions, with three-photon resonance (where the sum of the two smaller frequencies among the three driving fields equals to the third one) and broadband probe pulse. Our method remains effective on enantiodetection with the help of experimental techniques, such as the chop detection method, which is used to eliminate the influence of the other redundant levels existing in the real system of chiral molecules. Under these more general conditions, the enantiomeric excess of the chiral mixture is estimated by taking an easily available standard sample (usually the racemic mixture) as the reference.
Determination of enantiomeric excess is important and remains challenging. We propose theoretically a spectroscopic method for this issue based on the chirality-dependent ac Stark effects in cyclic three-level models under the three-photon resonance condition. The enantiomeric excess of the chiral mixture is determined by comparing the amplitudes of the two ac Stark peaks chosen in the Fourier transform spectrum of the induced polarizations, which are (approximately) proportional to the number of molecules of the two enantiomers, respectively. In contrast to current spectroscopic methods based on the interference between the electric and (usually weak) magnetic dipole transition moments and/or with the need for enantiopure samples, our method involves only electric dipole transitions and does not require the enantiopure samples. Therefore, it will give strong chiral signals and can be applied to the determination of enantiomeric excess for chiral molecules whose enantiopure samples are still challenging to achieve.
We propose a method for enantio-detection of chiral molecules based on a cavity-molecule system, where the left- and right-handed molecules are coupled with a cavity and two classical light fields to form cyclic three-level models. Via the cavity-assisted three-photon processes based on the cyclic three-level model, photons are generated continuously in the cavity even in the absence of external driving to the cavity. However, the photonic fields generated from the three-photon processes of left- and right-handed molecules differ with the phase difference π according to the inherent properties of electric-dipole transition moments of enantiomers. This provides a potential way to detect the enantiomeric excess of chiral mixture by monitoring the output field of the cavity.
We propose a theoretical method for enantio-discrimination of chiral molecules based on a cavity-molecules system, which consists of a cavity without external driving and an ensemble of chiral mixture (containing left- and right- handed molecules) confined in the cavity. The molecules in the chiral mixture are modeled as cyclic three-level models coupled with the quantized cavity field and two classical light fields. Via the cavity-assisted three-photon process based on the cyclic three-level model, the intracavity photons are generated continuously though there is no external driving to the cavity, and the fields of the photons generated from the the left- and right- handed molecules differ with the phase difference \pi. This provides a potential way to detect the enantiomeric excess of chiral mixture by monitoring the output field of the cavity.
We propose a theoretical method for enantio-discrimination based on the light deflection effect in four-level models of chiral molecules. This four-level model consists of a cyclic three-level subsystem coupled by three strong driving fields and an auxiliary level connected to the cyclic three-level subsystem by a weak probe field. It is shown that the induced refractive index for the weak probe field is chirality-dependent. Thus, it will lead to chirality-dependent light deflection when the intensities of two of the three strong driving fields are spatially inhomogeneous. As a result, the deflection angle of the weak probe light can be utilized to detect the chirality of pure enantiomers and enantiomeric excess of the chiral mixture. Therefore, our method may act as a tool for enantio-discrimination.
The optical bistability (OB) and multistability (OM) in chiral molecules are investigated by placing the sample into a unidirectional ring cavity. Because of broken mirror symmetry of the effective potential, the chiral molecules have a cyclic three-level Δ-configuration structure, in which one- and two-photon transitions can coexist. We find that the OB is achievable in this system on exact one-, two- and three-photon resonance conditions but absent in the three-level Λ-type system under the two-photon resonance. Moreover, the OM and the switching between OB and OM are also realized by choosing parameters properly. Interestingly, the left- and right-handed chiral molecules exhibit different bistable and multistable behaviors. It is shown that the threshold intensity of OB is strongly dependent on the percentage of the two enantiomers in the mixture. This provides an effective approach to probe molecular chirality and to determine enantiomer excess, which may find potential application in organic chemistry, pharmacology, biochemistry, etc..