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Journal of Nonlinear Optical Physics & Materials

Sustained elimination of molecular alignment dephasing via coherent control with phase-modulated pulse trains

Shuwu Xu, Lingxiao Wang, Haizhi Fang, Zhongjie Ren, Yunxia Huang

Published Apr 22, 2026• 0 citations
Publisher

Abstract

Suppressing molecular alignment dephasing is of significant practical value for applications involving nonrigid rotors. In this work, we propose a strategy that employs a train of time-delayed, phase-modulated laser pulses to achieve the elimination of alignment dephasing over an extended temporal window. Our findings demonstrate that for molecules with short rotational periods, such as CO or N 2 used in this study, dephasing can be completely eliminated for at least a dozen revival periods, thereby restoring alignment approaching the rigid-rotor limit. For molecules exhibiting longer revival periods, however, the effectiveness of this approach is somewhat diminished. While complete dephasing elimination is not achieved in such cases, the method remains sufficiently effective for most practical applications. We also qualitatively analyze the underlying reasons for the reduced performance in molecules with longer rotational periods.

DOI
10.1142/s0218863526500268

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Suppressing molecular alignment dephasing is of significant practical value for applications involving nonrigid rotors. In this work, we propose a strategy that employs a train of time-delayed, phase-modulated laser pulses to achieve the elimination of alignment dephasing over an extended temporal window. Our findings demonstrate that for molecules with short rotational periods, such as CO or N 2 used in this study, dephasing can be completely eliminated for at least a dozen revival periods, thereby restoring alignment approaching the rigid-rotor limit. For molecules exhibiting longer revival periods, however, the effectiveness of this approach is somewhat diminished. While complete dephasing elimination is not achieved in such cases, the method remains sufficiently effective for most practical applications. We also qualitatively analyze the underlying reasons for the reduced performance in molecules with longer rotational periods.
In this thesis, we discuss the possibilities for suppression of decoherence, the loss of purity in an open quantum system, and the related process of dephasing. In the first chapter, we review the literature on the subject of open system dynamics, decoherence and methods of decoherence suppression. In the second chapter, we focus on the specific case of a rotationally hot diatomic molecule as an example of an open quantum system, where molecular vibrational wavepackets are subject to dephasing due to rovibrational coupling. We report analytical and numerical results addressing whether the dephasing rate can be controlled by adjustment of the initial wavepacket phases. It appears that over long timescales, phase-only control is not possible, but for earlier timescales the possibility of phase-only control of dephasing remains. In addition, we point out that the time-dependence of the dephasing process depends significantly upon the degeneracy of the rotational environment states. In the final chapter, we discuss the same system, but apply a qualitatively different method of dephasing suppression based on the nonlinear resonance effect in a driving field. We extend previous work on the topic (Shapiro et al. [1]) by considering the effect of using a train of laser pulses as the driving field, in place of a two-colour continuous-wave laser field. We demonstrate that the pulse train method can be more effective at suppressing dephasing than the two-colour CW case.
We theoretically demonstrate that the molecular rotational state populations through an impulsive nonresonant Raman process can be manipulated by shaping the femtosecond laser pulse with a periodic phase-step modulation. We show that, by precisely controlling these parameters characterizing the periodic phase-step modulation, both the odd and even rotational state populations can be completely suppressed or reconstructed as that induced by the transform-limited laser pulse, and the relative excitation between the odd and even rotational state populations can also be obtained. Furthermore, we show that the field-free molecular alignment can be manipulated due to the modulation of the odd and even rotational state populations.
Molecules in selected rotational states can remain well aligned for extended periods of time in the presence of an appropriate periodic train of nonresonant laser pulses. Here, we show that these states can be prepared by slowly switching the electromagnetic field amplitude during a sequence of laser pulses. For low-temperature ensembles, a high degree of alignment can be achieved by designing pulse trains that take into account the distribution of avoided crossings between quasienergy curves. Additionally, we present calculations that illustrate several effects causing misalignment. A discussion of subtleties concerning systems with unbounded rotational spectra is also included.
We theoretically study the control of two-photon excitation to bound and dissociative states in a molecule induced by trains of laser pulses, which are equivalent to certain sets of spectral phase modulated pulses. To this end, we solve the time-dependent Schr\"odinger equation for the interaction of molecular model systems with an external intense laser field. Our numerical results for the temporal evolution of the population in the excited states show that, in the case of an excited dissociative state, control schemes, previously validated for the atomic case, fail due to the coupling of electronic and nuclear motion. In contrast, for excitation to bound states the two-photon excitation probability is controlled via the time delay and the carrier-envelope phase difference between two consecutive pulses in the train.
Quantum control over molecular alignment rephasing is experimentally investigated in gaseous CO2. The control process is achieved by illuminating the medium with a pair of pump-pulses separated in time by approximately an integer value of T0=1/8B0, where B0 is the rotational constant. Through a Raman-type process, each pulse alone produces rotational coherence leading to a periodic orientational anisotropy. It is the combination of the two pulses that yields to quantum interference, resulting in a modification of this anisotropy probed by a third delayed pulse. The effect is accurately analyzed for different time delays between the two pulses. A theoretical analysis supplies a clear understanding of the role played by the different rotational motions involved in the overall process. The relative orientation of the electric field vector for the two pulses is discussed in terms of an additional control parameter.
We present an analysis of two experimental approaches to controlling the directionality of molecular rotation with ultrashort laser pulses. The two methods are based on the molecular interaction with either a pair of pulses (a ``double-kick'' scheme) or a longer pulse sequence (a ``chiral-pulse-train'' scheme). In both cases, rotational control is achieved by varying the polarization of and the time delay between the consecutive laser pulses. Using the technique of polarization-sensitive resonance-enhanced multiphoton ionization, we show that both methods produce significant rotational directionality. We demonstrate that increasing the number of excitation pulses supplements the ability to control the sense of molecular rotation with quantum-state selectivity, i.e., predominant excitation of a single rotational state. We also demonstrate the ability of both techniques to generate counterrotation of molecular nuclear-spin isomers (here, ortho- and para-nitrogen) and molecular isotopologs (here, 14{}^{14}14N2{}_{2}2​ and 15{}^{15}15N2{}_{2}2​).