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The Journal of Chemical Physics

Controlling molecular alignment rephasing through interference of Raman-induced rotational coherence

E. Hertz, O. Faucher, B. Lavorel, R. Chaux

Published Oct 15, 2000• 14 citations
Publisher

Abstract

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.

DOI
10.1063/1.1309526

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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 experimental and theoretical investigation of the quantum control of ground-state rotational coherence in a linear molecule. A sequence of two temporally separated laser pulses creates a rotational superposition state in CO2{\mathrm{CO}}_{2}CO2​ whose evolution is monitored through a polarization technique. We study the influence of the phase difference between the two pulses. We show that the overlapping of the two wave packets, produced by each pulse, gives rise to quantum interference that affects the orientational anisotropy of the sample. Because of the large number of coherently excited levels, the interference produces well-separated temporal structures, whose magnitude can be controlled through the relative delay of the two pulses.
We present an experimental study of molecular alignment echoes driven by ultrashort structured light fields carrying orbital angular momentum (OAM). By combining two time-delayed pump pulses with spatially varying polarization, we produce a molecular "q plate" in a gaseous CO_{2} sample that can be rephased in a controllable manner. This approach enables the storage of OAM information within the rotational coherences of molecules and its retrieval at adjustable times. The concept is validated using vortex beams of various topological charges ℓ=±1, 2. These results extend the mechanism of molecular alignment echoes to vector light fields and establish a new route for optical processing of ultrashort OAM pulses, advancing molecular-based quantum interfaces for structured light.
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.
We demonstrate a scheme for the preparation of molecular alignment and angular momentum orientation using a hybrid combination of two limits of Raman scattering. First a weak, impulsive pump pulse initializes the system via the nonresonant dynamic Stark effect. Then, having overcome the influence of the vacuum fluctuations, an amplification pulse selectively enhances the initial coherences by transient stimulated Raman scattering, generating alignment and angular momentum orientation of molecular hydrogen. The amplitude and phase of the resulting coherent dynamics are experimentally probed, indicating an amplification factor of 4.54.54.5. An analytic theory is developed to model the dynamics.
We present a theoretical study of the induced transient birefringence of a low density homogeneous molecular gas in a resonant pump–probe experiment. The molecular coherent state induced by the resonant pump field is described by second-order perturbation theory. The induced birefringence can be detected by a delayed probe pulse propagating through the molecular medium after illumination by the pump pulse. In the case of a nonresonant probe, the birefringence is linearly proportional to the mean value of the electronic polarizability of the molecular gas. The birefringence signal is composed of distinct components due to population change and those of rotational, vibrational, and mixed vibrational–rotational origins. This is demonstrated by numerical simulations on Li2 gas. Moreover, the quantum beats contained in the birefringence, as a function of the time delay between the pump and probe pulses, is dominated by the pure rotational motion. Finally, the birefringence is sensitive to the shape of the applied pump pulse and dependent on the spectral phase of the pump pulse.
We introduce a new scheme for controlling the sense of molecular rotation. By varying the polarization and the delay between two ultrashort laser pulses, we induce unidirectional molecular rotation, thereby forcing the molecules to rotate clockwise/counterclockwise under field-free conditions. We show that unidirectionally rotating molecules are confined to the plane defined by the two polarization vectors of the pulses, which leads to a permanent anisotropy in the molecular angular distribution. The latter may be useful for controlling collisional cross-sections and optical and kinetic processes in molecular gases. We discuss the application of this control scheme to individual components within a molecular mixture in a selective manner.