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arXiv:1411.4511 · Physical Review A

Shaping coherent excitation of atoms and molecules by a train of ultrashort laser pulses

Anahit L. Gogyan, S. Guérin, Yu. P. Malakyan

Published Mar 3, 2010• 9 citations
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Abstract

We propose a mechanism to produce a superposition of atomic and molecular states by a train of ultrashort laser pulses combined with weak control fields. By adjusting the repetition rate of the pump pulses and the intensity of the coupling laser, one can suppress a transition while simultaneously enhancing the desired transitions. As an example, various superpositions of vibrational states of the ${\mathrm{K}}_{2}$ molecule are shown.

DOI
10.1103/physreva.81.033401

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We propose a mechanism to produce a superposition of atomic and molecular states by a train of ultrashort laser pulses combined with weak control fields. By adjusting the repetition rate of the pump pulses and the intensity of the coupling laser, one can suppress a transition while simultaneously enhancing the desired transitions. As an example, various superpositions of vibrational states of the K2{\mathrm{K}}_{2}K2​ molecule are shown.
Efficient excitation of an ensemble of atoms and molecules to a desired state is of fundamental importance not only for spectroscopy but also for quantum control of molecular dynamics in chemical and biological processes. The latter usually occurs on a much shorter time scale (in the femtosecond range) giving rise to steadily advanced problem how to excite one of the nearby levels, even when they are well within the broad fs pulse spectrum. We propose a new effective method for selective excitation of atoms based on the atomic coherence and discuss a sensitive method to verify experimentally the proposed mechanism using the technique of quantum beating in pump-probe experiments.
In this paper, I discuss the selective and efficient vibrational population transfer between electronic states of diatomic molecules by a weak, ultrashort pulse train. The spectrum of each individual pulse in the train is wide enough to simultaneously excite several vibrational levels of the molecule. However, selectivity is achieved by mismatching the frequency comb, associated with the pulse train, to the excited vibrational levels. Efficiency results from the coherent accumulation of population from one pulse in the train to the next.
An excitation scheme is proposed for transferring population between ground-vibrational levels of a molecule. The transfer is accomplished by pumping and dumping population with a pair of coherent ultrashort-pulse trains via a stationary state. By mismatching the teeth of the frequency combs associated with the pulse trains to the vibrational levels, high selectivity in the excitation, along with high transfer efficiency, is predicted. The pump-dump scheme does not suffer from spontaneous emission losses, it is insensitive to the pump-dump-train delay, and it requires only basic pulse shaping.
We present a general and highly efficient scheme for performing narrow-band Raman transitions between molecular vibrational levels using a coherent train of weak pump-dump pairs of shaped ultrashort pulses. The use of weak pulses permits an analytic description within the framework of coherent control in the perturbative regime, while coherent accumulation of many pulse pairs enables near unity transfer efficiency with a high spectral selectivity, thus forming a powerful combination of pump-dump control schemes and the precision of the frequency comb. Simulations verify the feasibility and robustness of this concept, with the aim to form deeply bound, ultracold molecules.
The main goal of this PhD work is an experimental study of coherent excitation of atomic and molecular wavepackets, i.e. superpositions of many quantum eigenstates, by shaped femtosecond pulses. Approaches allowing nearly complete population transfer between quantum eigenstates were well studied in the past within the two level approximation. In this work we focus on adiabatic and non-adiabatic methods of population transfer beyond the two-level approximation. Excitation of multi-level target states is possible due to broad spectrum of an ultrashort pulse which contains frequencies needed for multiple transitions to different states in the final superposition. At the same time, the spectrum of an ultrashort pulse can be modified, or ``shaped'', in order to affect the excitation process and control the amplitudes in the final superposition. Both non-adiabatic and quasi-adiabatic methods were first implemented and studied in electronic wavepackets in alkali atoms. The non-adiabatic approach revealed features linked to the strong-field perturbations of the energy level structure of the quantum system. An adiabatic method was implemented for the first time on a femtosecond time scale, and was thoroughly characterized. The control over complex amplitudes in the target superposition was demonstrated as well as completeness of the population transfer. In the second part of this work, we focused on coherent control of rotational wavepackets in diatomic molecules. Rotational excitation by a periodic train of femtosecond pulses was investigated in the context of ``delta-kicked'' rotor - a paradigm system for studying quantum chaos, and the effect of quantum resonance was demonstrated for the first time in a system of true quantum rotors. Control of uni-directional molecular rotation was proposed and demonstrated with a novel ``chiral pulse train'' - a sequence of femtosecond pulses with polarization rotating from pulse to pulse by a predefined angle. All the developed techniques offer new tools in coherent control of atomic and molecular wavepackets on an ultrashort time scale.
We have demonstrated that coherent molecular modulation can result in the collinear generation of mutually-coherent spectral sidebands that extend in frequency from the infrared to the far ultraviolet. Our technique is based on adiabatic preparation of a highly coherent molecular superposition-state, which is achieved by using narrow-linewidth lasers slightly detuned from a Raman resonance. The phases of the resultant Stokes and anti-Stokes sidebands are adjusted in order to synthesize desired single-cycle pulse trains at the target. In this article we review recent improvements and developments in this area, including: techniques for increasing the number of generated sidebands; synchronization of the pulse trains with the molecular motion in the given molecular system; laser self-focusing and spatial soliton formation due to the coherent interaction of light with oscillating molecules. In the future, this Raman source may produce sub-cycle optical pulses, and allow synthesis of waveforms where the electric field is a predetermined function of time, not limited to a quasi-sinusoidal oscillation.