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Physical Review A

Optical control of collisional population flow between molecular electronic states of different spin multiplicity

Ergin H. Ahmed, Xinhua Pan, John Huennekens, A. M. Lyyra

Published Jun 16, 2014• 7 citations
Publisher

Abstract

In this work we demonstrate optical control of the singlet-triplet probability distribution in the outcome of a collisional process involving lithium dimers and argon atoms. The control is achieved using the Autler-Townes effect to manipulate the spin character of a spin-orbit coupled pair of levels serving as a ``gateway'' between the singlet and triplet electronic state manifolds.

DOI
10.1103/physreva.89.061401

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In this work we demonstrate optical control of the singlet-triplet probability distribution in the outcome of a collisional process involving lithium dimers and argon atoms. The control is achieved using the Autler-Townes effect to manipulate the spin character of a spin-orbit coupled pair of levels serving as a ``gateway'' between the singlet and triplet electronic state manifolds.
We have demonstrated quantum control of the spin-orbit interaction based on the Autler-Townes (ac-Stark) effect in a molecular system using a cw optical field. We show that the enhancement of the spin-orbit interaction between a pair of weakly interacting singlet-triplet rovibrational levels, G (1)Π(g)(v=12,J=21,f)-1 (3)Σ(g)(-)(v=1,N=21,f), separated by 750 MHz in the lithium dimer, depends on the Rabi frequency (laser power) of the control laser. The increase in the spin-orbit interaction due to the control field is observed as a change in the spin character of the individual components of the perturbed pair.
In recent years, quantum optics has been expanded from atomic to molecular systems despite much weaker oscillator strengths and complex relaxation pathways that have presented serious challenges in the past. The richness of molecular excitation pathways and the variety of molecular interactions has made it possible to develop novel applications in this field. We have demonstrated how the Autler–Townes effect can be used to control molecular angular momentum alignment and how the Autler–Townes split line shape, combined with accurate control-laser electric field amplitude measurement, can be used to map the absolute magnitude of the molecular internuclear distance dependent electronic transition dipole moment function. In addition, the electric field amplitude in the control laser Rabi frequency can be used as a ‘tuning’ mechanism for the mixing coefficients of energy levels that are weakly perturbed by the spin–orbit interaction, i.e. to control the valence electron spin polarization and the spin multipli...
We have demonstrated Autler-Townes splitting in molecular lithium using continuous wave triple resonance spectroscopy. The Autler-Townes split double resonance lineshape consists of a superposition of several narrower twin peaks, one for each | M j |. The splitting is proportional to | M j | enabling separation of the individual Mj peaks. Thus, excitation of an individual magnetic sublevel can be used to align the angular momentum of nonpolar molecules all-optically. We also propose a novel quantum control scheme based on using a coupling laser to create dressed states of controllable molecular character. Preliminary calculations are presented for variable singlet triplet character.
We show that spin-orbit coupling in a quantum dot molecule allows for coherent manipulation of two-electron spin states using Raman transitions. Such two-electron spin states defined by the singlet and triplet states of two exchange-coupled quantum dots can have favorable coherence properties. In addition, two of the four metastable ground states in this system can be used as auxiliary states that could facilitate implementation of tasks such as mapping of spin states to that of a single propagating photon. We find that even weak spin-orbit effects---manifesting themselves as slightly different ggg factors for the electron and the hole---would allow for the coherent Raman coupling of the singlet-triplet states. We also discuss the possibilities for implementing quantum optical techniques for spin preparation and manipulation.
This paper describes an extension of the familiar coherence effects from atomic systems to the molecular regime. Such effects are inherent in the interaction of multiple laser fields with molecular systems. We have observed Autler–Townes splitting and the AC Stark shift in diatomic Lithium using the continuous wave all-optical triple resonance (AOTR) techniques. By using the Autler–Townes effect, we have partially resolved the magnetic sublevels of a molecular rovibrational level in a Doppler broadened sample, allowing all-optical alignment of the angular momentum in excited states of nonpolar molecules. The Autler–Townes effect in a molecular system extends the rovibrational state selectivity of the AOTR excitation technique to magnetic sublevels. PACS Nos.: 33.40tf, 42.50Hz
Coherent control beyond population control and spectral interferences is demonstrated on the interferences and intensity of the two Autler-Townes (AT) components in the photoelectron spectrum of K atoms, using a sequence of two intense time-delayed femtosecond laser pulses. Photoelectron spectra were taken at various delay times between the two laser pulses and at different laser intensities at a fixed delay time. With respect to the interferences in the AT doublet the role of time delay and laser intensity is interchangeable for $(n+0.5)\ensuremath{\pi}$ excitation. Strong laser fields or the optical phase of the delayed laser pulse allow the quantum mechanical phase of an atomic state to be manipulated in a symmetrical fashion. The observations are discussed in terms of a two-level model coupled to the continuum. For suitable combinations of the laser intensity of the first pulse and the time delay, the second laser pulse leaves the excited state population unchanged.