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Physical Review Letters

Quantum Control by Ultrafast Polarization Shaping

Tobias Brixner, Gerhard Krampert, Thomas Pfeifer, Reimer Selle, G. Gerber, M. Wollenhaupt, O. Graefe, C. Horn, D. Liese, Thomas Baumert

Published May 20, 2004• 277 citations
Publisher

Abstract

We demonstrate that the use of time-dependent light polarization opens a new level of control over quantum systems. With potassium dimer molecules from a supersonic molecular beam, we show that a polarization-shaped laser pulse increases the ionization yield beyond that obtained with an optimally shaped linearly polarized laser pulse. This is due to the different multiphoton ionization pathways in K2 involving dipole transitions which favor different polarization directions of the exciting laser field. This experiment is a qualitative extension of quantum control mechanisms which opens up new directions giving access to the three-dimensional temporal response of molecular systems.

DOI
10.1103/physrevlett.92.208301

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We demonstrate that the use of time-dependent light polarization opens a new level of control over quantum systems. With potassium dimer molecules from a supersonic molecular beam, we show that a polarization-shaped laser pulse increases the ionization yield beyond that obtained with an optimally shaped linearly polarized laser pulse. This is due to the different multiphoton ionization pathways in K2 involving dipole transitions which favor different polarization directions of the exciting laser field. This experiment is a qualitative extension of quantum control mechanisms which opens up new directions giving access to the three-dimensional temporal response of molecular systems.
This work focuses on the developments of ultrafast laser pulse shaping techniques and their application to diatomic molecular systems in order to reveal fundamental effects in light matter interaction. It describes the development of a new pulse shaper consisting of a sequence of four liquid crystal arrays and a polarizer. This pulse shaper is the first non- interferometric setup for unrestricted phase, amplitude, and polarization shaping. Moreover, a parametric encoding of the electrical field was developed which allowed for generating pulse sequences consisting of several sub-pulses. Each sub-pulse can be controlled in its physically apparent parameters: energy, position in time, phase, and chirps as well as state of polarization with ellipticity, orientation, and helicity. The feasibility of the setup is illustrated by systematic variations of single and double pulses, which are experimentally generated and measured. In these series of pulses, one of the pulses parameters is varied while the other parameters are kept constant. This proves the precise control of the pulse shape. Further, more complex pulses are shown to give an impression of the capabilities of this setup in combination with the parameterization. For some fields of application, it could be desirable to guide the phase, amplitude and polarization shaped pulses through an optical fiber. For example, life science could benefit from shaped pulses in the area of imaging and photodynamic therapy. In order to make the shaped pulses available at the distal end of an optical fiber, the effects of the fiber have to be compensated. In this work, the generation of phase, amplitude, and polarization shaped pulses after transmission trough an optical fiber is presented. This is demonstrated for two types of optical fibers – a standard single-mode fiber and a microstructured hollow core photonic crystal fiber. The procedure for the determined generation of parametrically shaped pulses differs for both types of optical fibers. The capabilities of parametrically shaped pulse sequences are – again – illustrated by a series of example pulses. The parametrically shaped pulses transmitted through the hollow core fiber are implemented in a coherent control experiment. For this purpose, the potassium dimer produced in a molecular beam serves as a test system. The multi-photon ionization is studied by a closed feedback loop optimization and shaper-assisted pump-probe spectroscopy. These experiments revealed the excitation path including the vibrational dynamics in the first and second excited state. Further, the relevance of the polarization control in the excitation process is highlighted. Moreover, coherent control techniques are applied to investigate the interaction of shaped femtosecond laser pulses with ultracold atoms and molecules. The focus of these experiments is the control of the photoassociation of a colliding atom pair to a bound molecule. This process is investigated using two-color pump-probe spectroscopy. The molecular transients are compared to theoretical calculations. The analysis of this data reveals the interaction process which is discussed in the time and frequency domain. In a second experiment, the multi-photon ionization of ultracold rubidium dimers is optimized in a closed feedback loop. In this optimization, a parameterization in the frequency domain is employed which extracts the relevant transition frequencies. In combination with a complementary experiment the excitation pathway was revealed.
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Quantum coherent control of ultrafast bond making and the subsequent molecular dynamics is crucial for the realization of a new photochemistry, where a shaped laser field is actively driving the chemical system in a coherent way from the thermal initial state of the reactants to the final state of the desired products. We demonstrate here coherent control over the relative yields of Mg2 molecules that are generated via photoassociation and subsequently photodriven into different groups of final states. The strong-field process involves non-resonant multiphoton femtosecond photoassociation of a pair of thermally hot magnesium atoms into a bound Mg2 molecule and subsequent molecular dynamics on electronically excited states. The branching-ratio control is achieved with linearly chirped laser pulses, utilizing the different chirp dependence that various groups of final molecular states display for their post-pulse population. Our joint experimental and theoretical study establishes the feasibility of high degree coherent control over quantum molecular dynamics that is initiated by femtosecond photoassociation of thermal atoms.
Abstract Quantum coherent control of ultrafast bond making and the subsequent molecular dynamics is crucial for the realization of a new photochemistry, where a shaped laser field is actively driving the chemical system in a coherent way from the thermal initial state of the reactants to the final state of the desired products. We demonstrate here coherent control over the relative yields of Mg 2 molecules that are generated via photoassociation and subsequently photodriven into different groups of final states. The strong-field process involves non-resonant multiphoton femtosecond photoassociation of a pair of thermally hot magnesium atoms into a bound Mg 2 molecule and subsequent molecular dynamics on electronically excited states. The branching-ratio control is achieved with linearly chirped laser pulses, utilizing the different chirp dependence that various groups of final molecular states display for their post-pulse population. Our joint experimental and theoretical study establishes the feasibility of high degree coherent control over quantum molecular dynamics that is initiated by femtosecond photoassociation of thermal atoms.
In a typical experiment aiming to control quantum dynamics phenomena, each molecule experiences the same temporal laser field, but with an amplitude that depends on the spatial location and orientation of the molecule in the laser beam. It is proved under commonly arising conditions that at least one optimal laser field exists which will control all molecules in the sample, regardless of their orientation or spatial location. The optimal laser field may consist of a multipolarization control containing up to three orthogonal, independently shaped components. The analysis also includes the prospect of multipartite control where the field couples distinct groupings of states (e.g., multiple vibronic states), but without direct coupling within a group of states. This conclusion shows that achieving quantum control is not a matter of striking a compromise over the sample diversity, but rather a task subject to optimization to reach the highest possible level of control for all molecules in the sample.