Coherent Control and Analysis of Atomic Multi-Photon Processes

Missouri University Of Science And Technology, Rolla MO

Investigators

Abstract

Coherent control aims at manipulating the outcome of quantum processes such as the excitation or ionization of atoms, chemical reactions, or collective many-particle processes by using light. Applications range from medical imaging techniques (magnetic resonance imaging) to structural and dynamical analysis of large biomolecules (magnetic resonance spectroscopy) to quantum information processing (quantum computing). Present techniques rely on the shaping of the microscopic properties of the light, such as the temporal structure of the light pulses as well as the electric field frequency, direction, and strength corresponding to the photons' energy, spin polarization, and density, respectively. In this project, the senior investigators and graduate students will advance established coherent control schemes and develop new ones to control the electron dynamics in photo-absorption processes in one of the simplest atomic systems – a lithium atom with a single active electron. In the new experiments, previous limitations will be overcome by shaping the macroscopic properties of the laser beam. Specifically, the laser field wavefronts will be altered, creating so-called electromagnetic "vortex" laser beams, which carry orbital angular momentum in the direction of propagation, thereby opening additional ionization pathways which are “forbidden” in conventional photo-absorption processes. Shaping ALL properties of laser pulses, including their wavefronts, will significantly expand the available scientific toolbox to induce and observe specific quantum pathways and it will unlock new dials for the coherent control of electron dynamics governing many processes in nature. On a more technical level, lithium atoms are trapped in an all-optical trap and subjected to femtosecond laser pulses. In a first step, the microscopic properties of the femtosecond laser fields will be altered, e.g., by creating tunable sequences of bichromatic laser pulses with variable polarization. This will enable enhancing specific multiphoton absorption pathways, studying time-dependent Rydberg dynamics, or creating exotic electronic wave packets. In the second step, the macroscopic properties of the laser beam will be shaped, and optical vortex beams carrying internal orbital angular momentum will be generated using holography. The goal is to couple this angular momentum to the electrons, thereby opening dipole-forbidden ionization pathways. The imprint of the optical vortices on the ejected electron wave packet will be analyzed employing a "reaction microscope", allowing the measurement of the momentum vectors of atomic fragments. Despite recent theoretical predictions, experimental evidence on non-dipole transitions in photoionization, i.e., on bound-free transitions, are, to date, entirely absent. This lack of data is not surprising considering the formidable experimental challenge to position a (microscopic) atom accurately in the center of a helical laser beam. The new experiments aim to overcome this obstacle and provide the first evidence on orbital angular momentum transfer in photoionization processes, thereby providing a benchmark for theoretical descriptions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.