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Conference on Lasers and Electro-Optics

Building electronic wave packets from scratch

R. R. Jones

Published Nov 27, 2002• 0 citations
Publisher

Abstract

Summary form only given. Unipolar half-cycle pulses (HCPs) can be used to alter the momentum distribution of atomic electrons over subpicosecond timescales. Previously, momentum kicks from HCPs were used to produce electronic wave packets from highly excited Rydberg eigenstates. In addition, these pulses have been used to kick bound electrons into the continuum, making it possible to monitor the time-dependent momentum distribution of electronic wave packets. Conversely, our most recent experiments have shown that HCPs can extract momentum from free electrons, facilitating their capture by positive ions. In the experiments, calcium atoms in the singlet 4s4p level are photoionized into the continuum using a 1.5ps, 390-nm laser pulse.

DOI
10.1109/iqec.1998.680294

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Summary form only given. Unipolar half-cycle pulses (HCPs) can be used to alter the momentum distribution of atomic electrons over subpicosecond timescales. Previously, momentum kicks from HCPs were used to produce electronic wave packets from highly excited Rydberg eigenstates. In addition, these pulses have been used to kick bound electrons into the continuum, making it possible to monitor the time-dependent momentum distribution of electronic wave packets. Conversely, our most recent experiments have shown that HCPs can extract momentum from free electrons, facilitating their capture by positive ions. In the experiments, calcium atoms in the singlet 4s4p level are photoionized into the continuum using a 1.5ps, 390-nm laser pulse.
Ultrashort, nearly unipolar electric field pulses are used to produce novel dynamic states in Rydberg atoms. A single 500 fsec field pulse redistributes essentially all of the population from a single stationary Rydberg state to hundreds of different final states. The dynamic evolution of the nonstationary wave packet is monitored using a second, time-delayed pulse. The time-dependent momentum-space probability distribution of the wave packet can be retrieved experimentally using a classical impulse approximation.
A kick from a unipolar half-cycle pulse (HCP) can redistribute population and shift the relative phase between states in a radial Rydberg wave packet. We have measured the quantum coherence properties following the kick, and show that selected coherences can be destroyed by applying a HCP at specific times. Quantum mechanical simulations show that this is due to redistribution of the angular momentum in the presence of noise. These results have implications for the storage and retrieval of quantum information in the wave packet.
We propose a method of producing high-purity angular-momentum-state Rydberg atoms by a pair of unipolar laser pulses. The first positive-polarity optical half-cycle pulse is used to prepare an excited-state wave packet while the second one is less intense, but with opposite polarity and time delayed, and is employed to drag back the escaping free electron and clip the shape of the bound Rydberg wave packet, selectively increasing or decreasing a fraction of the angular-momentum components. An intelligent choice of laser parameters such as phase and amplitude helps us to control the orbital-angular-momentum composition of an electron wave packet with more facility; thus, a specified angular-momentum state with high purity can be achieved. This scheme of producing high-purity angular-momentum-state Rydberg atoms has significant application in quantum-information processing.
Nearly unipolar, subpicosecond half-cycle pulses (HCPs) have been used to ionize Ba+{\mathrm{Ba}}^{+}Ba+ ngngng Rydberg ions. The double ionization probability vs HCP field strength data are compared with analogous single ionization measurements for neutral Ba 6snd6snd6snd Rydberg atoms prepared in the same apparatus. We find that neutral and ionic Rydberg states with the same binding energies follow approximately the same ionization curves. Our findings are in agreement with the predictions of an impulsive ionization model and classical trajectory Monte Carlo simulations.
Rydberg wave packets generated by a single half-cycle pulse (HCP) interacting with a stationary Rydberg state periodically localize within a narrow region in both momentum and coordinate space (i.e., phase space). This property is the key to shaping and manipulating wave packets. We investigate the dependence of this transient localization and focusing on the principal quantum number n and on the strength of the ultrashort HCP (``kick strength,'' $\ensuremath{\Delta}p).$ It is shown that the localization in momentum space of the wave packet, quantified by the momentum width, obeys a universal scaling rule which is controlled by an effective Planck constant ${\ensuremath{\Elzxh}}_{\mathrm{eff}}.$ Maximum focusing is reached in the classical limit $({\ensuremath{\Elzxh}}_{\mathrm{eff}}\ensuremath{\rightarrow}0).$
We study the dynamics of highly excited (Rydberg) atoms in laboratory-scale magnetic fields via their ionization by THz half-cycle pulses (HCPs). A half-cycle pulse is a unipolar electromagnetic pulse that can probe an atomic electron at all points along its orbit around the atomic core. The dynamical evolution of diamagnetic Zeeman states subjected to an ultrashort (picosecond) HCP is calculated for both hydrogen and sodium atoms. Diamagnetic Zeeman states are localized parallel and perpendicular to the magnetic field. We use a grid-based pseudo-potential method to determine these eigenstates of the combined Coulomb-diamagnetic potential. We study states with principal quantum number n between 15--30 in the ℓ-mixing regime at magnetic fields of 1--6T. These initial states are subjected to the electric field of an HCP and their subsequent time evolution is calculated by solving the time-dependent Schrodinger equation. The HCP width is varied between the short pulse (impulsive) and the long pulse limit, keeping the total momentum transferred a constant. We calculate the total ionized fraction, and also the spectrum of the ionized photoelectrons. We find that, in the impulsive limit, where the width of the pulse is much smaller than typical time scales in the system, the calculated amount of ionization and the photoelectron spectra are different depending on the localization of the initial state. These differences are shown to be due to the different initial momentum distributions of the parallel and perpendicular states. As the duration of the HCP is made longer, we find that ionization is suppressed as compared with the impulsive limit. The states localized perpendicular to the magnetic field are found to be much more sensitive to the HCP width than the parallel states, reflecting the fact that the two classes of states interact with different parts of the Coulomb-diamagnetic potential during the HCP. The ionization characteristics are shown to scale classically with the principal quantum number. The results are presented in such a manner that they can be verified by experiment.