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Journal of Physics B Atomic Molecular and Optical Physics

Polarization-dependent pump-probe studies in atomic fine-structure levels: towards the production of spin-polarized electrons

Emma J. Sokell, Sébastien Zamith, M. A. Bouchène, B. Girard

Published May 16, 2000• 21 citations
Publisher

Abstract

The precession of orbital and spin angular momentum vectors has been observed in a pump-probe study of the 4P fine-structure states of atomic potassium. A femtosecond pump pulse prepared a coherent superposition of the two fine-structure components. A time-delayed probe pulse then ionized the system after it had been allowed to evolve freely. Oscillations recorded in the ion signal reflect the evolution of the orientation of the orbital and spin angular momentum due to spin-orbit coupling. This interpretation gives physical insight into the cause of the half-period phase shift observed when the relative polarizations of the laser pulses were changed from parallel to perpendicular. Finally, it is shown that these changes in the orientation of the spin momentum vector of the system can be utilized to produce highly spin-polarized free electrons on the femtosecond scale

DOI
10.1088/0953-4075/33/11/303

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•Jan 1, 2006•0 citations
The precession of orbital and spin angular momentum vectors has been observed in a pump-probe study of the 4P fine-structure states of atomic potassium. A femtosecond pump pulse prepared a coherent superposition of the two fine-structure components. A time-delayed probe pulse then ionized the system after it had been allowed to evolve freely. Oscillations recorded in the ion signal reflect the evolution of the orientation of the orbital and spin angular momentum due to spin-orbit coupling. This interpretation gives physical insight into the cause of the half-period phase shift observed when the relative polarizations of the laser pulses were changed from parallel to perpendicular. Finally, it is shown that these changes in the orientation of the spin momentum vector of the system can be utilized to produce highly spin-polarized free electrons on the femtosecond scale
This paper presents a theoretical analysis of the possibilities of producing spin-polarized electrons with a sequence of two ultra-short time-delayed laser pulses. The first pulse, which is right circularly polarized, excites resonantly the fine structure n p level of potassium atoms leading to a spin-flip and thus polarizing the atom. The second pulse ionizes the system thereby resulting in spin-polarized electron release. We examine and compare several situations corresponding to different polarizations of the ionizing pulse (σ + ,σ - and π). For each case we derive analytical expressions for the angular and global electron spin-polarization rates as well as for the differential cross sections. The obtained rates can be very high (up to 100%) and can be controlled accurately on the femtosecond time scale by varying the time delay between the pulses.
We present a scheme to control the spin-polarization of photoelectrons/photoions using short laser pulses. It is based on the pump-probe method. By exciting fine structure manifolds of a system by the short pump pulse with a sufficient bandwidth, a superposition of fine structure states is created. After the pump pulse, the coherently superposed excited state evolves in time under the field-free condition with a period determined by the inverse of energy difference between them. In terms of the spin state, this suggests that the different spin state evolves differently in time, leading to the time-varying spin-polarization. Therefore, varying the time delay between the pump and probe pulses leads to the control of spin states upon photoionization. Specific theoretical results are presented for two-valence-electron atoms, in particular for Mg, which demonstrate that, under certain conditions, not only the degree of spin-polarization but also its polarity can be manipulated through time delay. Furthermore, we propose a new technique to accelerate the coupling time by the introduction of a dressing laser with a few ns duration. Since the underline physics is rather general and transparent, the presented scheme may be potentially applied to nanostructures such as quantum wells and quantum dots.
The complexity of ultrafast molecular photoionization presents an obstacle to the modeling of pump-probe experiments. Here, a simple optimized model of atomic rubidium is combined with a molecular dynamics model to predict quantitatively the results of a pump-probe experiment in which long-range rubidium dimers are first excited, then ionized after a variable delay. The method is illustrated by the outline of two proposed feasible experiments and the calculation of their outcomes. Both of these proposals use Feshbach 87Rb^{87}\mathrm{Rb}87Rb2{}_{2}2​ molecules. We show that long-range molecular pump-probe experiments should observe spin-orbit precession given a suitable pump pulse, and that the associated high-frequency beat signal in the ionization probability decays after a few tens of picoseconds. If the molecule was to be excited to only a single fine-structure state, then a low-frequency oscillation in the internuclear separation would be detectable through the time-dependent ionization cross section, giving a mechanism that would enable observation of coherent vibrational motion in this molecule.
We combine bichromatic polarization pulse shaping with photoelectron imaging tomography for time-resolved spatial imaging of ultrafast spin–orbit wave packet (SOWP) dynamics in atoms. Polarization-shaped two-color pump-probe sequences are generated by spectral amplitude and phase modulation of a femtosecond input pulse and used to excite SOWPs in the potassium fine-structure doublet. By selecting different spectral bands for pump and probe pulse, we achieve interference-free detection of the spatiotemporal SOWP dynamics. Using tomographic techniques, we reconstruct the three-dimensional photoelectron momentum distribution (3D-ED) created by the probe pulse. Time-resolved measurement of the 3D-ED reveals the orbital realignment dynamics induced by spin–orbit interaction in the neutral atom.
Optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization is studied experimentally and theoretically. For that purpose the coherent electron spin dynamics in a nnn-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state. The amplitude and phase shifts of the electron spin beat signal in an external magnetic field, that are induced by laser control pulses, depend on the pump-control delay and polarization of the control relative to the pump pulse. Additive and nonadditive contributions to pump-induced signal due to the control are isolated experimentally. These contributions can be well described in the framework of a two-level model for the optical excitation of the resident electron to the trion.
Recent advances in the study of Rydberg atoms have focused on the control, manipulation and detection of Rydberg wave packets using novel external fields such as half-cycle pulses. The radial degree of freedom has been controlled and used to process information encoded in Rydberg states. However, these previous experiments make use of only a single degree of freedom, restricting the potential of other degrees of freedom for quantum computing in Rydberg atoms. In this dissertation, we explore the control and detection of other degrees of freedom in an electron wave packet, such as the angular momentum quantum number ℓ, the magnetic quantum number m and the electron spin; so that the full range of quantum numbers can participate in information processing. We first propose an interferometric control of the population of angular momentum states using two time-delayed phase-locked ultrafast laser pulses. The population of arbitrary angular momentum states can be greatly enhanced by optimizing the time delay and the relative phases between two laser pulses. We then qualitatively measure the evolution of angular momentum components in Stark wave packets by a weak half-cycle pulse (HCP). This measurement utilizes a time-delayed HCP and is proved to be effective for detecting various aspects of wave packet dynamics, particularly, the evolution of non-stationary states. The technique relies on the fact that the HCP redistributes the eigenstate populations and induced the population variation which reflects the evolution of eigenstate phases. Finally, we find that the dynamics of m-states could be highly correlated with the internal degree of freedom of the electron, the spin. We study the effect of spin-orbit coupling on the wave packet dynamics and observe the angular precession of a Rydberg wave packet. The population redistribution from p to s states is highly sensitive to the polarization of the HCP and changes with the precession of the electron orbit. We obtain the selection rules and give an intuitive classical interpretation of angular momentum redistribution by an HCP. In short, new dynamics involving multiple degrees of freedom of an electron wave packet are discovered, observed and understood.