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Journal of Physics B Atomic Molecular and Optical Physics

On angular coherent control of photocurrent caused by photoionization in a bichromatic laser field

V A Pazdzersky, V I Usachenko, A V Ushnurtsev

Published Aug 24, 2001• 2 citations
Publisher

Abstract

A novel kind of coherent control of various atomic and molecular photoprocesses proceeding in a bichromatic electromagnetic (EM) laser field, angular coherent control , is proposed and demonstrated here theoretically by studying the particular case of negative-ion photodetachment in the field of a linearly polarized fundamental laser frequency and its second harmonic of arbitrary spatial configuration and orientation of the unit polarization vectors e 1 and e 2 of the respective harmonic fields. Due to the inherent spatial asymmetry of the applied bichromatic field, the ejection of detached photoelectrons is also generally asymmetric in respect of the detached photoelectron momentum replacement p →- p . As follows from our gauge-invariant analytical calculations within the framework of perturbation theory on EM interaction and a zero-range potential model for the negative ion, this field asymmetry results in a photocurrent, the magnitude and direction of which are strongly dependent on both the relative phase shift Φ between the harmonics of the ionizing bichromatic field and on the angle θ between the polarization vectors e 1 and e 2 . The latter strong dependence on the angle θ is the background for angular control of the direction and magnitude of the produced photocurrent by means of an appropriate choice and proper adjustment of the angle between the polarization vectors of the incident bichromatic field harmonics.

DOI
10.1088/0953-4075/34/17/311

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•Sep 7, 2015•0 citations
A novel kind of coherent control of various atomic and molecular photoprocesses proceeding in a bichromatic electromagnetic (EM) laser field, angular coherent control , is proposed and demonstrated here theoretically by studying the particular case of negative-ion photodetachment in the field of a linearly polarized fundamental laser frequency and its second harmonic of arbitrary spatial configuration and orientation of the unit polarization vectors e 1 and e 2 of the respective harmonic fields. Due to the inherent spatial asymmetry of the applied bichromatic field, the ejection of detached photoelectrons is also generally asymmetric in respect of the detached photoelectron momentum replacement p →- p . As follows from our gauge-invariant analytical calculations within the framework of perturbation theory on EM interaction and a zero-range potential model for the negative ion, this field asymmetry results in a photocurrent, the magnitude and direction of which are strongly dependent on both the relative phase shift Φ between the harmonics of the ionizing bichromatic field and on the angle θ between the polarization vectors e 1 and e 2 . The latter strong dependence on the angle θ is the background for angular control of the direction and magnitude of the produced photocurrent by means of an appropriate choice and proper adjustment of the angle between the polarization vectors of the incident bichromatic field harmonics.
The interference phenomena which can take place at the negative ion photodecay in a bichromatic electromagnetic (EM) field of linear and circular polarization consisting of coherent superposition of commensurable th and th harmonics of the same fundamental field are studied within the confines of the theory of perturbation on EM interaction and a zero-range potential model for the negative ion. The case of both odd and and that of odd and even (or vice versa) have been considered. It has been shown that, in the first case, the angular distribution of the emitted photoelectrons is always completely polar symmetric (in respect of the replacement , here p is the photoelectron momentum) and that the total photodecay probability is a periodic function of phase shift between harmonics. However, in the second case of odd and even , the angular distribution has polar asymmetry the degree of which depends on the values of and ratio of the electric vector strengths of the harmonics. Unlike the first case however, the total photodecay probability does not depend on phase shift at all. Moreover, the importance of taking into account the intermediate intercontinuum EM transitions (accompanied with the photoelectron momentum and energy change) has been shown too.
Coherent control of the photoelectron angular distribution (PAD) in ionization by a bichromatic circularly polarized field is theoretically studied in neon in the photon energy range 16--20 eV. The breakdown of the PAD axial symmetry with respect to the light propagation direction is analyzed. The coherent control of the PAD manifests itself through: (1) a change from a one-lobe to a three-lobe shape, respectively, for co- and counter-rotating harmonics, (2) a variation of the polar asymmetry with the light frequency, and (3) a rotation of the PAD around the direction of the beam depending on the relative phase between the harmonics. Tuning the lowest harmonic at the 2p53s2{p}^{5}3s2p53s or 2p54s2{p}^{5}4s2p54s intermediate excited states strongly influences the PAD. In contrast to atomic hydrogen and the alkali atoms, the cases of equal and opposite helicity are not related in neon. Features of a conjugate polarization setup, when one harmonic is linearly polarized while the other one is circularly polarized, are also discussed.
We demonstrate a left-right asymmetry control of the photoelectron angular distribution in multiphoton ionization of Li atoms by a bichromatic laser field. By delaying the fundamental (780 nm) and its second harmonic relative to each other in steps of 130 attoseconds, we can vary the relative phase between the two laser fields with subwavelength accuracy and thereby steer the ejected electrons. Good agreement is found between the measurements and calculations at the appropriate intensities of the two harmonics. Published by the American Physical Society 2024
We investigate the coherent control of the photoelectron angular distribution in bichromatic atomic ionization. Neon is selected as target since it is one of the most popular systems in current gas-phase experiments with free-electron lasers (FELSs). In particular, we tackle practical questions, such as the role of the fine-structure splitting, the pulse length, and the intensity. Time-dependent and stationary perturbation theory are employed, and we also solve the time-dependent Schr\"odinger equation in a single-active electron model. We consider neon ionized by a FEL pulse whose fundamental frequency is in resonance with either $2p\text{\ensuremath{-}}3s$ or $2p\text{\ensuremath{-}}4s$ excitation. The contribution of the nonresonant two-photon process and its potential constructive or destructive role for quantum coherent control is investigated.
Abstract We investigate features of electron emission from atoms in the field of the fundamental and second harmonic of a laser in conditions relevant to those realized in free-electron lasers. General expressions for the angular anisotropy and photoelectron spin polarization are obtained. These expressions are used for describing of the ionization of the valence shell of inert gas atoms and illustrated by calculations for neon. The possibility of control over the electron emission characteristics by varying the relative phase between harmonic (i.e., realization of one of the forms of the coherent control) is considered.
Photoelectron imaging techniques are used to experimentally investigate photodetachment of negative ions in a strong laser-field as well as the orbital alignment dynamics in the residual neutral.\n\nStrong-field photodetachment in a linearly polarized pulsed laser with 100 fs pulse length is used to create a wave packet in the neutral atoms of carbon, germanium and silicon. The negative ions of these three atomic species are isoelectronic with a half-filled ppp-shell. The photodetachment rate is highly dependent on the magnetic quantum number $m_\\ell$ with the orbital with $m_\\ell = 0$ having a detachment rate of an order of magnitude larger than orbitals with $m_\\ell$ non-zero, with the polarization axis acting as a quantization axis. The residual neutral is thus left in an aligned state after photodetachment. The aligned state constitutes of a coherent superposition of fine-structure states and the population in the different orbitals will accordingly display a time-dependent spatial oscillation. The created wave packet is subsequently probed through photoionization by a second strong-field laser pulse.\n\nSecondly, a tomographic method is implemented to observe the three-dimensional distribution of photoelectrons produced in strong-field photodetachment processes. The method makes no assumptions of the existence of any symmetry axes in the photoelectron distribution and is thus applicable to any laser polarization ellipticity. Traditional imaging methods are dependent on the photoelectron distribution to be cylindrically symmetric by using either linearly or circularly polarized light, a requirement which can be lifted by using a tomographic technique. The method is applied to multiphoton detachment of the negative ion of silver.\n\nFinally, strong-field photodetachment of homonuclear diatomic negative ions is performed. The possibility of the additional electron being ejected from either of the two atomic centers can significantly alter the photoelectron distribution compared to an atomic negative ion with the same detachment energy. Experiments are performed on C2−_2^-2−​, Si2−_2^-2−​ and Ag2−_2^-2−​ and compared to theoretical models.