Orbital angular momentum of free electrons

Phase (color) and amplitude (brightness) of electron wavefunctions with several values of the orbital angular momentum quantum number and a Laguerre-Gauss amplitude profile. (top left), (top right), (lower left) are all eigenstates of the orbital angular momentum operator, while the superposition of and (lower right) is not. Both of the upper wavefunctions have , while the lower wavefunctions have .

Electrons in free space can carry quantized orbital angular momentum (OAM) projected along the direction of propagation.[1] This orbital angular momentum corresponds to helical wavefronts, or, equivalently, a phase proportional to the azimuthal angle.[2] Electron beams with quantized orbital angular momentum are also called electron vortex beams.

Theory

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An electron in free space travelling at non-relativistic speeds, follows the Schrödinger equation for a free particle, that is where is the reduced Planck constant, is the single-electron wave function, its mass, the position vector, and is time. This equation is a type of wave equation and when written in the Cartesian coordinate system (,,), the solutions are given by a linear combination of plane waves, in the form of where is the linear momentum and is the electron energy, given by the usual dispersion relation . By measuring the momentum of the electron, its wave function must collapse and give a particular value. If the energy of the electron beam is selected beforehand, the total momentum (not its directional components) of the electrons is fixed to a certain degree of precision. When the Schrödinger equation is written in the cylindrical coordinate system (,,), the solutions are no longer plane waves, but instead are given by Bessel beams,[2] solutions that are a linear combination of that is, the product of three types of functions: a plane wave with momentum in the -direction, a radial component written as a Bessel function of the first kind , where is the linear momentum in the radial direction, and finally an azimuthal component written as where (sometimes written ) is the magnetic quantum number related to the angular momentum in the -direction. Thus, the dispersion relation reads . By azimuthal symmetry, the wave function has the property that is necessarily an integer, thus is quantized. If a measurement of is performed on an electron with selected energy, as does not depend on , it can give any integer value. It is possible to experimentally prepare states with non-zero by adding an azimuthal phase to an initial state with ; experimental techniques designed to measure the orbital angular momentum of a single electron are under development. Simultaneous measurement of electron energy and orbital angular momentum is allowed because the Hamiltonian commutes with the angular momentum operator related to .

Note that the equations above follow for any free quantum particle with mass, not necessarily electrons. The quantization of can also be shown in the spherical coordinate system, where the wave function reduces to a product of spherical Bessel functions and spherical harmonics.

Preparation

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There are a variety of methods to prepare an electron in an orbital angular momentum state. All methods involve an interaction with an optical element such that the electron acquires an azimuthal phase. The optical element can be material,[3][4][5] magnetostatic,[6] or electrostatic.[7] It is possible to either directly imprint an azimuthal phase, or to imprint an azimuthal phase with a holographic diffraction grating, where grating pattern is defined by the interference of the azimuthal phase and a planar[8] or spherical[9] carrier wave.

Applications

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Electron vortex beams have a variety of proposed and demonstrated applications, including for mapping magnetization,[4][10][11][12] studying chiral molecules and chiral plasmon resonances,[13] and identification of crystal chirality.[14]

Measurement

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Interferometric methods borrowed from light optics also work to determine the orbital angular momentum of free electrons in pure states. Interference with a planar reference wave,[5] diffractive filtering and self-interference[15][16][17] can serve to characterize a prepared electron orbital angular momentum state. In order to measure the orbital angular momentum of a superposition or of the mixed state that results from interaction with an atom or material, a non-interferometric method is necessary. Wavefront flattening,[17][18] transformation of an orbital angular momentum state into a planar wave,[19] or cylindrically symmetric Stern-Gerlach-like measurement[20] is necessary to measure the orbital angular momentum mixed or superposition state.

See also

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References

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  1. ^ Bliokh, Konstantin; Bliokh, Yury; Savel’ev, Sergey; Nori, Franco (November 2007). "Semiclassical Dynamics of Electron Wave Packet States with Phase Vortices". Physical Review Letters. 99 (19): 190404. arXiv:0706.2486. Bibcode:2007PhRvL..99s0404B. doi:10.1103/PhysRevLett.99.190404. ISSN 0031-9007. PMID 18233051. S2CID 17918457.
  2. ^ a b Bliokh, K. Y.; Ivanov, I. P.; Guzzinati, G.; Clark, L.; Van Boxem, R.; Béché, A.; Juchtmans, R.; Alonso, M. A.; Schattschneider, P.; Nori, F.; Verbeeck, J. (2017-05-24). "Theory and applications of free-electron vortex states". Physics Reports. 690: 1–70. arXiv:1703.06879. Bibcode:2017PhR...690....1B. doi:10.1016/j.physrep.2017.05.006. ISSN 0370-1573. S2CID 119067068. Lloyd, S. M.; Babiker, M.; Thirunavukkarasu, G.; Yuan, J. (2017-08-16). "Electron vortices: Beams with orbital angular momentum" (PDF). Reviews of Modern Physics. 89 (3): 035004. Bibcode:2017RvMP...89c5004L. doi:10.1103/RevModPhys.89.035004. S2CID 125753983.
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  4. ^ a b Verbeeck, J.; Tian, H.; Schattschneider, P. (2010). "Production and application of electron vortex beams". Nature. 467 (7313): 301–4. Bibcode:2010Natur.467..301V. doi:10.1038/nature09366. PMID 20844532. S2CID 2970408.
  5. ^ a b McMorran, Benjamin J.; Agrawal, Amit; Anderson, Ian M.; Herzing, Andrew A.; Lezec, Henri J.; McClelland, Jabez J.; Unguris, John (2011-01-14). "Electron Vortex Beams with High Quanta of Orbital Angular Momentum". Science. 331 (6014): 192–195. Bibcode:2011Sci...331..192M. doi:10.1126/science.1198804. PMID 21233382. S2CID 37753036.
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  16. ^ Clark, L.; Béché, A.; Guzzinati, G.; Verbeeck, J. (2014-05-13). "Quantitative measurement of orbital angular momentum in electron microscopy". Physical Review A. 89 (5): 053818. arXiv:1403.4398. Bibcode:2014PhRvA..89e3818C. doi:10.1103/PhysRevA.89.053818. S2CID 45042167.
  17. ^ a b Guzzinati, Giulio; Clark, Laura; Béché, Armand; Verbeeck, Jo (2014-02-13). "Measuring the orbital angular momentum of electron beams". Physical Review A. 89 (2): 025803. arXiv:1401.7211. Bibcode:2014PhRvA..89b5803G. doi:10.1103/PhysRevA.89.025803. S2CID 19593282.
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  20. ^ Harvey, Tyler R.; Grillo, Vincenzo; McMorran, Benjamin J. (2017-02-28). "Stern-Gerlach-like approach to electron orbital angular momentum measurement". Physical Review A. 95 (2): 021801. arXiv:1606.03631. Bibcode:2017PhRvA..95b1801H. doi:10.1103/PhysRevA.95.021801. S2CID 119086719.