1. Photo Orbital Angular Momentum in Astrophysics (By Martin Harwit ApJ, 2003, Vol. 597, 1266) 周建锋 清华大学天体物理中心
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3. 1. Spin Angular Momentum Photons are endowed with inherent spin angular momentum . This is demonstrated by the selection laws in atomic physics, where Photons all carrying the same spin are circularly polarized .
4. 1. Spin Angular Momentum Plane waves are the superposition of circularly polarized photons . They actually don’t exist physically, just as that white light photons don’t exist either. Plane waves carry no angular momentum. K (r) E H
5. 1. Detection of Spin Angular Momentum Beth Experiment in 1936 (Phys. Rev.) Left circularly polarized light Right circularly polarized light /2 birefringent plate
6. 1. Photon Orbital Angular Momentum Laser light with a Laguerre-Gaussian amplitude distribution is found to have a well-defined orbital angular momentum . (Allen, 1992, PRA) Usually, a photon just has its spin angular momentum . It is also possible to possess a larger angular momentum of . Beams of photons each with OAM as high as m =200 have been produced in lab. (Curtis et al., 2002, Opt. Commun.)
7. 2. Multipole Fields (Classical Electrodynamics) Spherical electromagnetic waves in free space, where can be completely described by superpositions of electric and magnetic multipole fields. For electric multipole (TM model), For magnetic multipole (TE model),
8. 2. Multipole Fields (Classical Electrodynamics) For electric multipole radiation, Each photon, with energy , here carries off angular momentum
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10. 2. Multipole Fields (Quantum Electrodynamics) In quantum electrodynamics, angular momentum J becomes an operator. Spin and orbital angular momentum may not always be clearly separable.
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