Photon's spin and orbital angular momentum

In summary, the spin density S is a property of the photon that is independent of the measurement axis and is associated with the local ellipticity of the polarization of light. On the other hand, the orbital AM density L is a property of the light beam as a whole and is dependent on the axis of measurement, which is used to calculate its angular momentum.
  • #1
intervoxel
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"Locally, the spin density S is an intrinsic (i.e. origin-independent) quantity, which is associated with the local ellipticity of the polarization of light. In turn, the orbital AM density L=r x P0 is a manifestly extrinsic (origin-dependent) and is produced by the corresponding canonical (orbital) momentum density P0."

Can anyone explain the phrase "which is associated with the local ellipticity of the polarization of light", please?
 
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  • #2
Am I correct in assuming it's the word "local" that's got your knickers in twist?
 
  • #3
Spin is a property of the photon, and usually modeled as left handed or right handed - from various combinations and phase delays you can build up any of the standard light polarizations, of which elliptical is the most general case.

Orbital angular momentum (OAM) is the property of the light beam as a whole; most light beams have zero OAM, but it is possible to generate angular momentum in the beam by how it is generated - the standard measurement uses the axis of the propagating beam as the origin for the OAM calculation.

OTOH intrinsic photon spin is independent of the measurement axis - you can only "see" a photon that is coming towards you, and the spin measured at the detector will be LH or RH without reference to any external axis.
 
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FAQ: Photon's spin and orbital angular momentum

What is the difference between spin and orbital angular momentum of a photon?

Spin and orbital angular momentum are two forms of angular momentum possessed by a photon, which is a particle of light. Spin angular momentum refers to the intrinsic rotational motion of a photon, while orbital angular momentum refers to the photon's movement around a central point or axis. In simpler terms, spin refers to the photon's internal rotation, while orbital angular momentum refers to its external motion.

How is the spin of a photon measured?

The spin of a photon can be measured using a technique called spin-sensitive optical scattering. This involves shining a laser beam onto a sample of material and observing the scattered light. The pattern of scattered light can reveal information about the spin of the photons in the beam.

Can a photon have both spin and orbital angular momentum?

Yes, a photon can have both spin and orbital angular momentum simultaneously. In fact, most photons have both forms of angular momentum, with the relative amounts depending on the specific properties of the photon such as its energy and polarization.

How does the spin and orbital angular momentum of a photon affect its properties?

The spin and orbital angular momentum of a photon can affect its polarization, helicity, and total angular momentum. For example, photons with different spin states can have different polarizations, and photons with orbital angular momentum can have a helical wavefront. Additionally, the total angular momentum of a photon is conserved in interactions with other particles, which can have important consequences in various physical phenomena.

Can photons with different spin or orbital angular momentum interact with each other?

Yes, photons with different spin or orbital angular momentum can interact with each other. However, the nature of the interaction depends on the specific properties of the photons and their surroundings. For example, photons with opposite spin can cancel each other out through destructive interference, while photons with different orbital angular momentum can exhibit a phenomenon called optical vortices, where their wavefronts interfere to create a spiral-like pattern.

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