Abstract
We review basic physics and novel types of optical angular momentum. We start
with a theoretical overview of momentum and angular momentum properties of
generic optical fields, and discuss methods for their experimental
measurements. In particular, we describe the well-known longitudinal (i.e.,
aligned with the mean momentum) spin and orbital angular momenta in polarized
vortex beams. Then, we focus on the transverse (i.e., orthogonal to the mean
momentum) spin and orbital angular momenta, which were recently actively
discussed in theory and observed in experiments. First, the recently-discovered
transverse spin angular momenta appear in various structured fields: evanescent
waves, interference fields, and focused beams. We show that there are several
kinds of transverse spin angular momentum, which differ strongly in their
origins and physical properties. We describe extraordinary features of the
transverse optical spins and overview recent experiments. In particular, the
helicity-independent transverse spin inherent in edge evanescent waves offers
robust spin-direction coupling at optical interfaces (the quantum spin Hall
effect of light). Second, we overview the transverse orbital angular momenta of
light, which can be both extrinsic and intrinsic. These two types of the
transverse orbital angular momentum are produced by spatial shifts of the
optical beams (e.g., in the spin Hall effect of light) and their Lorentz
boosts, respectively. Our review is underpinned by a unified theory of the
angular momentum of light based on the canonical momentum and spin densities,
which avoids complications associated with the separation of spin and orbital
angular momenta in the Poynting picture. It allows us to construct
comprehensive classification of all known optical angular momenta based on
their key parameters and main physical properties.
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