Light With Orbital Angular Momentum
The drawing shows light carrying different amounts of orbital angular
momentum, a newly exploited property of light that can be used to carry,
and retrieve, a wealth of information. The corkscrew patterns represent
equal-phase regions about the beam axis; in other words, regions of
the light wave that are "synchronized" or in the same phase
(such as the portions of the wave with maximum electric field) trace
out corkscrew-like patterns in the direction that the light beam travels.
The figures show light beams with different values of orbital angular
momentum, with higher values of orbital angular momentum leading to
tighter corkscrews. The colors from blue to red denote phase delays
from 0 to 360 degrees (0 to 2 pi).
Cross-sections of an (a) ordinary "vertically polarized"
light beam and (b) a beam with orbital angular momentum. In an ordinary
light beam, the electric fields (arrows) at every point of the cross
section are in the same phase; they are synchronized. In a beam with
orbital angular momentum, different points on the cross section are
in different phases; while the top (12 o'clock) position has a maximum
electric field that points up, the left and right (3 o'clock and 9 o'clock)
positions are at an electric field minimum--they are out of phase by
90 degrees from the top position. Meanwhile the electric field at the
bottom (6 o'clock) position points in the opposite direction as the
one on the top, representing a phase difference of 180 degrees from
the top. The color wheel, from blue to red, denotes a range of phase
from 0 to 360 degrees (0 to 2 pi). Note that in light with orbital angular
momentum, the direction of polarization (denoted by the direction of
the arrow) does not change across the cross section. The fields only
point up or down, with different phase delays in different places. (Images
courtesy Silvia Carrasco, Boston University).
Reported by: Torner,
Torres, Carrasco, Optics Express, February 7, 2005
News Update (available February 24, 2005)
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