PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 639 May 30, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

OPTICAL PERISTALSIS. Part of the digestion process consists of the
massaging movement of powerful esophageal muscles urging food
particles along the alimentary track.  The same sort of
"peristalsis" can also be carried out at the nanoscopic level with
small objects in the grip of cleverly crafted light pulses.  David
Grier and Brian Koss at the University of Chicago use the optical
tweezer  method of controlling particles with multiple laser beams,
but instead  of a static array of beams, they use computer-generated
holograms to convert a single beam of light into large numbers of
optical traps.  Each hologram may be considered to be a specialized
diffraction grating, producing intricately articulated networks of
hundreds of optical traps.   Objects can fall into these light traps
and then the traps can be moved, thus transporting the objects.  The
aim is to move and position sub-micron things in 3D space.
Applications include inserting the object into a microscopic
reservoir and pulling it back (parallelism is one of the technique's
strengths), or centering or rotating a biological cell in a
microscope's field of view.   Grier's work has led to a commercial
version of this holographic optical tweezers, one in which a pattern
of 200 optical traps can be refreshed or modified at a rate of 100
times per second.  (By the way, how forefront research is turned
into saleable products is an interesting story by itself.  For
example, the company Grier started, Arryx,
Inc.---http://arryx.com---has a scientific advisory board (SAB) with
notable scientists from Princeton, NIH, the Whitehead Institute,
Harvard, and Northwestern.)   In the "peristalsis" mode of
operation, particles are deliberately handed off from
one optical trap to another, as in a bucket brigade.  In a separate
"thermal ratchet" mode of operation, the transfer from trap to trap
might involve intervals of  free diffusion; this mode should be
useful for fractionating DNA molecules (see previous Update story at
http://www.aip.org/enews/physnews/2003/split/627-1.html ) as part of
the process of sequencing a gene.
Speaking as a physicist, Grier says the most important aspect of his
group's holographically generated tweezer patterns is the ability to
implement time-varying potential energy landscapes for moving tiny
objects in a "force-free" way.  Speaking as a  biophysicist, Grier
points to the ability to reach into a microscopic environment and to
position samples just where you want them. (Koss and Grier, Applied
Physics Letters, 2 June 2003; d- grier{at}uchicago.edu, 773-702-9176,
lab website at http://griergroup.uchicago.edu/~grier/hot/ )


A NEW OPTICAL GEOMETRIC PHASE has been measured for the first time,
by a group of physicists at Colgate University.  The new geometrical
phase is associated with light beams carrying orbital angular
momentum. This development can be considered yet another step toward
understanding and exploiting the weirdness of quantum reality for
performing novel feats of computation.   To see the meaning behind
the new effect, we shall break the explanation into parts,
considering in turn the issues of phase, orbital angular momentum in light,
and then geometrical
phase in light.  First, phase.  Many common periodic things have phase.  The
orientation or phase of a minute hand on a clock is the amount by
which the hand has swept around the clock face: a quarter past the
hour, half past the hour, etc.  Except when going into a new time
zone the phase of the clock regularly returns to its original
position every sixty minutes. The phase of a water wave specifies
where along the wave's crest-to-trough cycle it might be at any
moment. Now consider a different kind of phase.  Picture a sign with
an arrow on it, oriented north.  Starting at the equator, and
without changing its orientation, push the sign along the ground one
fourth of the way around the world. Next push the sign due north
until you reach the north pole, where, without changing the sign's
orientation, you move directly south again to return to your
starting point.  Even though you will have traced a closed loop the
sign will now have a westerly orientation.  In other words, because
of the intrinsic curved geometry of the path, a change in phase will
have occurred.  This kind of phase change can occur in a quantum
system.
Second, orbital angular momentum.   The ordinary forward momentum of
a particle of light is equal to Planck's constant divided by the
wavelength of the equivalent light wave. Furthermore, the light is
said to possess an intrinsic angular momentum, or "spin."  The spin
angular momentum can be oriented by polarizers so that the electric
field of the light wave is oscillating vertically up and down, or
horizontally back and forth.  Equivalently, if the light wave is
circularly polarized (the electric field precesses in corkscrew
fashion as the wave moves along) the two contrary states of the spin
would then correspond to the light wave's electric field precessing
clockwise (in a "right-handed" way) or anticlockwise (in a"left
handed" way).  For the purposes of  data processing a 0 or 1 bit can
be associated respectively with vertical and horizontal polarizations or,
equivalently, with
clockwise or anticlockwise polarizations. But what does it mean for
light to have "orbital" angular momentum?  What is it that orbits?
To ponder this issue, picture the electric field values for a vertical planar slice
of the light beam.  For vertically-polarized light, the electric
field at all the points on the slice are vertically oriented.  Look
at the sameslice at a later time and the fields are still vertically oriented.
For circularly polarized light, the fields in the slice will, at a
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