t is strange to think that light—
that most ephemeral of things—
can have any mechanical effect.
But it has long been known that the
universal palette can, in fact, push and
pull on physical objects. The idea was
put on sound mathematical footing with
the development of the theory of electromagnetism by James Clerk Maxwell,
who described what we now call radiation pressure.
Radiation pressure is the most intuitive form of optical force: Light incident
on a surface produces a force on that
surface. As P.N. Lebedev notes in his
experimental verification of this hypothesis, “The value of this beam pressure is
rather small.” This is something of an
understatement. The maximum pressure
exerted by the sun on a reflecting object
is on the order of 1 mN/m2. Measuring such a tiny force at the turn of the
century, as Lebedev somehow managed
to do, was an impressive feat.
The lack of attention that this subject
received over the subsequent few decades
can perhaps be explained by the fact that
it is difficult to disentangle the forces
due to the light from thermal forces
induced by the light beam. The thermal
forces often swamp any effect that one
might seek to measure. In addition, the
tiny forces that available light sources
could generate would also make experi-
ments challenging.
Ashkin lays the groundwork
Forty years ago, Arthur Ashkin
addressed the thermal problem using
a relatively new light source called the
laser. With it, Ashkin realized that he
could use particles that were transparent and therefore non-absorbing at the
[ A simple optical tweezers system ]
L3
CCD camera
L4
Sample
Light source
L1
L2
Beam block
The laser input into the system is from the left hand side, indicated by the red line. L1
and L2 form a beam-expanding telescope, as do L3 and L4. Note the system is in a horizontal geometry, in contrast with many optical tweezers that sit in a vertically positioned
microscope. The illumination for the sample is provided by a simple white light LED.
wavelength of the light source. This
de-coupling of optical and thermal
forces paved the way for a number of
significant breakthroughs. Ashkin was
especially interested in developing techniques to manipulate atoms. His work
on optical forces culminated in his first
observation of the laser cooling of atoms.
This application of the technique led to
two Nobel Prizes—one given to Steven
Chu, Bill Phillips and Claude Cohen-Tannoudji in 1997 for the development
of an atomic trap based on laser cooling,
and another given to Eric Cornell, Wolfgang Ketterle and Carl Wieman in 2001
for using advanced techniques based on
the same principles to create a Bose-Einstein condensate. Laser cooling research
is ongoing, with recent breakthroughs in
the development of Fermi gases.
Although Ashkin’s research played
an important role in the development
of these techniques, his best known
work is in optical forces on microscopic
particles. In this area, he should be
rightly seen as having founded a new
field. For the bulk of the 1970s, Ashkin
and his collaborators embarked on a
series of pioneering experiments that
showed the capabilities of these optical
forces. Of particular importance were
demonstrations of trapping using two
counter-propagating beams and using
a single beam that was obtained by
propagating a beam vertically and using
gravity to balance the radiation pressure
force. Much of Ashkin’s early work was
devoted to understanding basic forces
and ways to improve stability. However,
finding lasting applications or a committed research community proved difficult.
It wasn’t until 1986, when Ashkin
demonstrated a single-beam gradient
trap, that a more powerful technique was
born. The trap differed from the single-beam radiation trap in that it was able
to act in the same direction as gravity,
forming a true three-dimensional device.
In all previous optical trapping work, the
gradient force, in which a particle moves
along an intensity gradient toward the
point of highest intensity, acted only in
two dimensions, thereby confining the
beam on the beam axis. Trapping in
22 | OPN Optics & Photonics News
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