Monday, May 31, 2004

How optical tweezers work

In a “levitation trap” the laser beam balances the pull of gravity. To
“optically” trap a particle in three dimensions it is necessary to exert
a “longitudinal” force in the same direction as the laser beam and a
“transverse” force at right angles to the beam. The transverse force
is created by having the maximum laser intensity at the centre of the
beam. (a) If the particle is to the left, say, of the centre of the beam, it
will refract more light from the right to the left, rather than vice versa.
The net effect is to transfer momentum to the beam in this direction,
so, by Newton’s third law, the particle will experience an equal and
opposite force – back towards the centre of the beam. In this
example the particle is a dielectric sphere. (b) Similarly, if the beam is
tightly focused it is possible for the particle to experience a force that
pushes back towards the laser beam. (c) We can also consider an
energetic argument: when a polarizable particle is placed in an
electric field, the net field is reduced. The energy of the system will be
a minimum when the particle moves to wherever the field is highest –
which is at the focus. Therefore, potential wells are created by local
maxima in the fields. -- Optical tweezers: the next generation
Optical tweezers (laser tweezers, laser trap, optical trap, light gradient trap) is a modern micromanipulation tool. It is mainly used in biological and medical research. Optical tweezers catches, traps and moves certain microscopic objects by light. "Principles of optical tweezers"

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