Optical tweezers is the instrument which uses the momentum of light to apply
force on a microscopic particle and hence it can be used to trap it. The forces
applied by this instrument can be very less to order of pico-newton. These
small forces are very useful in the ﬁeld of biophysics, especially in the study of
biological cells. This instrument leads to non-invasive techniques of study and
manipulation of biological cells i.e. manipulation without any damage. That is
why popularity of optical tweezers increased very rapidly in last few decades.
The presence of radiation pressure on small particles was ﬁrst detected ex-
perimentally by Nicholas and Hull  in the beginning of 20th century but it
was invention of lasers which enable A. Ashkin to show that this force may
be used to control the dynamics of small neutral particles. He showed that,
particles can be accelerated, decelerated or stably trapped using these radiation
forces. This trap means particle forms a stable equilibrium point in space and
if a force try to deviate particle from its equilibrium position then a restoring
force always bring it back towards it.
The basic principle behind trapping is radiation pressure of light. For a
perfectly reﬂecting mirror,change in radiation pressure per second and hence
force by an incident beam of power P is given by
,where c is velocity of
light. So, light beam of 1W exerts a force of 7nW of surface, which is very
small and of no use. But because of lasers one can, now, achieve a tightly
focused beam. Due to small size and mass of particle the radiation pressure of
1W laser is, now, 10
times the gravitational force on the particle. This force
is large enough to aﬀect the dyamics of the particle.
Nowdays, These techniques of optically trapping and manipulation are very
popular in the ﬁelds where dynamics of single particle is involved. The ﬁeld
of molecular chemistry, superconductivity, cold atom physics and many more
get their beneﬁts from these techniques. The force is small so biological cells
and living organisms can also be trapped and manipulated without fear of any
damage. That is why, these techniques have huge advantage in ﬁeld of biophysics
and biology. infact at present, I am working on a apparatus which was used by
Apurba Paul to study healthy and infected red blood cells to detect malaria. As
we shall see, these techniques are simple and very advantageous in many ﬁelds.
0.1.1 Basic Forces involved in trapping
In beginning, A. Ashkin only seeked to accelerate a particle using that radiation
forces considering it is perfectily reﬂecting. He consider that a single photon
and if P is power of incident beam than total change in
momentum will be twice the number of photons multiply by momentum of
single photon, which turn out to be
, as I stated before. Using a laser tightly
focused in small spot of micrometer range and let it hit a particle of same size as
spot we can get acceleration 10
g, where g is acceleration due to gravity of earth.
He designed a experiment and succeded to get accelaration approximately equal
to his estimates.This proved that trapping is a radiation pressure phenomena.