Biomedical Engineering Reference
In-Depth Information
tion scheme of such objects with laser light has started to be widely used
[37,38]. This technique is called laser trapping, which enables us to capture a
microscopic object by a radiation pressure force without mechanical contact.
In this section, the mechanism of laser trapping and applications to biological
and medical investigation are described.
1.6.1
Photon Pressure
Photon pressure is the force that is exerted on an object under manipulation
by laser light [39]. The force is caused by momentum change of the photon
when it is scattered by the object. The momentum of each photon is expressed
as
h
is the wave vector of the photon and
h
is Planck's constant.
The direction of the momentum is equivalent to the propagation of light.
While the propagation direction of light changes through light scattering, the
momentum of the photon changes, so that a force is exerted on the object
according to the momentum conservation theorem. This is photon pressure.
Figure 1.32a shows a schematic diagram of photon pressure exertion, as
a result of light refraction at the surface of a spherical particle both on the
incidence to the particle and on exit. The light ray has a moment
k
/
2
π
,where
k
p
p
and
p
−
p
before and after travelling through the particle. The momentum change
causes the radiation pressure force
f
to be exerted on the particle, as shown
in Fig. 1.32a.
If the incident laser beam has an inhomogeneous intensity profile like a
Gaussian beam, the force exerted on the particle has two components; one is
the scattering force
F
scat
and the other is the gradient force
F
grad
[37,39].
The net force
on the particle is described as the summation of these two
forces, as follows:
F
F
=
F
scat
+
F
grad
.
(1.3)
Laser Beam
A
p
f
A
F
f
p'
A'
(a
)
(b
Fig. 1.32.
A schematic of laser trapping.
a
The radiation force exerted on a particle
by scattering.
b
The radiation force exerted by a converging laser beam acts to keep
the particle at the spot
