Biomedical Engineering Reference
In-Depth Information
The scattering force
F
scat
pushes the particle along the propagation direction
of light. In the case of a weak absorbing particle, such as a metallic particle,
the force generated by light absorption is dominant in the scattering force [40].
The gradient force
F
grad
is exerted on a particle according to the gradient
of the electromagnetic field around the particle. If the incident laser beam
has a Gaussian beam shape, the particle is attracted to the center of the
laser beam by the gradient force. By using this phenomenon, A. Ashkin,
of the AT&T Bell Laboratory, developed an optical levitation technique to
make a microscopic particle hover and translate from position to position,
with a weakly focused laser beam coming from beneath the particle [39,41].
The particle is captured in the lateral direction of the laser beam owing to
the gradient force. In the axial direction the particle is blown upwards by
the scattering force and is stabilized at the point at which the scattering
force is balanced by the gravity on the particle. With the use of optical
levitation, dielectric particles sized from several micrometers up to 100
m
μ
can be translated.
1.6.2
Three-Dimensional Laser Trapping
In 1986, A. Ashkin invented a method for trapping a small dielectric par-
ticle by the photon pressure exerted by a strongly focused laser beam under
a microscope [37]. In this technique the particle can be trapped in three
dimensions, and translated to an arbitrary location in a sample cuvette.
Figure 1.32b shows a schematic of radiation force generation on a particle
with a strongly converging laser beam. In the figure, the particle is assumed to
be larger than the spot size of the laser beam and to have a higher refractive
index than that of the surroundings. A ray depicted as A-A
in Fig. 1.32b
generates radiation force
f
A
on the particle, as shown in the figure. Also,
other rays generate radiation forces on the particle in the same manner. The
net radiation force on the particle is given by the summation of such forces
induced by all rays hitting the particle. The net radiation force becomes the
force
shown in the figure. The force pulls the particle into the laser beam
spot, even if the particle is located beneath the spot. The particle, which has
a higher refractive index than that of the surroundings, can be trapped in
three dimensions.
If the particle has a lower refractive index than that of the surroundings,
it is hardly trapped because it is pushed away from the laser beam. Also,
a metallic particle that has a size of the order of a micrometer is barely
trapped in three dimensions, because of reflection on the surface of the metal.
For lower refractive index particles and metallic particles, three-dimensional
laser trapping was demonstrated by Sasaki et al. [42]. In their scheme the
position of a laser beam spot was constantly moved to encircle the particle.
The particle was captured three-dimensionally in the light field surrounding
the particle, like being in a cage. In the case of a suciently small metallic
particle compared to the wavelength of the laser beam, the particle can be
F
