Biomedical Engineering Reference
In-Depth Information
10
μ
m
10
μ
m
(a)
(b)
Fig. 1.33.
Manipulation of a human red blood cell with a single laser beam. The
arrows indicate the position of the laser beam spot. A trapped cell was first located
near the center of the image (
a
). By shifting the spot position, the cell was moved
to the right in the image with the spot movement (
b
). Other cells were not moved
during the manipulation
trapped in three dimensions by radiation force [40,43]. For the trapping of
such particles, the gradient force generated from the laser beam spot that
pulls a particle must be stronger than the scattering force that expels the
particle from the spot.
In the case of trapping in two dimensions, a micrometer-sized metallic
particle on a substrate can be trapped by a strongly focused laser beam [44-
46]. This is explained by optical absorption of the metallic particle. When the
particle is located below the laser beam spot at the distance of the particle
diameter, the net force on the particle by optical absorption is exerted to pull
into the optical axis of the beam. In this case we can trap a metallic particle
on a substrate in the lateral direction, but we cannot lift the particle away
from the substrate in the axial direction.
Figure 1.33 shows the manipulation of a human red blood cell trapped
by a Nd : YAG laser beam (
λ
= 1064 nm, 2 W). In the microscope images,
first the red blood cell marked by an arrow is trapped around the center of
the image (a), then the cell is translated to the right by moving the spot
position of the laser beam (b). But other red blood cells in the image do not
change their positions, because the laser beam is not irradiated onto these
cells. Only the cell located at the laser beam spot can be manipulated with
the light. This is an advantage of laser trapping. In this experiment the laser
beam was focused with a microscope objective (
100, 1.3 numerical aperture
(NA)) and the spot size of the beam was around 1
×
mindiameter.
μ
1.6.3
Force Measurement
In laser trapping the spring constant of the trapping force is small, in the
μ
N/m order [47]. In comparison with a cantilever for an atomic force micros-
