Biomedical Engineering Reference
In-Depth Information
behind the pinhole. The pinhole eliminates undesired light from other parts
of the sample. Sample scanning is done by changing the applied voltage of
a piezo tube. A CCD camera is used to observe the sample surface under a
conventional microscope condition.
Feedback Stabilization of a Particle.
According to thermal energy con-
ditions, a probe particle moves continuously in a laser-trapping potential due
to collisions with water molecules (Brownian motion). Due to this Brownian
motion, the intensity of the light scattered from the particle changes over
time. This will cause noise on measured signals when we use the trapped
particle as a near-field probe. This noise affects the maximum resolving po-
wer of the laser-trapping NSOM. Consequently, it is necessary to reduce the
fluctuation of the probe position in order to enhance the resolution of the
NSOM. To reduce the fluctuation of the particle, a feedback stabilization
techniqueiseffective[53].
In the feedback stabilization technique, information about the particle
position is given back to the spot position of the trapping laser beam. The
position of the probe particle is monitored by the detection of the spot posi-
tion. The spot position can be detected sensitively with a quadrant detector
and two sets of deferential amplifiers to calculate the displacement signals of
the spot in the
x
-and
y
-directions. To shift the spot position of the trapping
laser beam, two galvanomirrors are used. The angles of the galvanomirrors
are changed to minimize the displacement signals in both
x
-and
y
-directions.
The displacement signals in the
x
-and
y
-directions are captured on a personal
computer.
Figure 1.38 shows displacement diagrams that depict movements of a
probe particle. Figure 1.38a is the result taken without feedback of the dis-
placement signal to the galvanomirror and Fig. 1.38b is the result taken with
5
50
0
0
-50
-5
-50
0
50
-5
0
5
Displacement [nm]
Displacement [nm]
(a)
(b)
Fig. 1.38.
Particle displacement in an optical trap (experimental data):
a
without
feedback stabilization of a probe particle;
b
with feedback stabilization
