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118 3 Optical Tweezers
Micromechanics

Laser scanningmanipulation was applied to measure the dragforce [3.33]
acting on a glass bead moving in mineral oil between two glass plates. The
rotation rate of a small particle induced by optical pressure was measured by
the cycle of the scattered light from optically trapped particles [3.34].

3.4.2 Industry

Particle Transport

The spatial patterningand directional transport of plural particles in wa-
ter were shown to be possible by single-beam laser trapping. For radioactive
substance or nucleus materials, the optical trappingof metallic oxide parti-
cles with various optical constants were performed to confine, position and
transport without physical contact in water by Omori et al. 3-D trappingwas
possible for a ThO 2 particle but only 2-D trappingwas observed for a UO 2
particle in water usingan He–Ne laser light at 633 nm. This is because a UO 2
particle has a relatively large refractive index and a large extinction coefficient
in the visible region [3.35].
Figure 3.44 shows the relationship between optical constant (refractive
index n and extinction coefficient k) and the maximum trappingefficiency
Q max for microspheres with a wavelength of 633 nm. The objective’s NA is
1.3 and the microsphere diameters are 2µm (a) and 10 µm (b). In this calcu-
lation, absorption was considered, therefore decreasing Q max with increasing
the diameter. It is also seen from the figure that 3-D trapping was possible
for the metallic oxide havinga refractive index less than 2.4 by an He–Ne
laser light (Q max < 0). They also demonstrated that laser trappingwas also
possible in air [3.36].

-1.5 0.3 -1.5 0.4
0.2
-2 0.1 0.2 -2 0.3 0.4
0 0.1 0.3
-2.5 -2.5 0.2
0.1 0 0.2
log 10 k -3.5 -0.1 -0.1 0 log 10 k -3.5 0.1
-3
-3
-4 -4 0
-0.1 -0.1
-4.5 -4.5
-5 -5
1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4
n n
Fig. 3.44. Relationship between optical constant and maximum trapping efficiency
Q max for microsphere with wavelength of 633 nm [3.35]