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3

Optical Tweezers

Solar radiation pressure causes manmade satellites to tilt in orbit and also
to induce the rotational burstingof meteorites and tektites in space. The
effect of optical pressure appears notable even in daily life when an object be-
comes smaller than several micrometers. Optical tweezers are tools that use
optical pressure in trappingmicroobjects includinglivingcells and microor-
ganisms, and also in directionally rotating artificial microobjects fabricated
by micromachining. Given their noninvasive nature, optical tweezers are use-
ful particularly in biological processes. Nowadays, these optical tweezers are
used to control and manipulate various types of micro/nanoobject in various
research and industrial fields.
In this chapter, we first analyze the trappingefficiency of optical tweezers
usinggeometrical optics and then compare the results with those obtained in
experiments. Finally, we show the various applications of optical tweezers.

3.1 Background

Figure 3.1 shows a photograph of Halley’s comet taken on March 21, 1986 at
Nobeyama near Tokyo. The tail of the comet is said to be directed alongthe
direction of solar radiation pressure. An optical pressure force is very weak but
can be visualized as the tail of a comet in space. This force was measured on
earth soon after the laser was invented. The measurement method is illustrated
in Fig. 3.2 [3.1]. Small mirrors (vanes) were suspended by a gold wire in a
vacuum chamber. Pulsed light emitted from a ruby laser hits the first vane,
and light reflected from the vane hits the second vane and passes through
an exit beam splitter. The reflection of the beam from both vanes generates
sufficient optical pressure force to tilt the vanes and the scale mirror fixed to
the lower end of the suspension wire, which changes light deflection on the
scale. The experimental results were obtained in vaccum (10 −5 Torr) under the
conditions listed in Table 3.1 to eliminate the thermal effect of air molecules
induced by light absorption.