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4

Optical Rotor

Optical tweezers have been successfully utilized in various scientific and en-
gineering fields such as biology, microchemistry, physics, optics and micro-
mechanics. Their ability to rotate microobjects remotely without the use of
bearings presents important opportunities in optical microelectromechanical
systems (optical MEMS) and biotechnology. This chapter describes the prin-
ciple, design, fabrication, and evaluation of an optical rotor to increase the
mixingperformance of microliquids to enable future fluidic applications. The
optical rotor will be used as a mixer in micrototal analysis systems (µ-TAS).

4.1 Background

In space, small particles are blown away rotationally by the radiation pres-
sure of the sun, the so-called windmill effect. In micromechanics the following
methods are known for rotatinga microobject usinga single laser beam: one
in which a circularly polarized laser beam is used [4.1] and another in which
the rotatingnonuniform intensity profile of a higher-order-mode laser beam is
used [4.2]. However, the rotation speeds of both methods are very slow, about
6.7 × 10 −1 − 6.7 × 10 −2 rpm [4.1] and 6 rpm [4.2].
Trappingand manipulation of micrometer-sized particles were demon-
strated firstly by Ashkin usinga laser beam through a microscope objec-
tive [3.2]. Presently, optical tweezers have been successfully applied in various
fields. The optical pressure can also be used to rotate the dissymmetrical mi-
croobjects shown in Fig. 4.1, which are a polystyrene particle (refractive index
n =1.6, density ρ =1.07 gcm −3 ), a broken glass (n =1.5,ρ =2.2gcm −3 ), a
glass rod having concave end on the top and elongated cylindrical body, a bro-
ken ZnO (n =2.0,ρ =5.67 gcm −3 ), a broken Si (n =3.5,ρ =2.33 gcm −3 ),
and a broken GaP (n =2.12,ρ =4.13 gcm −3 ) for example. However, we
cannot control the rotational direction for arbitrarily shaped broken micro-
objects.