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FEATURE
ARTICLE
Optical Tweezers
Lee Woei Ming, Zhang Dianwen, Tao Shaohua and Yuan Xiaocong
In nature, light from the Sun provides plants with the
means of making food, and hence energy that sustains
the cycle of life on Earth. In science, light from
laser beams is providing biologists with a new means of
studying the intricate details and functions within single
living cells. It has become a tool to discover life itself.
Laser beams used to trap and manipulate microscopic
particles are called optical tweezers. The first optical
tweezer was demonstrated by Dr Arthur Ashkin [1] and
co-workers in the 1980s at Bell Labs. They demonstrated
that light can move matter because photons carry linear
momentum (a photon of wavelength λ has a momentum
p = hc/λ, where h is the Planck’s constant, and c
is the speed of light). In their experiment, a highly focused
Gaussian beam was used to create an optical gradient
force on a particle with a higher refractive index
than the surrounding medium. As light refracts when it
passes through the micro-objects, the refraction of light
will cause a change in the photon linear momentum, and
due to the conservation of momentum, a force will result
which acts on the particle in the opposite direction. This
force is directed towards the area of the highest intensity;
hence the particle is trapped in the center of the beam.
Figure 1: 1st type of optical tweezers
Another type of optical tweezer is based on the orbital
angular momentum property of a Laguerre-Gaussian
(LG) beam, TEM∗ 0l , also known as an optical vortex [2].
In this case, the particles are trapped in the region of zero
intensity at the center of the LG beam [see Fig. 2]. The
optical traps created by the helical LG beams of light (due
to a phase singularity) can also exert optical torques on
the trapped objects. The special intensity profiles of these
beams are created by engineering the phase of the laser
beam’s wavefront.
Figure 2: 2nd type of optical tweezers
Figure 3: Irregular shaped optical tweezers
In NTU, we have begun a research project on optical
tweezers only in the past year. Our initial focus is to build
a strong foundation for the fundamental understanding
of optical tweezers. At the moment, we are able to generate
irregular shaped beams to be used as a new form
OPTICAL TWEEZERS September 2003 25