more complicated trap structures with
multiple beams, new techniques had to
be introduced.
Multibeam techniques
Currently, there are two multibeam
techniques that are widely used—
scanning and holographic. In scanning
techniques, a beam is scanned very
rapidly across the particles of interest.
As long as the beam returns to a particle
before it diffuses from the trapping site,
the particle remains trapped, even if the
incident light only illuminates it for a
fraction of a second. Perhaps the most
famous example of this (if not the most
practical) is the microTetris demonstration carried out at the Vrije University.
In holographic techniques, an input
Gaussian beam has its phase modulated
into that of a target intensity at the focal
plane of the microscope objective. In
this way, complex beam patterns can
be created that are static but dynami-
cally altered. Holographic beam shaping
has the advantage that particles can be
easily manipulated in three dimensions
and that patterns of light that are not
simple arrays of spots are relatively easy
to produce. As a counterpoint to micro-
Tetris, Miles Padgett’s group in Glasgow
used holographic optical tweezers to
demonstrate particles performing the
smallest “Strip-the-Willow” dance in the
world. See “References and Resources” for
links to these videos.
Application-based research
Increasingly, applications are driving optical tweezers research. While there have
been a growing number of experiments in
areas such as colloidal dynamics, statistical mechanics, hydrodynamics, Brownian
Courtesy of J. Guck, University of Cambridge
Optical
fiber
Laser beams
Optical
fiber
Cell
Low laser power
High laser power
A cell is trapped between two counter-propagating beams in a dual-beam fiber trap. The laser wavelength used is 1,064 nm.
(Left) With low laser power of 200 m W, the cell has no measureable deformation. (Right) However, at higher powers of 1. 4 W,
the cell is appreciably stretched. The amount of stretching is linked to the properties of the cytoskeleton.
24 | OPN Optics & Photonics News
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