Optical Trapping
Optical trapping, developed at Bell Labs in the mid 1980s
and known sometimes as "laser tweezers", is a non-contact technique
for the manipulation of microparticles by using radiation pressure
generated by the scattering of a tightly focused laser beam. Due to its
unusual properties, such as the ability to manipulate an object totally
enclosed within another object, there are now many wide-ranging
applications of optical trapping. Here it fulfils a specific need for a method
of non-destructive particle manipulation for use in light scattering studies,
where a small object is probed by one laser beam while being held by
another one. In this way valuable information can be obtained about, for
example, the internal structure of living cells and changes taking place
within cells undergoing development or subjected to treatments. The
instrument used for trapping microparticles was designed and built at University of Hertfordshire and
was the first self-contained, portable
"optical trapping microscope".
Since it was first constructed it has been the basis of development work in optical
trapping as well as studies on specific applications, such as microbial
cell sorting and isolation. Our research also includes the theory of
strongly focused Gaussian beams.
Examples of optical trapping in action
(left) Saccharomyces cerevisiae yeast cells rearranged on a slide using optical
tweezers. When carried out by moving a computer mouse controlling the trap position and
switching the laser on and off, an operation of this kind resembles dragging icons
on a computer screen, and takes only a minute or so.
Animated GIF image - 660k
(right) A small cluster of budding Saccharomyces cerevisiae cells
rotated using optical tweezers around a point of attachment to a glass slide.
The cluster could be rotated indefinitely without breaking the bond.
MPEG clip - 56k
(if the file does not play, try saving to disc first)
(left) A Myxomycete (slime
mould) spore rotated using radiation pressure - "the water-wheel effect".
The spore is trapped by its wall and slight optical absorption together
with reflection from the reticulate wall allow transfer of momentum from
the laser beam, causing the spore to spin - see the MPEG
clip - 430k.
(right) Pair of images showing an organelle moved using optical tweezers
within an intact filamentous cell of a green alga Zygnema. The organelle (arrowed)
originally visible near the right-hand side of the cell was transferred towards the
left.
MPEG clip - 330k
Contact
Dr Zbigniew Józef Ulanowski
STRI
University of Hertfordshire
Hatfield, Herts AL10 9AB
UK
T: +44 (0) 1707 284604
F: +44 (0) 1707 284185
E: z.ulanowski 
herts.ac.uk