Construction of an Inverted Optical Tweezers    

Karen Cydylo, University of Connecticut; Hamsa Sridhar, Kings Park High
School; John No�, Marty Cohen and Harold Metcalf. Laser Teaching Center,
Department of Physics and Astronomy, Stony Brook University

While several previous experiments with optical tweezers have been performed in
the Laser Teaching Center, this is the first attempt to build an inverted optical
tweezers.  Optical tweezers trap particles on the size of a micron, and are
commonly used in biophysics to study pico-newton forces on, and the motion of,
biological molecules.  Optical tweezers use the radiation pressure of light to
trap particles at the center of a tightly focused laser beam.  Incident light
rays both reflect and refract when they interact with the particles, resulting in
forces due to changes in momentum. The gradient force, which always points
towards the point of highest light intensity, is what keeps particles confined in
the trap. There is also an undesired radiation pressure force that pushes the
particles out of the trap, away from the incident light. In a normal tweezers
setup the radiation pressure force and gravity both work against the trapping
force. In inverted optical tweezers the laser beam is focused upward onto the
particles, so that the radiation pressure force and gravity are opposed. As a
result it is much easier to achieve true three-dimensional trapping in an
inverted tweezers.

The goal of this project is to create a useful inverted tweezers setup from
existing components. The heart of the setup is a donated Nikon inverted
microscope that became available during the project. This provides a very stable
mechanical system and an effective illumination device (condenser). For the laser
we originally used a Sharp LT024 near-infrared (780 nm) diode. The diode laser
has an elliptical beam, but this can be circularized using a pair of cylinder
lenses. The beam is focused by an 100X oil-immersion objective to create the
optical trap.  The specimen is illuminated from above and observed with a GBC
CCTV camera focused at infinity.

After assembling this setup and achieving an appropriate beam size and shape it
was noticed that the laser output power was much lower than expected, only 3 mW
versus about 20 mW. Since 3 mW was unlikely to be sufficient power to achieve
trapping, the setup was rebuilt using a 19.5 mW red He-Ne laser (632.8 nm). The
HeNe beam is already circular, and only has to be enlarged by a pair of spherical
lenses with a focal length ratio of 5:1.

We have successfully imaged with this inverted microscope-camera setup, and have
observed Brownian motion in yeast cells. The HeNe laser setup is complete and
aligned and we hope to achieve trapping soon. In future experiments with this
setup a cut transparent plastic film will be placed in the laser beam, thus
creating an "optical vortex" beam that can rotate the trapped particles.

This work was supported by a grant from the National Science Foundation (Phy-0243935).