The Design of a Near-Infrared Optical Tweezers Setup

Peter Amendola, Paul Tchertchian, Harold Metcalf and John Noe
Laser Teaching Center, Department of Physics and Astronomy
State University of New York at Stony Brook 11794-3800

An improved near-infrared optical tweezers setup for the examination and manipulation of micron-sized particles has been designed and constructed. The design is based on standard optical mounts and a pre-drilled optical table.

An "optical tweezers" traps tiny neutral particles by means of the gradient force resulting from a tightly focused laser beam. In appearance it resembles a microscope with a video camera for viewing and a laser beam directed downwards through the objective lens on to the sample being viewed. Applications include cellular microsurgery and the study of polymers.

The gradient force is the net momentum change that arises from the refraction of light rays in a spherical particle whose index of refraction is greater than that of its surrounding medium. It always points towards the direction of greatest light intensity, that is, the focal spot of the light. In order to achieve trapping, the gradient force must be strong enough to overcome the random motion of the particles and other forces like the radiation pressure of the light, which tends to push the particle away from the light source. This requires a strong light source and a high-powered microscope objective.

The laser in our setup was a near-infrared diode laser (wavelength 780 nm) with a power output of up to 50 mW. The initially elliptical beam was made circular by means of two cylinder lenses. A slanted dichroic mirror mounted at the top of the microscope directed the beam downwards through the 100x oil-immersion objective, while allowing visible light from the sample to pass straight through to the video camera, which was protected by an infrared filter. Mounts for the laser, mirrors and lenses were standard parts from the Thorlab catalog.

At the present time the setup is complete and the microscope is being tested with 0.75 micron latex sphere samples and small micro-organisms from pond water.

This research was supported by the Simons Foundation.