Hamsa Sridhar, Kings Park High School, Kings Park, NY; Harold Metcalf and John Noe, Laser Teaching Center, Department of Physics and Astronomy, Stony Brook University
The purpose of this project was to construct a working inverted optical tweezers setup, to optimize the trap strength by maximizing the gradient force exerted on the particles, and to gauge the trapping efficiency by calculating the drag force exerted in all three dimensions when trapping and manipulating yeast cells in a fluid medium.
A 23mW red He-Ne laser was used. Plane mirrors reflected and steered the beam while spherical plano-convex lenses resized it. A 50x objective (NA = 0.85) focused the beam onto the stage of a Nikon inverted microscope, creating an optical trap for the cells, enclosed in a 100 micron-deep sample chamber. White illumination light passed through the chamber into a CCD camera, which was connected to a monitor for imaging.
To achieve optimal trapping, the gradient force was maximized by optimizing the intensity of the rays that entered the objective at the steepest possible angles. The laser beam's intensity profile was measured, the width of the appropriate central component of the beam to enter the aperture of the objective was determined, and the beam was resized accordingly. Intensity losses were minimized through careful alignment procedures.
Yeast cells were successfully tweezed three- dimensionally. To measure the trap efficiency, the drag velocity in the xy-plane and the z-axis were calculated. To account for aberrations caused by refraction, an equation was created to model the distance the focus travels versus the distance the beam is steered for various angles of incidence. The drag force exerted was calculated.