The NASA Glenn Research Center has identified photonic control (optical micromanipulation using optical tweezers) as a nonintrusive tool for manipulating nanoscopic and microscopic material, without contamination, into the required configurations for the fabrication of sensors that will be integrated into the intelligent engines of the future. A complete understanding of the forward-scattered light from optically trapped particles provides information about material in optical traps that is not directly observable. Such signatures measured from the forward-scattered light of a trapped particle can be used as a guide for particle manipulation and placement.
The NASA Glenn Research Center and Cleveland State University collaborated to compare two theoretical models of the forward scattered light plus the trapping beam light from an optically trapped polystyrene microsphere. The theoretical models were formulated at Cleveland State University, and experimental tests of those models were performed at Glenn. The predicted optical trapping properties of a gaussian beam in the liquid surrounding the particle were compared with that of a gaussian beam that was apertured and focused by a high-numerical-aperture oil-immersion microscope objective lens and aberrated by the interface between the coverslip and the water in the sample chamber (i.e., an apertured, focused, abbreviated (AFA) beam).
Time-sequence series of eight photographs illustrating changes in the forward scattered light as a 10-μm-diameter particle is drawing into an optical trap, from 0.000 sec (before the sphere moves toward the trap) to 2.615 sec (when the sphere has come to rest in a stable trapping position).
Long description of figure 1.
In the Glenn experiment, a downward-propagating laser beam was tightly focused by a 100× microscope objective lens to form an optical trap. The forward-scattered light from a 10-μm-diameter polystyrene latex sphere as it was drawn into the optical trap was projected onto a screen, recorded, and examined. It was found that before the sphere was drawn into the trap, the forward-scattered light initially showed a bright featureless spot. As the sphere was drawn into the trap, a series of concentric interference rings formed and appeared to propagate radially outward from the center of the pattern as the sphere moved into the trap. The number of concentric rings in the pattern increased and became dimmer as the focal waist of the beam moved inside the sphere. The AFA beam model correctly predicted that six or seven bright and dark fringes would form as the sphere was drawn into the trap. The initial beam before trapping can be seen at 0.000 sec. Some of the changes in the interference fringes are seen from 0.915 to 2.305 sec. The dim set of interference fringes from the optically trapped particle can be seen at 2.615 sec.
Optically trapped 10-µm-diameter polystyrene microsphere.
Long description of figure 2.
Proper characterization and modeling of the forward-scattered light from an optically trapped object will greatly enhance NASA’s ability to create unobtrusive sensors out of new nanoscale and microscale materials. Proper understanding of the signature of forward-scattered light allows for automation of assembly processes even when objects that are manipulated in the optical trap are not directly observable. Knowledge of the signature of the forward-scattered light of optically trapped objects will aid in characterizing the unobtrusive sensors of the future.
Lock, James A.; Wrbanek, Susan Y.; and Weiland, Kenneth E.: Scattering of a Tightly Focused Beam by an Optically Trapped Particle. Applied Optics, vol. 45, no. 15, 2006, pp. 3634-3645.
Find out more about the research of Glenn’s Optical Micromanipulation Lab: http://www.grc.nasa.gov/WWW/OptInstr/Nano_OpticalMicroManipulationLab.html
Glenn contact: Susan Y. Wrbanek, 216-433-2006, Susan.Y.Wrbanek@nasa.govLast updated: December 14, 2007
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