This paper reports an experimental study of the low laser intensity Kerr Effect produced by optical trapping of fluorescently labeled 100 nm diameter polystyrene particles in aqueous suspension. Optical trapping was made by a tightly focused and periodically blinking IR laser beam. A green laser beam, aligned co-linear with the IR laser, was used as the fluorescence excitation light. The fluorescence signals from particles trapped by the blinking IR laser were measured by a lock-in amplifier to improve the signal to noise ratio required to detect the very minute (sub-thermal fluctuation) changes in local particle density induced by optical trapping. The use of confocal detection ensured that the fluorescent signals measured were only from the diffraction-limited focal region of the two laser beams. By independently measuring the fluorescence intensity as a function of particle concentration and dn/dC (the change in refractive index due to change in concentration), we were able to determine the Kerr coefficients for laser trap powers in the range of 10.6 mW to 85 mW. Non-linear behavior in the refractive index vs. laser intensity relationship indicates that higher order Kerr coefficients are needed to describe the Kerr effect. Kerr coefficients obtained by using a circularly polarized IR laser were similar to those obtained by a linearly polarized laser, indicating that the induced electric dipole-dipole interactions did not contribute to the electric field-induced concentration changes giving rise to the Kerr effect.

Date Published: 5 September 2007
PDF: 7 pages
Proc. SPIE 6644, Optical Trapping and Optical Micromanipulation IV, 664408 (5 September 2007); doi: 10.1117/12.739784

Published in SPIE Proceedings Vol. 6644:
Optical Trapping and Optical Micromanipulation IV
Kishan Dholakia; Gabriel C. Spalding, Editor(s)