Alice Crawford, Brooke Cranswick,† Rani Kishore, Kristian Helmerson

Physics Laboratory, National Institute of Standards and Technology,

100 Bureau Drive, Gaithersburg, MD 20899


We are investigating how optical trapping forces differ for wavelengths near the resonance of the trapped particle as opposed to wavelengths far from resonance. In most applications to date, wavelengths far from an absorption peak of the trapped particles are used for the trap. Theory predicts that the trapping forces exerted on a Rayleigh particle are enhanced when frequencies red-detuned from the particles absorption peak are used to trap. The trap stiffness may be three to fifty times larger for frequencies near resonance than for frequencies far off resonance (Agayan et. al. Applied Optics 41 2002 p. 2318).†† The ability to selectively trap only particles with a given absorption peak may have many practical applications and is of much interest, however experimental data is sparse.

††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††† We are using nanoshells, particles with a dielectric core and metallic coating, in order to experimentally investigate near resonance trapping. The plasmon resonances of the nanoshells can be tuned by adjusting the ratio of the diameter of the dielectric core, d1, to the thickness of the metallic coating, d2 (C. L. Nehl et. al. Nano Letters 4 2004 p 2355).† Our nanoshells are fabricated by Naomi Halasí group† at Rice University and consist of a silica core with a gold coating.† A typical shell has d1 = 60 nm, d2 = 20 nm and a fairly broad absorption peak centered at 848 nm. The center of the absorption peak† is† changed by adjusting d1/d2. We are using shells with several different d1/d2 ratios in conjunction with a diode laser and a fiber laser that emit at 850 and 1064 nm respectively. Trap stiffness is measured for different combinations of trapping frequency and d1/d2 using back focal plane (BFP) detection.