Nanometer gold-silica composite particles manipulated by optical tweezers

We have studied the behavior of nano or micro size composite particles submitted to optical trapping forces and a comparison was made with homogeneous particles of similar dimension. The forces were measured using the power spectrum signal analysis. Most of the results presented were obtained using a lateral effect position sensitive detector (PSD), which allowed the fluctuations of the particle position in the optical trap to be monitored. A 4-quadrant photodiode was also used for the same purpose. We bring clear experimental evidence that the trapping force was increased by a factor of about 2-3 for composite particles made of a colloidal gold core encapsulated in a silica shell, with respect to homogeneous silica or latex beads. These results were discussed in the frame of the various approaches currently used for modeling optical tweezing forces.