An optical bottle method is developed to determine the potential-energy profile of colloidal Rayleigh nanoparticles in an optical trap. The three-dimensional distribution of fluorescent particles in the trap is measured by laser scanning confocal fluorescence microscopy. At sufficiently low concentrations at which interactions between the particles are negligible, the single-particle trapping potential-energy profile is determined from the equilibrium number-density profile by use of the Boltzmann distribution. Fluorescence imaging as well as calculations based on a discrete dipole approximation show that effects due to scattering forces are negligible for polystyrene particles of size less than 10% of the wavelength of the trapping laser, thus justifying the assumption of conservative forces in the equilibrium potential-energy determinations. The new optical bottle method measures the entire two-dimensional trapping-potential profile for an individual nanoparticle without the restriction that only one particle be contained in the trap, thus obviating the need for high laser power.
© 2013 Optical Society of America
Original Manuscript: June 19, 2013
Manuscript Accepted: August 10, 2013
Published: October 3, 2013
Jinxin Fu, Qiwen Zhan, Min Yao Lim, Zhiyuan Li, and H. Daniel Ou-Yang, "Potential energy profile of colloidal nanoparticles in optical confinement," Opt. Lett. 38, 3995-3998 (2013)