Quantitative Optical Trapping of Single

We report a quantitative analysis of the forces acting on optically trapped single gold nanorods. Individual nanorods with diameters between 8 and 44 nm and aspect ratios between 1.7 and 5.6 were stably trapped in three dimensions using a laser wavelength exceeding their plasmon resonance wavelengths. The interaction between the electromagnetic field of an optical trap and a single gold nanorod correlated with particle polarizability, which is a function of both particle volume and aspect ratio. Gold nanoparticles are promising tools for optical applica-tions in materials science and the life sciences due to their unusual optical properties which are caused by localized surface plasmon resonances.1,2 At the plasmon resonance wavelength, collective oscillations of the conduction electrons lead to enhanced light absorption and scattering, so that particles down to a few nanometers in size can be detected with dark-field3 and luminescence microscopy.4 Varying the size and shape of the particles tunes the plasmon resonances over a wide range of wavelengths.