Dynamics of an optically bound structure made of particles of unequal sizes.

This theoretical study based on the coupled dipoles model focuses on the dynamics of two optically bound dielectric spheres of unequal sizes confined in counter-propagating incoherent Bessel beams. We analyzed the relative motion of the particles with respect to each other and defined conditions where they form a stable optically bound structure (OBS). We also investigated the motion of the center of mass of the OBS and found that its direction depends on the particle separation in the structure. Besides the optical interaction between objects, we also considered a hydrodynamic coupling in order to obtain more precise results for moving an OBS.

Non-spherical gold nanoparticles trapped in optical tweezers: shape matters.

We present the results of a theoretical analysis focused on three-dimensional optical trapping of non-spherical gold nanoparticles using a tightly focused laser beam (i.e. optical tweezers). We investigate how the wavelength of the trapping beam enhances trapping stiffness and determines the stable orientation of nonspherical nanoparticles in the optical trap which reveals the optimal trapping wavelength. We consider nanoparticles with diameters being between 20 nm and 254 nm illuminated by a highly focused laser beam at wavelength 1064 nm and compare our results based on the coupled-dipole method with published theoretical and experimental data. We demonstrate that by considering the non-spherical morphology of the nanoparticle we can explain the experimentally observed three-dimensional trapping of plasmonic nanoparticles with size higher than 170 nm. These results will contribute to a better understanding of the trapping and alignment of real metal nanoparticles in optical tweezers and their applications as optically controllable nanosources of heat or probes of weak forces and torques.

Three-dimensional optical trapping of a plasmonic nanoparticle using low numerical aperture optical tweezers.

It was previously believed that larger metal nanoparticles behave as tiny mirrors that are pushed by the light beam radiative force along the direction of beam propagation, without a chance to be confined. However, several groups have recently reported successful optical trapping of gold and silver particles as large as 250 nm. We offer a possible explanation based on the fact that metal nanoparticles naturally occur in various non-spherical shapes and their optical properties differ significantly due to changes in localized plasmon excitation. We demonstrate experimentally and support theoretically three-dimensional confinement of large gold nanoparticles in an optical trap based on very low numerical aperture optics. We showed theoretically that the unique properties of gold nanoprisms allow an increase of trapping force by an order of magnitude at certain aspect ratios. These results pave the way to spatial manipulation of plasmonic nanoparticles using an optical fibre, with interesting applications in biology and medicine.