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.

Within the Rayleigh approximation, we investigate the behavior of an individual ellipsoidal metal nanorod that is optically confined in three dimensions using a single focused laser beam. We focus on the description of the optical torque and optical force acting upon the nanorod placed into a linearly polarized Gaussian beam (scalar description of the electric field) or a strongly focused beam (vector field description). The study comprises the influence of the trapping laser wavelength, the angular aperture of focusing optics, the orientation of the ellipsoidal nanorod, and the aspect ratio of its principal axes. The results reveal a significantly different behavior of the nanorod if the trapping wavelength is longer or shorter than the wavelength corresponding to the longitudinal plasmon resonance mode. Published experimental observations are compared with our theoretical predictions with satisfactory results.

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.

It is generally accepted that the interaction between particles mediated by the scattered light (called optical binding) is very weak. Therefore, the optical binding is usually neglected in a multi-particle trapping in distinct optical traps. Here we show that even the presence of only two dielectric particles confined in the standing wave leads to their significantly different behavior comparing to the case of a single trapped particle. We obtained persuading coincidence between our experimental records and the results of the deterministic and stochastic theoretical simulations based on the coupled dipole method.

Standing wave optical trapping offers many useful advantages in comparison to single beam trapping, especially for submicrometer size particles. It provides axial force stronger by several orders of magnitude, much higher axial trap stiffness, and spatial confinement of particles with higher refractive index. Mainly spherical particles are nowadays considered theoretically and trapped experimentally. In this paper we consider prolate objects of cylindrical symmetry with radius periodically modulated along the axial direction and we present a theoretical study of optimized objects shapes resulting in up to tenfold enhancement of the axial optical force in comparison with the original unmodulated object shape. We obtain analytical formulas for the axial optical force acting on low refractive index objects where the light scattering by the object is negligible. Numerical results based on the coupled dipole method are presented for objects with higher refractive indices and they support the previous simplified analytical conclusions.