Determining arrangements of optically bound nanoparticle clusters in three dimensions in a Gaussian beam standing wave optical trap

Grimm, Philipp Martin
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The invention of optical tweezers in 1986 has enabled controlled trapping and manipulating of dielectric particles in the microscopic and nanoscopic regime. More recently, using a specialized optical trap, a novel ultra-strong particle-particle interaction, based on scattered fields and induced dipoles was discovered, namely lateral optical binding. It can be used to achieve self-assembly of nanoparticles into contactless clusters with stable configurations. Experiments have shown that coupling of these clusters to the external electromagnetic field depends on the cluster geometry. The observation was attributed to asymmetries in cluster constituents, such as different particle radii, but a simultaneous experimental observation of cluster geometry and particle radii remained challenging. In this thesis a new method is introduced which measures simultaneously the configuration of a pair of optically bound nanoparticles in three dimensions as well as the ratio of particle radii. This ratio is approximated in two different ways, by analyzing the particle widths in darkfield microscopy images and by analyzing the power of the light scattered from the nanospheres. After validating the procedure and data evaluation for a single immobilized bead it was applied to optically bound particle pairs in a Gaussian beam standing wave optical trap. Both particle size estimations provide similar results. It can be concluded that the difference in brightness observed for distinct nanoparticles originates from a difference in their radii and not from their relative displacements along the optical axis. Nevertheless, two particles with significant difference in radius tend to assemble at slightly different axial positions. This deviation from ideal lateral optical binding may cause additional geometry dependency on the coupling of the cluster to the external optical field and should be included into simulations on optical binding dynamics. Finally, an astonishing symmetry break even for particle pairs with similar radii was observed. The center of mass of these clusters shows a shift a few times as large as the exciting wavelength and particle separation distance away from the trap center to a new, well-defined equilibrium position. This observation challenges the current theoretical explanation of the lateral shifts which requires an asymmetry in the cluster constituents.