Browsing by Subject "Optical binding"
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Item Determining arrangements of optically bound nanoparticle clusters in three dimensions in a Gaussian beam standing wave optical trap(2015-08) Grimm, Philipp Martin; Florin, Ernst-Ludwig; Fink, ManfredThe 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.Item Ultra-precise manipulation and assembly of nanoparticles using three fundamental optical forces(2012-12) Demergis, Vassili; Florin, Ernst-Ludwig; Shubeita, George T; Fink, Manfred; Makarov, Dmitrii E; Korgel, Brian AThe invention of the laser in 1960 opened the door for a myriad of studies on the interactions between light and matter. Eventually it was shown that highly focused laser beams could be used to con fine and manipulate matter in a controlled way, and these instruments were known as optical traps. However, challenges remain as there is a delicate balance between object size, precision of control, laser power, and temperature that must be satisfied. In Part I of this dissertation, I describe the development of two optical trapping instruments which substantially extend the allowed parameter ranges. Both instruments utilize a standing wave optical field to generate strong optical gradient forces while minimizing the optical scattering forces, thus dramatically improving trapping efficiency. One instrument uses a cylinder lens to extend the trapping region into a line focus, rather than a point focus, thereby confining objects to 1D motion. By translation of the cylinder lens, lateral scattering forces can be generated to transport objects along the 1D trapping volume, and these scattering forces can be controlled independently of the optical gradient forces. The second instrument uses a collimated beam to generate wide, planar trapping regions which can con fine nanoparticles to 2D motion. In Part II, I use these instruments to provide the first quantitative measurements of the optical binding interaction between nanoparticles. I show that the optical binding force can be over 20 times stronger than the optical gradient force generated in typical optical traps, and I map out the 2D optical binding energy landscape between a pair of gold nanoparticles. I show how this ultra-strong optical binding leads to the self-assembly of multiple nanoparticles into larger contactless clusters of well de ned geometry. I nally show that these clusters have a geometry dependent coupling to the external optical field.