Browsing by Subject "Optical trap"
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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 Development of quantitative three-dimensional thermal noise imaging of biopolymer filaments(2012-05) Kochanczyk, Martin David; Florin, Ernst-Ludwig; Shubeita, George; Marder, Michael; Bengtson, Roger; Zhang, John XBiopolymer networks perform many essential functions for living cells. Most of these networks show a highly nonlinear mechanical response that is well-studied on the macroscopic scale. While much work has been done to connect the macroscopic responses of networks to microscopic parameters, such as filament stiffness, cross-linking geometry and pore size, there is a lack of experimental techniques that can measure these properties in situ. This thesis presents the development of a quantitative scanning probe imaging technique, which can explore soft matter in an aqueous environment. An optical tweezer-based microscope, called a photonic force microscope, was designed and constructed. A stability analysis method, called Power Spectrum Integration Analysis, was developed and was used to show that the photonic force microscope achieves nanometer precision in the measurement of probe position with a bandwidth of 1MHz. A novel single filament assay was developed that allowed for the isolation and probing of individual biopolymer filaments. A scanning probe technique, called thermal noise imaging, which uses the diffusive motion of an optically trapped nanoparticle as a fast, natural scanner, was used to scan microtubules grafted on one end. The resulting thermal noise images were strongly influenced by the thermally driven, transverse fluctuations of the filaments. Analytical tools, which include Brownian dynamics simulations of probe and filament, were developed to assist quantitative analysis of thermal noise images. The persistence length of individual microtubules was extracted, and the length dependence persistence length for taxol stabilized microtubules was confirmed. The transverse fluctuations of a microtubule grafted on both ends were imaged. Finally, thermal noise images of collagen filaments inside a three-dimensional collagen network were recorded, and variations of the filament diameter were extracted. This thesis establishes thermal noise imaging as a quantitative tool for studying soft material on the nanometer scale, as well as paves the way for investigating force distributions inside biopolymer networks.Item High-precision laser beam shaping and image projection(2012-05) Liang, Jinyang, 1985-; Becker, Michael F.; Heinzen, Daniel J.; Tunnell, James W.; Evans, Brian L.; Bank, Seth R.Laser beams with precisely controlled intensity profiles are essential for many areas. We developed a beam shaping system based on the digital micromirror device (DMD) for ultra-cold atom experiments and other potential applications. The binary DMD pattern was first designed by the error diffusion algorithm based on an accurate measurement of the quasi-Gaussian incident beam from a real-world laser. The DMD pattern was projected to the image plane by a bandwidth-limited 4f telescope that converted this pattern to the grayscale image. The system bandwidth determined the theoretical limit of image precision by the digitization error. In addition, it controlled the spatial shape of the point spread function (PSF) that reflected the tradeoff between image precision and spatial resolution. PSF was used as a non-orthogonal basis set for iterative pattern refinement to seek the best possible system performance. This feedback process, along with stable performance of DMD, the blue-noise spectrum of the error diffusion algorithm, and low-pass filtering, guaranteed high-precision beam shaping performance. This system was used to produce various beam profiles for different spatial frequency spectra. First, we demonstrated high-precision slowly-varying intensity beam profiles with an unprecedented high intensity accuracy. For flattop and linearly-tilted flattop beams, we achieved 0.20-0.34% root-mean-square (RMS) error over the entire measurement region. Second, two-dimensional sinusoidal-flattop beams were used to evaluate image precision versus system bandwidth. System evaluation confirmed that this system was capable of producing any spatial pattern with <3% RMS error for the most system bandwidth. This experiment extended the beam shaping to any system bandwidth and provided a reference to estimate the output image quality based on its spatial spectrum. Later experiment using a Lena-flattop beam profile demonstrated the arbitrary beam profile generation. We implemented this system for applications on the homogenous optical lattice and dynamic optical trap generation. The DMD pattern was optimized by the iterative refinement process at the image feedback arm, and projected through a two-stage imaging system to form the desired beam profile at the working plane. Experiments demonstrated a high-precision beam shaping as well as a fast and dynamic control of the generated beam profile.Item Single-photon atomic cooling(2009-08) Price, Gabriel Noam; Raizen, Mark G.This dissertation details the development and experimental implementation of single-photon atomic cooling. In this scheme atoms are transferred from a large-volume magnetic trap into a small-volume optical trap via a single spontaneous Raman transition that is driven near each atom's classical turning point. This arrangement removes nearly all of an atomic ensemble's kinetic energy in one dimension. This method does not rely on a transfer of momentum from photon to atom to cool. Rather, single-photon atomic cooling achieves a reduction in temperature and an increase in the phase-space density of an atomic ensemble by the direct reduction of the system's entropy. Presented here is the application of this technique to a sample of magnetically trapped ⁸⁷Rb. Transfer efficiencies between traps of up to 2.2% are demonstrated. It is shown that transfer efficiency can be traded for increased phase-space compression. By doing so, the phase-space density of a magnetically trapped ensemble is increased by a factor of 350 by the single-photon atomic cooling process.