Browsing by Subject "Optical trapping"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
Item Fundamental tests of physics with optically trapped microspheres(2011-05) Li, Tongcang; Raizen, Mark G.; Heinzen, Daniel J.; Reid, Alan W.; Sitz, Greg O.; Gordon, VernitaThis dissertation details our experiments on studying the Brownian motion of an optically trapped microsphere with ultrahigh resolution, and cooling of its motion towards the quantum ground state. We have trapped glass microspheres in water, air and vacuum with optical tweezers. We developed a detection system that can monitor the position of a trapped microsphere with Angstrom spatial resolution and microsecond temporal resolution. We studied the Brownian motion of a trapped microsphere in air over a wide range of pressures. We measured the instantaneous velocity of a Brownian particle. Our results provide direct verification of the Maxwell-Boltzmann velocity distribution and the energy equipartition theorem for a Brownian particle. For short time scales, the ballistic regime of Brownian motion is observed, in contrast to the usual diffusive regime. We are currently developing a new detection system to measure the instantaneous velocity of a Brownian particle in water. In vacuum, we have used active feedback to cool the three center-of-mass vibration modes of a trapped microsphere from room temperature to millikelvin temperatures with a minimum mode temperature of 1.5 mK, which corresponds to the reduction of the root mean square (rms) amplitude of the microsphere from 6.7 nm to 15 pm for that mode. The mean thermal occupation number of that mode is reduced from about 6.8$\times 10^8$ at 297 K to about 3400 at 1.5 mK.Item Measuring the nonconservative force field in an optical trap and imaging biopolymer networks with Brownian motion(2011-12) Thrasher, Pinyu Wu; Florin, Ernst-LudwigOptical tweezers have been widely used by biophysicists to measure forces in single molecular processes, such as the force of a motor molecule walking and the force of a DNA molecule winding and unwinding. In these and similar force measurements, the usual assumption is that the force applied to a particle inside the tweezers is proportional to the displacement of the particle away from the trap center like Hookean springs, which would imply that the force field is conservative. However, the Gaussian beam model has indicated that the force field generated by optical tweezers is actually nonconservative, yet no experiments have measured or accounted for this effect. We introduce an experimental method -- the local drift method -- that can measure the force field in optical tweezers with high precision without any assumptions about the functional form of the force field. The force field is determined by analyzing the Brownian motion of a trapped particle. We successfully applied this method to different sizes of particles and measured the three dimensional force field with 10 nm spatial resolution and femtonewton precision in force. We find that the force field is indeed nonconservative. The nonconservative contribution increases radially away from the optical axis for both small and large particles. The curl vector field -- a measurement of the nonconservative force field -- reverses direction from counter-clockwise for small particles in the Rayleigh regime to clockwise for large particles in the ray optics regime, consistent with the different scattering force profiles in the two distinct scattering regimes. Together with the thermal fluctuations of the trapped particle, the nonconservative force can cause a complex flux of energy into the system. Optically-confined Brownian motion is further used to probe nanostructures such as a biopolymer network. This technique -- thermal noise imaging -- uses a Brownian particle as a "natural scanner" to explore a biopolymer network by moving the Brownian particle through the network with optical tweezers. The position fluctuations of the probe particle reflect the location of individual filaments as excluded volumes. The resolution of thermal noise imaging is directly coupled to the size of the probe particle. A smaller probe is capable of exploring smaller pore sizes formed by dense network. Previously, a 200 nm polystyrene particle had been used to probe an agar network. In this work, 100 nm gold probe particles are used to enhance the resolution. A 100 nm particle explore a network with mesh 2³ times smaller and therefore enhance the network resolution by 2³ times. A 100 nm particle also improves the imaging speed by a factor of 2 because of its faster diffusion. Three-dimensional thermal noise images of agarose filaments are obtained and a resolution of 10 nm for the position of the filaments is achieved. In addition, a gold particle is trapped with significantly less power than a polystyrene particle of the same size, indicating the possibility for using even smaller gold particles to further improve the resolution.Item Submicroscopic characterization of biopolymer networks in solution by Thermal Noise Imaging(2013-05) Bartsch, Tobias Fabian; Florin, Ernst-Ludwig; Shubeita, George T; Aldrich, Richard W; Demkov, Alex A; Fink, ManfredBiopolymer networks display a wide range of interesting mechanical properties that are essential for living organisms. For example, a highly nonlinear elastic response to strain gives biopolymer networks the ability to comply with small stresses but to resist large ones. These macroscopic mechanical properties have their origin in the properties of the individual filaments and their connectedness, like cross-linking geometry and pore size distribution. While the macroscopic properties of biopolymer networks have been extensively studied, there has been a lack of experimental techniques that can simultaneously determine mechanical and architectural properties of networks in situ with single filament resolution. This work introduces Thermal Noise Imaging (TNI) as a novel quantitative method to address these issues. TNI is a three-dimensional scanning probe technique that utilizes the confined thermal motion of an optically trapped particle as a three-dimensional, noninvasive scanner for soft, biological material. Using a photonic force microscope (PFM) custom built for this research, the position of the probe can be detected with nanometer precision and megahertz bandwidth. Two sets of single molecule experiments are described that demonstrate the microscope's exceptional precision and stability. Micrometer scale thermal noise images inside a collagen network are shown and quantitative information about cross-linking geometry is extracted from the data. Further, by imaging microtubules grafted to a support it is shown that the acquired data yield information about the transversal fluctuations of the imaged fibers and about fiber elasticity. These results pave the way for an investigation of force distributions inside biopolymer networks on the single filament level.Item Towards the creation of high-fidelity Fock states of neutral atoms(2013-08) Medellin Salas, David de Jesus; Raizen, Mark G.This dissertation presents the implementation of a technique to generate atomic Fock states of Lithium 6 with ultra-high fidelity, called laser culling. Fock states, atomic states with a definite number of particles, are a mandatory step for studying few-body quantum phenomena such as quantum tunneling, quantum entanglement, and serve as building blocks for quantum simulators. The creation of ultra-high fidelity Fock states begins with a degenerate Fermi gas in an optical dipole trap. Being fermions, lithium-6 atoms fill the energy levels of the dipole trap with 2 atoms per energy level. Introducing a magnetic field gradient creates a linear potential that tilts the potential produced by the optical dipole trap. The initially bound energy levels become quasi-bound states, each with a different lifetime. By exploiting the difference between these lifetimes, one can generate a single pair of atoms in the ground state of the trap with fidelities that can exceed 99.9%. This dissertation first presents the details of the design and construction of an apparatus for laser culling, and then reports on the progress made towards the creation of atomic Fock states with ultra-high fidelity.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.