Direct measurement of the nonconservative force field generated by optical tweezers

The force field of optical tweezers is commonly assumed to be conservative, neglecting the complex action of the scattering force. Using a novel method that extracts local forces from trajectories of an optically trapped particle, we measure the three dimensional force field experienced by a Rayleigh particle with 10 nm spatial resolution and femtonewton precision in force. We find that the force field is nonconservative with the nonconservative component increasing radially away from the optical axis, in agreement with the Gaussian beam model of the optical trap. Together with thermal position fluctuations of the trapped particle, the presence of the nonconservative force can cause a complex flux of energy into the optical trap depending on the experimental conditions

Microtubule dynamics depart from wormlike chain model

Thermal shape fluctuations of grafted microtubules were studied using high resolution particle tracking of attached fluorescent beads. First mode relaxation times were extracted from the mean square displacement in the transverse coordinate. For microtubules shorter than 10 um, the relaxation times were found to follow an L^2 dependence instead of L^4 as expected from the standard wormlike chain model. This length dependence is shown to result from a complex length dependence of the bending stiffness which can be understood as a result of the molecular architecture of microtubules. For microtubules shorter than 5 um, high drag coefficients indicate contributions from internal friction to the fluctuation dynamics.Comment: 4 pages, 4 figures. Updated content, added reference, corrected typo

Development of a Fast Position-Sensitive Laser Beam Detector

We report the development of a fast position-sensitive laser beam detector with a bandwidth that exceeds currently available detectors. The detector uses a fiber-optic bundle that spatially splits the incident beam, followed by a fast balanced photo-detector. The detector is applied to the study of Brownian motion of particles on fast time scales with 1 Angstrom spatial resolution. Future applications include the study of molecule motors, protein folding, as well as cellular processes

Ultrastrong Optical Binding of Metallic Nanoparticles

We demonstrate nanometer precision manipulation of multiple nanoparticles at room temperature. This is achieved using the optical binding force, which has been assumed to be weak compared to the optical gradient and scattering forces. We show that trapping by the optical binding force can be over 20 times stronger than by the gradient force and leads to ultrastable, rigid configurations of multiple nanoparticles free in solution – a realization of “optical matter.” In addition, we demonstrate a novel trapping scheme where even smaller nanoparticles are trapped between larger “anchor” particles. Optical binding opens the door for the observation of collective phenomena of nanoparticles and the design of new materials and devices made from optical matter

Legislative Documents

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