At fast timescales, the self-similarity of random Brownian motion is expected to break down and be replaced by ballistic motion. So far, an experimental verification of this prediction has been out of reach due to a lack of instrumentation fast and p
recise enough to capture this motion. With a newly developed detector, we have been able to observe the Brownian motion of a single particle in an optical trap with 75 MHz bandwidth and sub-{AA}ngstrom spatial precision. We report the first measurements of ballistic Brownian motion as well as the first determination of the velocity autocorrelation function of a Brownian particle. The data are in excellent agreement with theoretical predictions taking into account the inertia of the particle and the surrounding fluid as well as hydrodynamic memory effects.
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 thre
e 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.
We demonstrate that the data presented in the manuscript by Y. Roichman et al. are not sufficient to show that the circulation of a trapped particle exists in a static optical trap.
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.
