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Biomedical subjects

Alexander Rohrbach

Publications and source records attributed to Alexander Rohrbach.

7 recordsLinked to original sources
PubMed2006

Interferometric tracking of optically trapped probes behind structured surfaces: A phase correction method.

We investigate the influence of an additional scatterer on the tracking signal of an optically trapped particle. The three-dimensional particle position is recorded interferometrically with nanometer precision by using a quadrant photodiode in the back focal plane of a detection lens. A phase disturbance underneath the sample leads to incorrect position signals. The resulting interaction potential and forces are therefore erroneous as well. We present a procedure to correct for the disturbance by measuring its interferometric signal. We prove the applicability of our phase correction approach by generating a defined displacement of the trapped probe.

Journal Article↗
PubMed2005

Stiffness of optical traps: quantitative agreement between experiment and electromagnetic theory.

The first quantitative agreement between measured and calculated stiffnesses of optically trapped particles in the subwavelength regime is presented. It is shown for all three dimensions that the measured extent of harmonic optical trapping potentials for dielectric spheres comes very close to the theoretically predicted extent, provided all known instrumental parameters are considered. The recently predicted strong asymmetry of the trapping potential due to the electric field's linear polarization has been verified in all three directions. This effect vanishes for spheres with diameters d approximately lambda, which exhibit the strongest trap stiffnesses.

Journal Article↗
PubMed2005

Control of relative radiation pressure in optical traps: application to phagocytic membrane binding studies.

We show how to control the relative radiation pressure and thereby the stable trap position of an optically trapped bead by variation of the mean incident axial photon momentum. The thermal position fluctuations of a trapped bead are recorded by a three-dimensional back-focal-plane interferometry. The interferometric detection signals are in agreement with predictions based on an extended Mie theory. Depending on the application, the unique and linear range of such a detection system can be optimized by controlling the trap position of the bead. We use this method to investigate in three dimensions the binding of beads to membranes of living cells during phagocytosis. We found that independent of the bead coating (IgG, complement, LPS, avidin) the most frequent initial mechanical response of the cell was a downward pulling of the bead into the cell. The time delay between binding and response was on average 2 s.

Animals↗
PubMed2005

Three-dimensional bead position histograms reveal single-molecule nanomechanics.

We describe a method to investigate the structure and elasticity of macromolecules by a combination of single molecule experiments and kinematic modeling. With a photonic force microscope, we recorded spatial position histograms of a fluctuating microsphere tethered to full-length myosin-II. Assuming only that the molecule consists of concatenated rigid segments, a model derived from robot kinematics allows us to relate these histograms to the molecule's segment lengths and bending stiffnesses. Both our calculated position distributions and the experimental data show an asymmetry characteristic of a mixed entropic-enthalpic spring. Our model that fits best to experimental line profiles has two intramolecular hinges, one at the bound head domain, and another about 50 nm down the myosin tail, with a summed bending stiffness of about 3 k(B)T/rad.

Algorithms↗
PubMed2003

Three-dimensional tracking of small spheres in focused laser beams: influence of the detection angular aperture.

Back-focal-plane interferometry is a method capable of determining the three-dimensional position of a particle with high precision (< 3 nm) at high sampling rates (1 MHz). We investigated theoretically the performance of such a system for dielectric spheres with diameters D = 0.53-3 microm and for metallic spheres with D < or = 300 nm. Good sensitivity and linearity were achieved for a detection angular aperture sin(alpha) of no more than 0.5. A value of sin(alpha) > 0.7 should be used only for dielectric spheres with diameters approximately equal to the laser wavelength. Harmonic optical traps can be calibrated by measurement of the thermal motion of the sphere. We performed Brownian dynamics simulations and subsequent thermal noise analyses to prove that the wrong sin(alpha) incorrectly suggests an increased and nonharmonic axial trapping potential.

Journal Article↗
PubMed2002

Trapping forces, force constants, and potential depths for dielectric spheres in the presence of spherical aberrations.

We present and verify a theoretical model that predicts trapping forces (escape forces), force constants (trap stiffnesses), and trapping potential depths for dielectric spheres with diameters smaller than or equal to the wavelength of the trapping light. Optical forces can be calculated for arbitrary incident light distributions with a two-component approach that determines the gradient and the scattering force separately. We investigate the influence of spherical aberrations that are due to refractive-index mismatch on the maximum trapping force, the force constant, and the potential depth of a trap, which are important for optical tweezer applications. The relationships between the three parameters are explained and studied for different degrees of spherical aberration and various spheres (refractive indices n(s) = 1.39-1.57, radii a = 0.1-0.5 microm, lambda(0) = 1.064 microm). We find that all three parameters decrease when the distance to the coverslip increases. Effects that could make the interpretation of experimental results ambiguous are simulated and explained. Computational results are compared with the experimental data found in the literature. A good coincidence can be established.

Models, Theoretical↗