We experimentally demonstrate a class of tractor beams created by coherently superposing coaxial Bessel beams. These optical conveyors have periodic intensity variations along their axes that act as highly effective optical traps for micrometer-scale objects. Trapped objects can be moved selectively upstream or downstream along the conveyor by appropriately changing the Bessel beams' relative phase. The same methods used to project a single optical conveyor can project arrays of independent optical conveyors, allowing bidirectional transport in three dimensions.
We demonstrate how holographic video microscopy can be used to detect, count, and characterize individual micrometer-scale protein aggregates as they flow down a microfluidic channel in their native buffer. Holographic characterization directly measures the radius and refractive index of subvisible protein aggregates and offers insights into their morphologies. The measurement proceeds fast enough to build up population averages for time-resolved studies and lends itself to tracking trends in protein aggregation arising from changing environmental factors. Information on individual particle’s refractive indexes can be used to differentiate protein aggregates from such contaminants as silicone droplets. These capabilities are demonstrated through measurements on samples of bovine pancreas insulin aggregated through centrifugation and of bovine serum albumin aggregated by complexation with a polyelectrolyte. Differentiation is demonstrated with samples that have been spiked with separately characterized silicone spheres. Holographic characterization measurements are compared with results obtained with microflow imaging and dynamic light scattering.
The spin angular momentum in an elliptically polarized beam of light plays several noteworthy roles in optical traps. It contributes to the linear momentum density in a non-uniform beam, and thus to the radiation pressure exerted on illuminated objects. It can be converted into orbital angular momentum, and thus can exert torques even on optically isotropic objects. Its curl, moreover, contributes to both forces and torques without spin-to-orbit conversion. We demonstrate these effects experimentally by tracking colloidal spheres diffusing in elliptically polarized optical tweezers. Clusters of spheres circulate determinisitically about the beam's axis. A single sphere, by contrast, undergoes stochastic Brownian vortex circulation that maps out the optical force field.Optical forces arising from the polarization and polarization gradients in vector beams of light constitute a new frontier for optical micromanipulation. Linearly polarized light has been used to orient birefringent objects in conventional optical tweezers [1][2][3] and circular polarization has been used to make them rotate [1,[3][4][5][6][7]. More recently, optically isotropic objects also have been observed to circulate in circularly polarized optical traps [8][9][10], through a process described as spin-to-orbit conversion [10][11][12][13][14]. Here, we present a general formulation of the linear and angular momentum densities in vector beams of light that clarifies how the amplitude, phase and polarization profiles contribute to the forces and torques that such beams exert on illuminated objects. This formulation reveals that the curl of the spin angular momentum can exert torques on illuminated objects without contributing to the light's orbital angular momentum, and that this effect dominates spin-to-orbit conversion in circularly polarized optical tweezers. Predicted properties of polarization-dependent optical forces are confirmed through observations of a previously unreported mode of Brownian vortex circulation for an isotropic sphere in elliptically polarized optical tweezers.The vector potential describing a beam of light of angular frequency ω may be written aswhere u(r) is the real-valued amplitude, ϕ(r) is the realvalued phase andǫ(r) is the complex-valued polarization vector at position r. This description is useful for practical applications because u(r), ϕ(r) andǫ(r) may be specified independently, for example using holographic techniques [3,[15][16][17]]. Poynting's theorem then yields the time-averaged momentum densitywhere µ is the permeability of the medium and c is the speed of light in the medium. The momentum density gives rise to the radiation pressure that the light exerts on illuminated objects and may be expressed in terms of the experimentally accessible parameters aswhere I(r) = u 2 (r) is the intensity and whereis the spin angular momentum density in a beam of light with local helicityThe projection of σ(r) onto the propagation direction k(r) is related to the Stokes parameters of the beam [18] by σ(r) ·k(r) = S 3 (...
In-line holographic microscopy images of micrometer-scale fractal aggregates can be interpreted with an effective-sphere model to obtain each aggregate’s size and the population-averaged fractal dimension. We demonstrate this technique experimentally using model fractal clusters of polystyrene nanoparticles and fractal protein aggregates composed of bovine serum albumin and bovine pancreas insulin.
Frequency shifts and dissipation of a compound torsional oscillator induced by solid 4 He samples containing 3 He impurity concentrations (x3 = 0.3, 3, 6, 12 and 25 in units of 10 −6 ) have been measured at two resonant mode frequencies (f1 = 493 and f2 = 1164 Hz) at temperatures (T ) between 0.02 and 1.1 K. The fractional frequency shifts of the f1 mode were much smaller than those of the f2 mode. The observed frequency shifts continued to decrease as T was increased above 0.3 K, and the conventional non-classical rotation inertia fraction was not well defined in all samples with x3 ≥ 3 ppm. Temperatures where peaks in dissipation of the f2 mode occurred were higher than those of the f1 mode in all samples. The peak dissipation magnitudes of the f1 mode was greater than those of the f2 mode in all samples. The activation energy and the characteristic time (τ0) were extracted for each sample from an Arrhenius plot between mode frequencies and inverse peak temperatures. The average activation energy among all samples was 430 mK, and τ0 ranged from 2×10 −7 s to 5×10 −5 s in samples with x3 = 0.3 to 25 ppm. The characteristic time increased with increasing x3. Observed temperature dependence of dissipation were consistent with those expected from a simple Debye relaxation model if the dissipation peak magnitude was separately adjusted for each mode. Observed frequency shifts were greater than those expected from the model. The discrepancies between the observed and the model frequency shifts increased at the higher frequency mode.
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