Tractor beams are traveling waves that transport illuminated objects in the retrograde direction relative to the direction of propagation. The theory of photokinetic effects identifies design criteria for long-range general- purpose tractor beams. These criteria distinguish first-order tractor beams that couple to induced dipole moments from higher-order tractor beams that rely on coupling to higher-order multipole moments to achieve pulling. First-order tractor beams are inherently longer-ranged and operate on a wider variety of materials. We explore the physics of first-order tractor beams in the context of a family of generalized solenoidal waves.
The video stream captured by an in-line holographic microscope can be analyzed on a frame-by-frame basis to track
individual colloidal particles' three-dimensional motions with nanometer resolution, and simultaneously to measure their
sizes and refractive indexes. An efficient particle-tracking algorithm automates initial position estimation with sufficient
accuracy to enable unattended holographic particle tracking and characterization. In this work, we demonstrated this
approach to flow visualization in a microfluidic channel and also to flow cytometry of micrometer-scale colloidal
We present a novel method for single frame particle image velocimetry of micron scale
spheres based on holographic video microscopy. Our approach takes advantage of the blurring
that recorded holograms suffer when a sphere moves during the exposure period of the camera.
By measuring the angular variance in intensity of the blurred hologram, we extract a modelindependent
metric for the particle velocity. We find this to be accurate for speeds that
permit characterization of other properties of the sphere, such as radius and refractive index
through Lorenz-Mie mocroscopy. Singl-frame holographic velocimetry yields information on
the dynamics of a particle, without sacrificing any other measurements.
Mechanical equilibrium at zero temperature does not necessarily imply thermodynamic equilibrium at finite
temperature for a particle confined by a static, but non-conservative force field. Instead, the diffusing particle
can enter into a steady state characterized by toroidal circulation in the probability flux, which we call a Brownian
vortex. The circulatory bias in the particle's thermally-driven trajectory is not simply a deterministic response
to the solenoidal component of the force, but rather reflects an interplay between advection and diffusion in
which thermal fluctuations extract work from the non-conservative force field. As an example of this previously
unrecognized class of stochastic machines, we consider a colloidal sphere diffusing in a conventional optical
tweezer. We demonstrate both theoretically and experimentally that non-conservative optical forces bias the
particle's fluctuations into toroidal vortexes whose circulation can reverse direction with temperature or laser
Optical traps use forces exerted by specially structured beams of light to localize microscopic objects in three
dimensions. In the case of single-beam optical traps, such as optical tweezers, trapping is due entirely to gradients
in the light's intensity. Gradients in the light field's phase also control optical forces, however, and their quite
general influence on trapped particles' dynamics has only recently been explored in detail. We demonstrate
both theoretically and experimentally how phase gradients give rise to forces in optical traps and explore the
sometimes surprising influence of phase-gradient forces on trapped objects' motions.
Holographic optical trapping uses forces exerted by computer-generated holograms to organize microscopic materials
into three-dimensional structures. Achieving and verifying accurate three-dimensional placement requires
methods for assessing the accuracy of the projected traps' geometry, as well as methods for measuring trapped
objects' three-dimensional positions. Volumetric imaging of the projected trapping pattern solves one problem.
Holographic video microscopy addresses the other. The combination is exceptionally effective for organizing,
inspecting and analyzing soft-matter systems.
Colloidal particles driven by an optical vortex constitute a model driven-dissipative system in which both macroscopic
and microscopic aspects of transport can be studied. We find that the single-particle diffusion in an optical
vortex can be either normal super-diffusive or sub-diffusive depending on the number of particles in the vortex
and on the timescale over which the diffusion is measured. For a three particle system we find that the particles
dynamics can be either steady-state periodic or with weakly chaotic characteristics depending on the relative
efect of modulations in the intensity along the vortex and hydrodynamic interactions between the spheres. We
introduce the use of the N-fold bond orientational order parameter to characterize particle circulating in a ring
by one macroscopic quantity. for a three particle system we show that for short time scales the single-particle
super-diffusion corresponds to a super-diffusive motion of the order parameter. At longer time scales we find that
the order parameter asymptotes to the expected normal di.usion behavior for the steady state system, while
fractional dynamics develop in the weakly chaotic system. Moreover, we confirm a prediction that related the
power laws governing the fractional dynamics with those governing the weakly chaotic behavior.
We describe a class of diffractive optical elements that intended for use in holographic optical trapping systems to project ring-like optical traps. The ring traps' hologram is design by shape-phase algorithm, that incodes amplitude information in Fourier space into shape. The flexibility of the shape-phase algorithm leeds to several advantages over commonly used optical vorteces. While, optical vorteces carry orbital angular momentum which determines the vortex diameter, ring trap dimensions do not depend on the angular momentum. In particular, we formed a ring trap without angular momentum, or tangetial-force. Also unlike most optical vortices or ring-like Bessel beams, these ring traps have strong enough axial intensity gradients to trap objects in three dimensions. We investigated the three dimensional vortex-like trapping experimentaly and by volumetric imaging.