Trapping and guiding microscopic particles with light-induced forces
Date
2016
Authors
Eckerskorn, Niko Oliver
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Abstract
Contact-free trapping and manipulation of light absorbing
micrometer and nanometer-scale particles in air and in vacuum
utilises radiation pressure, which results from momentum transfer
from photons, and a pressure-dependent thermal force, caused by
momentum transfer from gas molecules to the confined particles.
Both forces are linearly proportional to the illuminating laser
intensity, and both push the particles towards regions of lower
intensity. While the radiation pressure of light was predicted
and described more than a century ago, the theory of thermal
forces, the so called photophoretic force, is still under
development. It depends on a number of poorly described factors,
such as the temperature gradient across the illuminated particle
and thermal creep of heated gas along the particle surface due to
temperature and pressure gradients.
In this thesis I use doughnut-shaped structured laser beams to
levitate and guide light-absorbing micron-size particles aiming
to uncover the optically induced forces in air at variable
pressure ranging from 10-2000 millibar. First, I designed and
built a counter-propagating optical pipeline to uncover the
influence of polarisation on the particle movement. Second, I
designed and constructed a vertically directed diverging vortex
beam trap, a `funnel' trap, to conduct a quantitative evaluation
of the photophoretic force and trapping stiffness by levitating
graphite particles and carbon-coated glass shells of calibrated
sizes in a carefully characterised vortex beam. Third, from the
measured size of the particles and the position of the particle
in the beam on the one hand, and the known density of the
particles and the intensity distribution of the funnel trap on
the other hand, I characterised the optically induced thermal
forces in the axial and transverse directions. Fourth, I compared
the contribution of thermal force to the light-pressure force and
their dependence on atmospheric pressure. Based on the results of
my experiments I determined the parameter space for guiding
particles with hollow-core vortex and Bessel beams, taking into
account the particle speed, size, and offset from the laser axis,
all linked to the optical beam properties such as beam
divergence, optical polarisation and power. The results of this
thesis are used in the development of a touch-free optical system
for pin-point delivery of macromolecules to the X-ray focal spot
at the Free Electron Laser facility at the DESY (Deutsches
Elektronen-Synchrotron) synchrotron in Hamburg, Germany, for
coherent diffractive imaging experiments on nanometer-scale
morphology. I conclude with a discussion of avenues for future
work in contact-free manipulating of particles with structured
laser beams to enhance significantly the efficiency of
nanometer-scale morphology of proteins and biomolecules.
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Keywords
optical trapping, photophoresis, optical funnel, trapping, DESY, coherent diffractive imaging
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Thesis (PhD)
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