1. s"
Computational Models of Optical Traps
Casey Cannon, Tom Donnelly, and Roland Smith
Motivation
Our research group is interested in studying stochastic heating, a
heating process with applications to laser-driven nuclear fusion.
We expect this heating mechanism to produce hot electrons
when a high-intensity laser pulse interacts with the electrons in a
spherical target. The interaction between the spherical target and
the laser pulse must happen in vacuum and the sphere cannot be
in contact with any support structure. One strategy is to use an
optical trap to hold the target sphere in place.
Harvey Mudd College Physics Department
Air" Vacuum"
Trapping"
Beam"
Trapping"
Beam"
High"
Intensity"
Pulse"
Goals
• Model the trapping of reflective spheres. Traditionally, optical
traps have been used with transparent spheres. We are
interested in trapping metal/reflective spheres because they
have a large electron density. We want to be able to explore
this regime.
• Measure the radius of trapped spheres. We expect the heating
mechanism to be significantly different for different size
spheres, so it is important to accurately know the size of the
spheres.
• Have the sphere trapped at least ~10 cm away from the
expensive optic used to focus the laser, so that no optics are
damaged by the high-intensity pulse.
Figure'1:'An aerosol of spheres is sprayed in the vicinity of a continuous-
wave trapping laser. At least one of these spheres is caught. The chamber is
brought to vacuum and a high-intensity pulse is fired at the trapped sphere.
Casey"Cannon
ccannon@hmc.edu
(805) 302-7179
Optical Trapping Mechanism
In Figure 2 we have a continuous-wave laser oriented vertically in
air, and a spherical target is a certain distance away from the z-
axis, the beam’s central axis. In Figure 2a, the spherical target is
transparent, and in Figure 2b, it is reflective. The transparent
sphere lenses photons inwards while the reflective sphere
deflects them outwards. The change in the photons’ momentum
pushes the sphere up, counteracting gravity. Depending on if
there are more photons incident on the inner half of the sphere
versus the outer half, the sphere will be pushed either towards or
away from the beam axis. The curve at the bottom of the figures
indicates the laser intensity distribution that will impart a
restoring force on the sphere to trap it.
Figure"2"(a) " " " " ""(b)"
A transparent (a) and reflective (b) sphere are being trapped by lasers with
intensity profiles given by the curves at the bottom of the figures.
Results/Future Work
I have successfully created a computational model for trapping
reflective spheres. I have performed various tests to make sure
my model is working properly. I have found reasonable
parameters for trapping spheres using a donut mode beam and
the Bessel beam. I have also studied responses of trapped
spheres to temporal changes in power. Since the responses
depend on sphere size, this can be used to measure sphere
radius. Future work includes characterizing the stability of the
traps and modeling of thin spherical shells.
Acknowledgements
Ashkin, Arthur. “Acceleration and trapping of particles by radiation
pressure.” Physical review letters 24.4 (1970): 156
"Green laser pointer TEM00 profile" by Zaereth - Own work. Licensed
under CC0 via Wikimedia Commons - https://commons.wikimedia.org/
wiki/File: Green_laser_ pointer_TEM00_profile.JPG#/media/
File:Green_laser_pointer_TEM00_profile.JPG
McGloin, D., and K. Dholakia. “Bessel beams: diffraction in a new light.”
Contemporary Physics 46.1 (2005): 15-28
g
Figure"3"(a) " " """"(b)" " " "(c)"
The intensity profiles of different laser modes have advantages and
disadvantages for trapping spheres. A Gaussian beam (a) can trap
transparent spheres as in Figure 2a while a donut (b) or Bessel (c)
beam can trap reflective spheres as in Figure 2b. The donut beam
spreads out as it propagates but can propagate indefinitely in a vacuum.
The Bessel beam does not spread out, but its intensity eventually dies
off.