OPTICAL TRAPPING: Trapping near resonance could reduce biological damage
(doc)
The two forces associated with optical trapping are the scattering force in the
direction of beam propagation and the gradient force in the direction of the
gradient of light due to the highly focused beam. The gradient force must
overcome the scattering force to allow stable, conventional single-beam optical
trapping, and it does so when the particle is trapped far from resonance on the
red side of the spectrum (assuming the focus is very tight and optical power is
high enough). For these reasons (and also because lasers are more commercially
available here), optical trapping and tweezing is most frequently carried out
far from any optical-absorption resonance.
Typically, polystyrene or silica beads on the order of 2 µm in size, which
exhibit resonances in the UV or blue wavelength regions, are trapped using
lasers from the green to the IR part of the spectrum. However, near-resonant
trapping is used extensively in laser cooling and trapping of much smaller
neutral atoms–so why shouldn’t these properties apply to somewhat larger
particles? To discover the answer, and to better understand the mechanisms
behind wavelength-dependent and near-resonant trapping, Brooke Hester from the
National Institute of Standards and Technology (NIST; Gaithersburg, MD)–a Ph.D.
candidate in the Chemical Physics program at the University of Maryland who is
doing her dissertation research at NIST–performed trapping experiments using a
home-built microscope trapping apparatus (see figure).1
Why near-resonant trapping?
Both the gradient and scattering optical forces are predicted to be strongest
near the resonance absorption of a trapped Rayleigh particle (a particle much
smaller than the trap wavelength). But the gradient force (the force responsible
for holding the particle near the focus) is maximized when it is red-shifted
from the resonance peak. Basically, the force responsible for trapping should be
the strongest if the correct trap wavelength is chosen. If this is true, says
Hester, then near-resonant optical trapping of single Rayleigh particles would
require less optical power than the typical far-resonant optical trapping.
Most optical-tweezing experiments use a trapped particle as a marker attached to
some other entity of interest such as a DNA strand or protein. Properties of the
particle of interest can be measured by exposing it to some force or other
stimuli, and its reaction can be tracked by recording the response of the
attached, optically trapped object. Because these experiments take place in
biological environments, the lower the laser power, the better–especially in the
case of optical tweezers that focus a large amount of optical power into a tiny
volume. So if researchers can use these Rayleigh particles in a near-resonant
trapping configuration, they could prevent damage to delicate biological
specimens.
A microscope apparatus is being used at NIST to understand differences between
near-resonance and far-from-resonance trapping and tweezing of particles. The
study could improve the prospects for near-resonant trapping at lower power
levels, to limit damage to biological specimens. (Courtesy of NIST)
In addition, if higher forces are present near resonance, this effect could be
exploited for optical-sorting experiments in which a researcher could trap
certain near-resonant particles but not others whose resonance is far from the
wavelength of the laser trap.
Observations and findings
In the experimental setup, a single-focus TEM00
Ti:sapphire Trestles laser from
Del Mar Photonics (San Diego, CA) is used for optical trapping. The tiny
particles used in the experiment are gold nanoshells, fabricated by Naomi Halas
and her Nanophotonics group at Rice University. Chemical techniques are used to
coat nanoscopic silica spheres with a thin and even layer of gold. The thickness
of the gold shell and the size of the silica sphere determine the location and
width of the resonance peak in the extinction spectrum of the gold nanoshell.
These dimensions can be manipulated with extreme sensitivity, allowing the
researchers at Rice to synthesize nanoshells with resonances from the visible to
the IR.
For this study, the resonances of the four nanoshell species are 650, 755, 790,
and 1100 nm. Preliminary experimental results show that lower laser power is
required to trap a nanoshell nearer the red side of the spectrum compared to
far-from-resonance trapping. For example, for a nanoshell with a resonance at
790 nm, lower laser power is required for trapping between 790 and 860 nm
compared to trapping at 1064 nm, and this required power decreases as the
trapping wavelength approaches 790 nm. The trapping wavelength range is limited
only by the wavelength range of the laser.
Hester says that further experiments will quantify this effect near to, far
from, and at resonance, and will focus on the effects of nanoshell heating in
optical-tweezer experiments.
–Gail Overton
REFERENCE
1. B. C. Hester et al., OSA Frontiers in Optics conference, poster paper JWC60,
San Jose, CA (Oct. 14, 2009).