download report - Sapienza

Scientific Report 2007-2009
Condensed matter physics and biophysics
C30. Holographic optical tweezers:
hands of light on the mesoscopic world
The mesoscopic world lies in between our macroscopic
world and the microscopic world of atoms and molecules.
Although phenomena are mainly governed by the laws of
classical physics it looks much different than the world
we live in. When shrinking the length scales from meters
down to microns, the balance between forces changes
dramatically: there is no inertia and viscous forces dominate,
thermal agitation moves objects around in perpetual
Brownian motion, surface forces are very strong
and light pressure can exert a significant force. Optical
forces are indeed ideally suited to manipulate matter
at the mesoscale which is characterised by length
scales ranging from ten nanometers to hundreds of micrometers,
femtonewton to nanonewton forces, and time
scales from the microsecond on. In 1986 Arthur Ashkin
demonstrated that a tightly focused laser beam can stabily
trap a micron sized dielectric object in 3D. Since
their appearance optical tweezers have been applied to
study mesoscopic phenomena in biology, statistical mechanics
and colloidal science. The commercial availability
of Spatial Light Modulators (SLM) opened new horizons
to optical micromanipulation. SLMs are typically
computer controlled liquid crystal minidisplays allowing
to arbitrarily shape a wavefront by imposing a pixel by
pixel phase shift on an incoming laser beam. Such an
engineered wavefront can be focused into a tiny hologram
image made of bright light spots in 3D, each spot
serving as an independent point trap. What makes holographic
optical trapping (HOT) very powerful is that it
provides a contactless micromanipulation technique with
many body, dynamic, 3D capabilities.
Figure 1: Frames from a movie † showing the interactive
micro-manipulation of 8 silica beads (2 µm diameter) in water.
The beads are arranged on the vertices of a 5 µm side
cube which is then rigidly rotated. Bottom row shows the
corresponding frames (holograms) displayed on the SLM.
We have contributed to HOTs technology by designing
a novel iterative procedure for computer generated holograms
with an unprecedented degree of efficiency and
Figure 2: Two colloidal particles confined in a liquid film
thinner then their diameter attract each other with a strong
and long ranged capillary interaction.
uniformity [1]. Using light as a tool for multi particle
manipulation we have developed light driven devices and
sensors for lab on chip applications such as an optical
driven pump or multipoint velocity or viscosity probes
for microfluidic channels [2]. When particle positions are
tracked with digital video microscopy, light forces can be
accurately calibrated so that HOTs also provide a unique
tool to probe forces in controlled geometries. Trapping
and isolating a pair of colloidal particles far away from
other beads and confining walls, we could directly investigate
very long ranged forces, such as the capillary or
hydrodynamic interactions arising in thin fluid films [3].
HOTs also provide a very convenient tool to investigate
the statistical mechanics of small systems by providing
a reconfigurable optical energy landscape for Brownian
motions. For example, trapping aerosol droplets with a
time varying strength, we demonstrated that parametric
resonance can be excited in a Brownian oscillator [4].
Although single or dual trap optical tweezers have
already boosted research in single cell and single
molecule biophysics, we are only beginning to explore
the full potential of 3D, multi-trap, dynamic holographic
micromanipulation of biological structures.
References
1. R. Di Leonardo et al., Opt. Express 15, 1913, (2007).
2. S. Keen et al., Lab on a Chip 9, 2059, (2009).
3. R. Di Leonardo et al., Phys. Rev. Lett. 100, 106103
(2008).
4. R. Di Leonardo et al., Phys. Rev. Lett. 99, 010601
(2007).
Authors
R. Di Leonardo 2 , G. Ruocco
http://glass.phys.uniroma1.it/dileonardo/
Sapienza Università di Roma 83 Dipartimento di Fisica