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