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Research
Our research revolves around the use of Near-field
Scanning Optical Microscopy (NSOM) to investigate
the properties of condensed matter systems at the
nanoscale. The main themes behind our research are
to better understand how light interacts with matter
in dimensions smaller compared with the wavelength
of light and from the interaction to infer the properties
of the system under study.
Two homebuilt NSOM systems, one operating at room
temperature and the other one designed to work at
liquid helium temperature, are the core of our laboratory.
Using these systems we can perform a variety of non-invasive
and non-destructive experiments.
Photodefined nanowires in high critical temperature
superconductors
This part of our research deals with the Metal-Insulator
transition existing in high temperature superconductors,
in particular in the YBCO system. Our efforts are
dedicated to the use of a technique introduced by
us. It consists in using the NSOM to define structures
with typical dimensions of about 100 nm. The interaction
of light of an adequate energy with the YBCO system
induces the motion of oxygen atoms, locally simulating
larger oxygen contents. In this way, starting from
an insulating sample it is possible to transform the
regions illuminated by the NSOM into a superconductor.
We are currently performing inelastic light scattering
using the NSOM and a solid immersion lens system to
determine the oscillator strength of the optical transition
responsible for the photoconductivity and photoinduced
superconductivity. We are also studying different
geometries for the photoinduced features that will
aid in the understanding of the behavior of the defined
Josephson junctions. This will allow us to improve
the comparison of our data with theoretical predictions.
We are confident that a thorough understanding of
the properties of these structures and the interactions
among them will provide much needed information regarding
the nature of the electronic system in high temperature
superconductors.
Spectroscopic investigation of quantum systems
This effort is dedicated to the understanding of the
quantum phenomena occurring in quantum dots. Light
from the NSOM is used to excite the system under study.
By performing a battery of tests (mainly photoluminescence,
photoluminescence excitation, inelastic light scattering),
the electronic energy and wavefunction are determined.
Using this substantial knowledge of the system we
manipulate the system to try to understand fundamental
issues in interacting quantum systems. Our main project
is to define the dipole-dipole interaction between
dots. In this experiment we glue a small dot of CdTe
to the NSOM tip, and bring it close to a larger one
of the same material. By measuring the energy transferred
between them as a function of their separation and
sizes we will be able to quantify the Forster-like
interaction between dots. This interaction is critical
in processes involving energy relaxation in the dot
when other dots are present. It also plays a crucial
role in FRET transitions when dots replace fluorescent
molecules.
Morphology of biomembranes
Our main effort in this area is dedicated to build
a NSOM based system that will detect the presence
of fluorescent molecules in biomembranes (either naturally
fluorescent or a fluorescent analog for one of the
constituents of the membrane). After detection, the
system will track the motion of the molecule in the
membrane. The cornerstone of the technique is an approach
that was developed in our lab, that allow to track
fluorescent molecules in a quasi bi-dimensional structure
having diffusion coefficients as large as 5 x 10-12
m2/s. The understanding of the diffusion properties
of different molecules in the biomembrane is one of
the most fundamental components towards a more complete
com prehension of the functionality and morphology
of cell membranes. The measurements will open up a
window of investigation in the 50-200 nm raft's size
range. Visualization of restricted lateral diffusion
by NSOM will elucidate the shape of these regions
about which little is known in the < 100 nm range.
We are also investigating formation of domains in
the solid phase of the membranes. These domains arise
as the acyl chains of the phospholipids tip when the
temperature is reduced below the liquid crystalline-gel
transition temperature. The measurements in the gel
state are based on the anisotropic index of refraction
of lipid bilayers. One of the principal optical axis
for the lipid bilayer lies along the direction of
the acyl chains in the lipid molecule, which it is
known to be oriented at 0 ~ 30o with respect to the
perpendicular to the membrane. The different polarizabilities
of the molecule along the acyl chains and perpendicular
to them gives rise to a difference between n|| and
n^.
Search for New Forces at the Submicron Range
A ''simple'' experiment for a tough question. We
all learned that the gravitational force varies as
the inverse of the square of the separation between
the bodies. And we know that it works great for macroscopic
or astronomical distances. What happens, however,
if the separation between the bodies is small, less
than a micron? We don't know if the Newtonian expression
for the gravitational force holds. Indeed, high energy
physics predictions include a Yukawa like correction
for the Newtonian potential for separations smaller
than ~ 1mm.
The principle of our experimental approach is quite
simple: Bring a metallic sphere close to a mechanical
oscillator that has been covered with two isotopes
of the same element.
By measuring the deflection of the oscillator, we
expect to be able to shed some light into gravitation
at small separations (and even, if we get on the good
side of nature, find unexpected interactions). For
realizing the experiment we are using a 6 mm sphere
covered with Au which is rapidly positioned over the
two halves of a Si micro electromechanical system.
Each of the halves is covered with a different isotope
of Ni, 64Ni and 58Ni. Using isotopes of the same element
minimizes the difference between Casimir and electrostatic
forces, while the gravitational force is a function
of the isotopeÂ_s mass.
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Publications
Krause DE, Decca RS, López D and Fischbach E (2007)
Experimental Investigation of the Casimir Force Beyond the Proximity-Force Approximation.
Phys Rev Lett 98 050403.
Bezerra VB, Decca RS, Fischbach E, Geyer B, Klimchitskaya GL, Krause DE, López D,
Mostepanenko VM and Romero C (2006)
Comment on: On the Temperature Dependence of the Casimir Effect.
Phys Rev E, 73 028101.
López D, Decca RS, Fischbach E and Krause DE (2005)
MEMS Based Force Sensor: Design and Applications.
Bell Labs Tech. J. 10, 61.
Decca RS, López D, Chan HB, Fischbach E, Krause DE and Jamell CR (2005)
Constraining New Forces in the Casimir Regime Using the Isoelectronic Technique.
Physical Review Letters, 94, 240401.
Decca RS, López D, Fischbach E, Klimchitskaya GL Krause DE and Mostepanenko VM (2005)
Precise Comparison of Theory and New Experiment for the Casimir Force Leads to Stronger Constraints on Thermal Quantum Effects and Long-Range Interactions.
Annals of Physics (N.Y.), 318, 37.
Decca RS, Lopez D, Chan HB, Fischbach E, Klimchitskaya GL, Krause DE and Mostepanenko VM (2004)
Precise Determination of the Casimir Force and First Realization of a "Casimir Less" Experiement.
Journal of Low Temp. Phys. 135, 63.
Lopez D, Pardo F, Bolle C, Decca RS and Bishop D (2004)
MEMS Technology for the Advancement of Science.
Journal Low Temp. Phys. 135, 51.
Decca RS, Fischbach E, Klimchitskaya GL, Krause
DE, Lopez D and Mostepanenko VM (2003)
Improved Tests of Extra-Dimensional Physics and
Thermal Quantum Field Theory From New Casimir Force
Measurements.
Physical Review D 68, 116003.
Decca RS, Lopez D, Fischbach E and Krause
DE (2003)
Measurement of the Casimir Force Between Dissimilar
Metals.
Physical Review Letters 91, 050402.
Fischbach E, Krause D, Decca RS and Lopez
D (2003)
Testing Newtonian Gravity at the Nanometer Distance
Scale Using the Iso-Electronic Effect.
Physics Letters A 318, 165.
Lee C-W, Decca RS, Wassall SR and Breen J
(2003)
Domain formation on in the L_Ő state of 1,2-Dipalmitoylphosphatidylcholine
Bilayers.
Physical Review E 67, 061914.
Decca RS, Lee C-W, Lall S and Wassall SR
(2002)
Single Molecule Tracking Scheme Using a Near-Field
Scanning Optical Microscope.
Rev. Sci. Instr. 73, 2675-2679. |
