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Research Area
Novel
Photonic Materials Based on Charge . Ordered Manganese
Oxides
Advisor: Dr. Vera Smolyaninova
Current
demands in the areas of computing and communication
require increasing speed of computation and information
transfer. One solution is to replace "slow" electronic
devices with "fast" photonic ones. This requires the
development of optical analogues of electronic integrated
circuits capable of routing, controlling and processing
optical signals. One of the promising approaches is
based on photonic band-gap materials, which allow control
of dispersion and propagation of light. A photonic band-gap
may appear in a three-dimensional photonic crystal (a
material with periodic modulation of refractive index
with a period of the order of the wavelength of light).
Inside a photonic crystal transmission or rejection
of light in a given wavelength range and wave guiding
of light along linear and bent defects of a periodic
photonic crystal structure can be achieved. The necessary
condition for the photonic band gap to appear is high
(more than 2 to 1) refractive index contrast within
the photonic crystal. Creation of a photonic band-gap
material is a very difficult technological task.
Mixed valence manganese oxides exhibit unique photosensitivity
properties, such as permanent photo- induced reflectivity
changes due to local photoinduced insulator to metal
transition. As a result, nanostructuring of these materials
may be performed using well-known optical holography
techniques. Unique and novel feature of this experimental
program is based on the extreme sensitivity of the manganite
materials to external fields, such as magnetic field,
temperature, external illumination, etc. Recently, permanent
photoinduced reflectivity changes in manganites were
demonstrated by Dr. Smolyaninova. This sensitivity will
be used to .develop. the holograms: application of the
external fields will result in substantial enhancement
of the weak initial refractive index contrast of the
holograms, resulting in the potential appearance of
photonic band gaps. On the other hand, such photoinduced
photonic crystal structures may be easily annealed using,
for example, intense light, so that new structures can
be written in the same region of the sample instead
of the old ones. Thus, unique writable and erasable
photonic crystal materials may potentially be created.
Such materials would be invaluable in numerous photonic
device applications.
The REU participants will be involved in every step
of this research program. In order to create photonic
structures, 500-1000 nm thin films of Bi1-xCaxMnO3 are
needed. These thin films will be grown in the Material
Research Lab at Towson University using a Pulsed Laser
Deposition (PLD) system. Students will be trained to
use the PLD system. For the photonic device application
thin films of good quality, which posses the charge
ordering, are needed. These thin films have to be characterized
in order to find appropriate growth conditions for desirable
properties. Since these materials have a sharp upturn
in the temperature dependence of the resistivity, transport
measurements will be a key measurement in the sample
characterization. Thin films will be characterized over
a wide temperature range using a closed cycle cryostat.
Participants will be trained in transport measurement
techniques in the temperature range from room to liquid
helium temperatures.
As a first step toward development of photonic crystals,
one-dimensional (1D) photonic structures, which consist
of a series of illuminated channels, will be created.
Optical reflectivity of these channels will be studied
using a near field scanning optical microscope (NSOM),
which is being developed at Towson University by Dr.
Smolyaninova. Students will be trained to use this unique
scanning technique. Temperature and magnetic field dependence
of electrical conductivity of illuminated channels will
be studied, since local photoinduced insulator to metal
transition is expected. A study of temperature and magnetic
field dependence of the conductivity of such channels
will be useful for controlling the optical contrast,
since metallic regions are supposed to have higher optical
reflectivity. Since magnetism and conductivity are closely
connected in manganites, local changes in magnetic structure
are expected. Local magnetic structure will be studies
with commercial magnetic force microscope (MFM) in the
Nanotechnology Laboratory at Towson University.
After successful development of 1D photonic structures,
this work can be extended to 2D photonic structures
using NSOM. The reflectivity, local magnetic structure
and conductivity of these structures will be tested
using various scanning techniques (NSOM, MFM, and STM).
In addition, prototype switchable photonic crystal devices
will be created and tested.
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