Optical Properties of Metal/Dielectric Composites with Passive and Active Host Matrices
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Date
2013-06
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Addis Ababa University
Abstract
The Thesis is devoted to the theoretical and numerical study of the enhancement of
the local electric field and the induced optical bistability (IOB) in metal ellipsoidal
and spherical dielectric nanoparticles covered by a metal shell with nonlinear dielectric
functions. In the electrostatic approximation, specification of the IOB domain reduces
to an analysis of the algebraic cubic equation for the local electric field in the particle
as a function of the external electric field. The comparatively simple analysis of
roots of a cubic equation, proposed in the Thesis, allowed us to find the range of
the amplitudes of incident electromagnetic wave and their frequencies where the IOB
exists and study its dependence on the parameters of the problem. The results of
numerical computations for metal and metal covered semiconductor nanoparticles are
presented graphically.
It is shown that the local field in metal spherical particles with dielectric core in
an external varying electric field has two maxima at two different frequencies. The
second maximum becomes more important with increment in the metal fraction. Due
to the nonlinear dielectric function of the core, the composite of these inclusions may
have two optically induced bistability domains at different frequencies. At rather high
metal fraction, two bistability domains merge and form one entire bistability domain.
The parameters of these domains are studied numerically. In the Thesis we focuses
on the second bistability domain. This domain exists in a comparatively narrow
frequency range and its onset fields are lower than those of the first bistability domain.
The lowest bistability onset fields are obtained in the entire domain. This peculiarity
of the optical induced bistability in the metal composite with small dielectric cores
can be attractive for possible applications.
In the Thesis it is shown that the enhancement factor of the local electric field in
metal spherical or cylindrical nanopacticles with dielectric cores imbedded in a dielectric
matrix have two maxima on two resonant frequencies. The second maxima for
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the inclusions with large dielectric cores covered by a thin metal shell is comparatively
small. With increasing in a metal fraction in the inclusion, both enhancement factors
grow. For large metal fractions, the maxima of the enhancement factors become the
same order of magnitude and considerably large. At a special relation between the
dielectric constants of the core and the host matrix, the two maxima merge and show
further increment. The numeric calculations show that the real and imagine parts of
the refractive index of the composites of metal inclusions having ”small” dielectric
cores have new interesting features.
The optical properties of metal/dielectric composites (metal with dielectric core
and pure metal inclusions) in passive and active host matrices are studied. It is shown
that the real and imaginary parts of the refractive index of the composites with metal
covered inclusions have two maxima at two resonant frequencies. Both types of the
composite show a strong anomalous dispersion of the real part of refractive index.
The active host matrices can considerably reduce the absorption and provide the
conditions for propagation of weakly damping electromagnetic waves on the resonant
frequencies. The weakly spreading wave packets of light of a negative group velocity
can be experimentally observed in these composites
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Keywords
Optical Properties of Metal