Browsing by Author "Shewamare, Sisay"

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    Effect of an Intermediate Metastable State on the Escape Rate of a Brownian Particle
    (Addis Ababa University, 2001-06) Shewamare, Sisay; Bekele, Mulugeta (PhD)
    vVe considered a Brownian particle trying to escape from a single-well potential with a run away on one side aided by the thermal energy of the medium in which it is moving, The analytical expression of escape rate of a Brownian particle is calculated for a model original potential (inverted N) and for a modified model potential having an intermediate metastable state using supersymmetric and mean first passage time methods. The modified model potential is parameterized by location and slope of the intermediate metastable state. We compared the escape rate for the modified model potential with that for the original a model potential and found that their ratio takes optimum value at a particular position of the intermediate state. In addition, this ratio takes optimum value at a particular slope value of the intermediate state
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    Optical Properties of Metal/Dielectric Composites with Passive and Active Host Matrices
    (Addis Ababa University, 2013-06) Shewamare, Sisay
    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 xv xvi 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