National Repository of Grey Literature 26 records found  1 - 10nextend  jump to record: Search took 0.01 seconds. 
Linear terahertz response of semiconductor nanostructures
Šándor, Jindřich ; Ostatnický, Tomáš (advisor) ; Němec, Hynek (referee)
In this work we study an interaction between electrons in Dirac points and terahertz electromagnetic radiation. First, we find eigenstates of the effective hamiltonian in Dirac points. Then we introduce the interaction Hamiltonian and subsequently we find an evo- lution relation for a density matrix in the first order of perturbation theory. Eventually we derive matrix elements of a conductivity tensor. We do all this in two different gauges of an electromagnetic potential. 1
Quantum interference in nanocrystals
Spitzkopf, David ; Ostatnický, Tomáš (advisor) ; Žonda, Martin (referee)
Quantum interference control has is a well described effect in bulk and infinite well super-lattice crystals. In this thesis, we describe this effect in nanocrystals. We use a 8-band model for the description of the electron band structure in the crystal. We then break its inversion symmetry with a linear potential. We then apply external electro- magnetic field and calculate its effect on the crystal with the help of perturbation theory. The fields applied are of two kinds, one harmonic, the other instant in time. We follow with the calculation of the generated current, with detailed numerical tests of several pa- rameters of the model. Conductivity for a non-linearity of the 3rd order is then formally calculated. 1
Magneto-optic response of layered media
Kunt, Ota ; Ostatnický, Tomáš (advisor) ; Antoš, Roman (referee)
Optical response of materials strongly depends on their magnetic properties. This phenomenon is also used in materials in the form of thin layers and multi- layers. Using cited literature we summarize theory needed to calculate magneto- optical response of multilayers. The calculation is based on Maxwell equations and magnetic materials are described with effective permittivity tensor. Jones formalism is used to describe polarised light and Yeh formalism is used to de- scribe multilayer response. A program using presented theory was developed and calculations for concrete structure were made. Multilayer parameters were cho- sen to correspond with those of a sample whose magneto-optical response was measured at the Department of Chemical Physics and Optics, MFF UK. 1
Light propagation in magnetic materials
Wagenknecht, David ; Ostatnický, Tomáš (advisor) ; Veis, Martin (referee)
Transparent magnetic materials, among the others semiconductors, have recently come to the focus of interest in both basic and applied research because they have great potential in optical applications and they can be used in optical spectroscopy to investigate fundamental physical phenomena. The theoretical calculations introduce that these materials have some extraordinary properties like asymmetric reflectance when light impact the sample from the opposite angles. In this bachelor thesis, transfer matrix formalism is derived based on Maxwell equations, taking into account special form of effective permittivity. The reflection coefficient for one layer demonstrates, that the asymmetric reflectance appears for transversal- magnetic polarization of light and transversal magnetization in the studied material. The derived formulae are used to calculate the difference in reflectivities for the waves impacting the structure from opposite angles. Ga1−xMnxAs with different concentrations of manganese is assumed to be the essential transparent magnetic layer in the sample. 1
Optical response of magnetic materials
Wagenknecht, David ; Ostatnický, Tomáš (advisor)
David Wagenknecht: Abstract of a diploma thesis Optical response of magnetic materials, 2014 Magnetooptical properties of anisotropic semiconductors are studied to describe asymmetry of Ga1−xMnxAs, because theoretical calculations predict extraordinary behaviour of reflectivity. Analytical formulae to describe materials with non-diagonal permittivity are derived and they are used for the numerical calculations to describe the optical response of the samples available for the measurement. The transversal Kerr effect is calculated and it exhibits asymmetry in both rotation of the plane of polarization and ellipticity of circularly polarized light due to asymmetry in reflectivity. Moreover, longitudinal and polar magnetization are studied because of the influence on the observability of the phenomena. Results are not only used to discuss conditions, which must be satisfied to prove the asymmetry, but also the actual experimental setup is designed to prepare the measurement. 1
Terahertz conductivity of Dirac electrons
Spitzkopf, David ; Ostatnický, Tomáš (advisor) ; Slobodeniuk, Artur (referee)
We explore the conductivity of Dirac electrons in the terahertz range, especially in graphene. We are trying to find a boundary condition that is suitable both for the eigen states around the Dirac points and the nanocrystalline structure of graphene. Ultimately we analyze the conductivity of these bound states. 1
Nonlinear interactions of terahertz radiation
Kadlec, Josef ; Ostatnický, Tomáš (advisor) ; Němec, Hynek (referee)
Conductivity of semicoductor nanostructures has its maximum in terahertz spectral range. Linear response is described in reasonable detail. With high intensity light sources it is also needed to be concerned with nonlinear response. In this thesis, there is firstly described already existing quantum model of linear conductivity which is using perturbation theory. This model is then extended by adding another perturbations, getting us quantum model for calculating nonlinear conductivity of arbitrary order. Model is then applied for calculation of third order nonlinear conductivity spectra for cubic nanocrystal. There is described spectra for varying parametres, such as tempera- ture, volume, electron density or scattering rate. In the end it is compared with semiclassical compuptation using Monte-Carlo simu- lation. 1

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