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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Optické nelinearity terahertzového záření / Nonlinear interactions of terahertz radiation

Kadlec, Josef January 2020 (has links)
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
2

Studium transportu náboje v polovodičových nanostrukturách pomocí časově rozlišené multi-terahertzové spektroskopie / Charge transport in semiconductor nanostructures investigated by time-resolved multi-terahertz spectroscopy

Kuchařík, Jiří January 2019 (has links)
Terahertz conductivity spectra contain information on charge transport mechanisms and charge confinement on nanometer distances. In this thesis, we make a substantial progress in understanding of terahertz conductivity in several regimes. First, we theoretically investigate linear terahertz conductivity of confined electron gas: while the spectra of degenerate electron gas exhibit geometrical resonances, the response in non-degenerate case smears into a single broad resonance due to the wide distribution of charge velocities. Then, we theoretically and experimentally analyze various TiO2 nanotube layers: their linear charge transport properties strongly depend on the fabrication process, which influences the internal structure of the nanotube walls. In the main part of the thesis, we develop a framework for evaluation of the nonlinear terahertz response of semiconductor nanostructures based on microscopic Monte-Carlo calculations. The nonlinear regime is highly non-perturbative even in moderate fields as illustrated by efficient high harmonics generation. We investigate measurable nonlinear signals for various semiconductor nanostructures; metallic nanoslits filled with nanoelements are the most promising for the experimental observation of terahertz nonlinearities. These nonlinearities per unit charge are...
3

Vlastnosti supravodičů v oblasti terahertzových frekvencí / Properties of superconductors in the terahertz frequency region

Šindler, Michal January 2012 (has links)
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4

Studium spintronických jevů v magneticky uspořádaných strukturách pomocí terahertzové spektroskopie / Study of spintronic phenomena in magnetically ordered stuctures using terahertz spectroscopy

Kubaščík, Peter January 2021 (has links)
The main objective of this thesis was to bring the first experimental evidence about the spin-Hall magnetoresistance (SMR) in the Terahertz (THz) spectral range. The time-domain THz spectroscopy (TDTS) was chosen as the main method, and we presented a new experimental scheme, which allows us to observe SMR or magnetoresistive effects with similar symmetry at a wide range of THz frequencies very efficiently. We focused on the study of SMR in the bilayers consisting of either a prototypical ferrimagnetic isolator or of heavy metal (FI/HM) and metallic stacks of ferromagnetic CoFeB and heavy metal Pt layer (FM/HM). While SMR shows a rapid decrease already at the lowest THz frequencies (< 1 THz) in the FI/HM structures, the SMR in FM/HM bilayers persists above 30 THz. These observations are then explained by a different mechanism of SMR. The second part of the thesis is devoted to the construction of the new TDTS setup and an easy-to-use model of the THz setup using the Gaussian description of THz radiation. The simulated results have been compared to corresponding experimental measurements using a spintronic THz emitter (STE). The last part of the thesis describes the emission of intensive THz pulses from large-area STE.
5

Terahertzové záření v nanostrukturách / Terahertz radiation in nanostructures

Klimovič, Filip January 2019 (has links)
V této teoretické práci se zabýváme kvantově mechanickými jevy, jež jsou spjaté s vodi- vostními elektrony uzavřenými v kvantových tečkách. Nejprve je odvozen model nanokrys- talu jakožto potenciálové jámy. Při tom se ukazuje, že pouze objem, ne tvar, je významným parametrem modelu pro účely terahertzové spektroskopie. Studované geometrie jsou tak vzájemně zaměnitelné a výběr mezi nimi m·že zjednodušit dané úlohy. Pro zkoumání depo- larizačních efekt·, které jsou zahrnuty v depolarizačním faktoru v Maxwell Garnettově teorii efektivního prostředí, je zvolena sférická symetrie. V rámci poruchy prvního řádu je vyřešena Poissonova rovnice pro elektrony rozmístěné uvnitř koule podle vlnové funkce a je určen depo- larizační faktor. Zatímco v klasické limitě tento nabývá p·vodní hodnoty, pro nanokrystaly se zvyšuje a maxima je dosaženo v ne-degenerovaném režimu, kdy je obsazen pouze základní stav. Navýšení depolarizačního faktoru posouvá plasmonovou rezonanci směrem k vyšším frekvencím. 1
6

Studium ultrarychlé odezvy elektronů v nanostrukturovaných a neuspořádaných polovodičových systémech pomocí časově rozlišené terahertzové spektroskopie / Ultrafast response of electrons in nanostructured and disordered semiconductor systems studied by time-resolved terahertz spectroscopy

Zajac, Vít January 2017 (has links)
of Doctoral Thesis Title: Ultrafast response of electrons in nanostructured and disordered semiconductor systems studied by time-resolved terahertz spectroscopy Author: Vít Zajac Department / Institute: Institute of Physics of the Czech Academy of Sciences Supervisor of the doctoral thesis: doc. RNDr. Petr Kužel, Ph.D., Institute of Physics of the Czech Academy of Sciences Abstract: This thesis deals with charge transport in semiconducting nanomaterials on the picosecond time scale studied by time-resolved terahertz spectroscopy. The problematics of the effective response of composite materials is reviewed and the VBD effective medium model is formulated. The wave equation for the THz probing pulse propagating through inhomogeneously excited percolated and non-percolated semiconducting nanomaterials is solved. This theory is used to investigate charge transport in samples of nanoporous-Si-derived nanocrystals and in epitaxial Si nanocrystal superlattices. The experimental spectra are successfully modeled with the use of Monte Carlo calculations of charge carrier mobility in nanocrystals of corresponding sizes and degrees of percolation within the VBD approximation. It is found that nanocrystals from different regions of the nanocrystal size distribution of the sample dominate the signal in THz and...
7

Laditelné materiály a struktury pro terahertzovou spektrální oblast / Tunable materials and structures for terahertz spectral range

Skoromets, Volodymyr January 2013 (has links)
This thesis is devoted to an experimental study of dielectric properties of incipient fer- roelectrics. The terahertz time-domain spectroscopy was used to investigate the complex permittivity spectra in both single crystals and various strained thin film structures versus temperature and applied electric bias. Namely, it allowed characterizing the ferroelectric soft-mode dynamics and its coupling to a central mode. An electric-field tunability of bulk single crystals of SrTiO3 was determined up to room temperature for the first time. The phenomenon is governed by soft-mode stiffening under applied field. As an application we proposed and characterized a tunable one-dimensional photonic structure with a thin SrTiO3 plate inserted as a defect layer. The importance of the soft-mode dynamics was stressed also in the study of bulk high density KTaO3 ceramics. A systematic study was performed of a KTaO3 thin film and especially of a set of strained multilayers consisting of SrTiO3/DyScO3 bilayers grown on DyScO3 substrate. Strain-induced ferroelectric transition was observed in these films governed by the soft mode coupled to a lower-frequency relaxation. A general model was developed describing the whole family of the studied samples. Effect of the composition stoichiometry of SrTiO3 films grown on DyScO3 was...
8

Terahertzová spektroskopie v časové doméně a vizualizace biologických objektů / Terahertz Time-Domain Spectroscopy and Visualization of Biological Objects

Nedvědová, Marie January 2022 (has links)
This thesis deals with the methods of Terahertz (THz) spectroscopy to observe the kinetics of haemostatic materials used for supporting the native mechanism of haemostasis. The theoretical part follows the physical principles of THz time-domain spectroscopy (THz TDS), mentions the advantages and limitations of this method and its application possibilities for the characterization of biomedical materials. Further, there are specified properties of actual haemostats, described principles of their function and usage in practice, including their interaction with the living tissue. There were performed experiments monitoring the kinetics of physiologic reaction of the tissue adhesive based on the cyanoacrylates and absorbable haemostats. The mechanisms of monitored reactions were explained based on the physical-chemical principles that are used also for the kinetic models’ derivation. Modelling of the measured data results in the estimation of the parameters characterizing the observed samples. The most interesting parameter is the time constant of the reaction because of the possibility to compare reaction rates of different types of haemostats. The detailed analysis of this parameter is performed using the means of statistical methods. Tissue adhesive samples were measured by other spectroscopic and microscopic methods to compare the findings with the experimental results of the THz TDS. Data were processed using algorithms designed especially for this experiment and analysed using mathematical methods.
9

Antény pro oblasti (sub)milimetrových vln / (Sub)millimeter-Wave Antennas

Pítra, Kamil January 2014 (has links)
Disertační práce se zabývá návrhem a optimalizací kruhově polarizované anténa pro oblast terahertzových kmitočtů. V práci se věnuji zjednodušené teorii terahertzového zdroje a návrhu vhodné antény pro tento zdroj. Návrh je zaměřen na dosažení kruhové polarizace z lineárně polarizovaných antén. Abych potlačil šíření povrchové vlny na elektricky tlustém dielektrickém substrátu, věnuji se návrhu a optimalizaci specifických periodických struktur. Návrh těchto struktur je poměrně komplikovaný, protože neexistuje přímočarý vztah mezi vlastnostmi struktur s elektromagnetickým zádržným pásmem (EBG) a geometrií buňky. Abych vhodně koncentroval vyzařovanou energii do úzkého svazku, věnuji se návrhu a optimalizaci částečně odrazného plochy (PRS), které působí jako planární čočka pro terahertzovou anténu.

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