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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
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Type-II thin film superconductors studied by terahertz radiation
Tesař, Roman ; Skrbek, Ladislav (advisor) ; Lipavský, Pavel (referee) ; Němec, Hynek (referee)
Title: Type-II thin film superconductors studied by terahertz radiation Author: RNDr. Roman Tesař Department: Department of Low Temperature Physics Supervisor: prof. RNDr. Ladislav Skrbek, DrSc. Consultant: RNDr. Jan Koláček, CSc. Abstract: Utilization of type-II superconductors for future practical applications such as fluxonics requires detailed knowledge of their physical properties, espe- cially at high frequencies within the THz spectral region. We have investigated interactions of thin-film NbN samples deposited on Si substrate and of a high quality epitaxial film of the NbN superconductor grown on a birefringent R-cut sapphire substrate with monochromatic linearly polarized laser beam both below and above the critical temperature Tc. For photon energies lower than the optical gap, detailed measurements of transmission in zero field provide BCS-like tem- perature curves with a pronounced peak below Tc which disappears as the energy of incident radiation is increased above the gap. In externally applied magnetic fields up to 10 T oriented perpendicularly to the sample, i.e., in the Faraday exper- imental geometry, the temperature behavior of transmission is modified because the gap is suppressed and vanishes at the upper critical field and, additionally, the presence of quantized vortices changes the shape...
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Charge transport in semiconductor nanostructures investigated by time-resolved multi-terahertz spectroscopy
Kuchařík, Jiří ; Němec, Hynek (advisor) ; Pereira, Mauro Fernandes (referee) ; Richter, Ivan (referee)
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...
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Elektronická struktura materiálů na bázi grafenu
Nádvorník, Lukáš ; Orlita, Milan (advisor) ; Němec, Hynek (referee)
In last two years, the proposal to create artificial graphene in standard semiconducting 2D systems via surface patterning has emerged. This way, an alternative system would be created, allowing us to study phenomena related to Dirac-type particles in a fully carbon free system. The main idea of the concept assumes the creation of an additional potential in a quantum well by nanopatterning of the specimen surface or by using local electrodes. The additionally introduced modulation can transform the conventional (i.e. parabolic) energy dispersion into separated minibands with possible appearance of Dirac cones. In the theoretical part, we introduce four basic criteria that estimate appropriate technological parameters and the required experimental conditions. Experimentally, we study the cyclotron resonance of prepared heterostructures AlGaAs/GaAs with induced hexagonal potential via the etching lateral holes. The observed multi-mode resonance response is discussed with respect to the expected appearance of Dirac cones.
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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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Charge transport in semiconductor nanostructures investigated by time-resolved multi-terahertz spectroscopy
Kuchařík, Jiří ; Němec, Hynek (advisor) ; Pereira, Mauro Fernandes (referee) ; Richter, Ivan (referee)
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...
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Terahertz radiation in nanostructures
Klimovič, Filip ; Ostatnický, Tomáš (advisor) ; Němec, Hynek (referee)
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
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Type-II thin film superconductors studied by terahertz radiation
Tesař, Roman ; Skrbek, Ladislav (advisor) ; Lipavský, Pavel (referee) ; Němec, Hynek (referee)
Title: Type-II thin film superconductors studied by terahertz radiation Author: RNDr. Roman Tesař Department: Department of Low Temperature Physics Supervisor: prof. RNDr. Ladislav Skrbek, DrSc. Consultant: RNDr. Jan Koláček, CSc. Abstract: Utilization of type-II superconductors for future practical applications such as fluxonics requires detailed knowledge of their physical properties, espe- cially at high frequencies within the THz spectral region. We have investigated interactions of thin-film NbN samples deposited on Si substrate and of a high quality epitaxial film of the NbN superconductor grown on a birefringent R-cut sapphire substrate with monochromatic linearly polarized laser beam both below and above the critical temperature Tc. For photon energies lower than the optical gap, detailed measurements of transmission in zero field provide BCS-like tem- perature curves with a pronounced peak below Tc which disappears as the energy of incident radiation is increased above the gap. In externally applied magnetic fields up to 10 T oriented perpendicularly to the sample, i.e., in the Faraday exper- imental geometry, the temperature behavior of transmission is modified because the gap is suppressed and vanishes at the upper critical field and, additionally, the presence of quantized vortices changes the shape...
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Optical spectroscopy of magnetically ordered materials
Surýnek, Miloslav ; Němec, Petr (advisor) ; Němec, Hynek (referee)
Spintronics is a dynamically developing branch of electronics which for transfer, processing and storing of information use not only electron charge but also its spin. Materials appropriate for a construction of spintronic devices should sustain the spin order for a sufficiently long time enabling a manipulation of spins. Simultaneously, in these materials the spin of electrons should be possible to transport fast and long enough across the device. In this work, heterostructure of GaAs/AlGaAs fulfilling these requirements is investigated by a pump-probe technique. In particular, long spin lifetimes of electrons in the heterostructure are studied using the resonant spin amplification method and with a use of the pulse picker. The nuclear spin polarization and the effect of an applied electric current on the studied heterostructure are also investigated in this work. Spectral filters are used to improve the existing experimental setup for the pump-probe technique and to measure a magnetooptical spectrum of the investigated heterostructure.
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Time-resolved spectroscopy of semiconductor nanostructures
Kořínek, Miroslav ; Trojánek, František (advisor) ; Herynková, Kateřina (referee) ; Němec, Hynek (referee)
Title: Time-resolved spectroscopy of semiconductor nanostructures Author: RNDr. Miroslav Kořínek Department: Department of Chemical Physics and Optics Supervisor: doc. RNDr. František Trojánek, Ph.D., Department of Chemi- cal Physics and Optics, Faculty of Mathematics and Physics, Charles University in Prague Abstract: This Ph.D. thesis is focused on the study of relaxation and re- combination processes in silicon nanocrystals embedded in dielectric (SiO2, SiC and Si3N4) matrices that were fabricated within the EU 7th Framework Pro- gramme Silicon Nanodots for Solar Cell Tandem (NASCEnT). Different host ma- terials, in which silicon nanocrystals are incorporated, not only provide different transport properties, but also differently affect the nanocrystal surfaces. A large part of the thesis is devoted to research of SiO2-embedded silicon nanocrystals having various sizes (in the order of nanometers) and various inter-nanocrystal separation, that differ also in the manner of an additional annealing. Experimen- tal techniques of optical spectroscopy are used to monitor the photoexcited charge carrier dynamics on a wide time scale from picoseconds to milliseconds. The initial (picosecond) dynamics is characterized very well using a relevant rate equation. We propose a theoretical description of how the...
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