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Spectroscopy of semiconductors with ultra-high time resolution
Hanák, Martin ; Trojánek, František (advisor) ; Kozák, Martin (referee)
This thesis is aimed at building and optimization of a new experiment setup for measuring time-resolved transmittance using a pump-probe method. Test measurements were then conducted on a sample of SiV centers in diamond. The theoretical part contains a general description of luminescence of color centers. It then specifically focuses on SiV centers in diamond and describes their structure, electronic and spectral properties in detail. The description of our setup and its optimization is given at the beginning of the experimental part. The dependencies of differential transmittance of SiV centers in diamond on excitation intensity and wavelength were measured and the lifetime of SiV centers was then determined from our measurement results. 1
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Selection of femtosecond laser pulses
Bažíková, Sára ; Trojánek, František (advisor) ; Zázvorka, Jakub (referee)
Construction of the laser is one of the most important physical advances of modern times. Its use in practice is extremely common and desirable, therefore, it is necessary to push the boundaries of possibilities of this technique still further. Lasers generating ultrashort pulses are widely used in material research for the study of nonlinear effects and ultrafast processes. These lasers have a high repetition rate, which must be reduced for some applications. In this thesis we used femtosecond titanium -sapphire laser from Spectra Physics, whose repetition rate is 80 MHz. The aim of this thesis is to reduce this frequency by using a pulse selector. The key mechanism that utilizes the pulse selector is an acoustooptic modulator that is based on the principle of Bragg diffraction and it is able to select pulses with a frequency of 1 Hz-4 MHz. The end of thesis is measurement of the properties of this selector and its optimization.
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Study of optical nonlinearities in semiconductors and semiconductor nanostructures
Chlouba, Tomáš ; Trojánek, František (advisor) ; Filip, Radim (referee) ; Herynková, Kateřina (referee)
In the main part of this thesis I study the relaxation mechanisms of charge carriers in silicon nanocrystals in SiO2 matrix. One of the potential applications of these structures lies in photovoltaics, specifically in construction of all-silicon tandem solar cells. I studied the dynamics of carriers in these structures by methods of ultrafast spectroscopy which helped to unravel the microscopic behaviour of carriers, their transport, localization etc. Furthermore I investigated the doping of such structures as the technology of doping is crucial for manufacture of pn- junctions which are the core component of solar cells. At the end I delve into the dissipative Jaynes-Cummings model by mathematical modeling and theoretical calculations which describes among others microlasers and as such comes under a field of cavity quantum electrodynamics.
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Ultrafast processes in semiconductor nanocrystals
Dzurňák, Branislav ; Trojánek, František (advisor) ; Dohnalová, Kateřina (referee)
This diploma thesis deals with optical properties of silicon nanocrystals implanted in silicon oxide substrate. We examined samples with various concentrations of nanocrystals. We measured Raman spectra of our samples and identified size of nanocrystals and distance between them as function of depth. We measured absorption and reflection of samples and calculated the energy of band gap. For various implantation doses we examined temperature dependence of luminescence spectra. Two peaks were observed for excitation wavelength of 408 nm, the first shifts towards longer wavelength with increasing temperature, the second one stays unchanging. We observed low-temperature resonant luminescence in order to identify fonon structure. During observation of time-resolved luminescence two components of fast luminescence appeared - the slower in scale of nanoseconds, the faster in scale of picoseconds. We devoted to study of the faster component. For the faster component we observed up-converted luminescence. Intensity dependence of this component is quadratic. We also observe degradation of luminescence owing to strong laser beam. In conclusion we discussed origin of each component of luminescence spectra.
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Time-resolved laser spectroscopy of nanomaterials
Kozák, Martin ; Trojánek, František (advisor) ; Šimurda, Miroslav (referee)
This diploma thesis is oriented to study of nonlinear properties of semiconductor nanocrystals using ultrashort femtosecond pulses. There is a detailed description of theory and experimental setup of the z-scan method, which can be used to measure the nonlinear refractive index and the two-photon absorption coefficient. In theoretical part we describe the influence the quality of laser beam, quality and thickness of the sample on measurements. There are numerical simulations of the measurements influenced by these parameters. This theory is used for processing of the measured dates by a program, which is attached to this thesis on CD. In experimental part we measured nonlinear refractive indeces of diamond nanocrystals fabricated by the PECVD method, silicon and CdS nanocrystals in glass matrices. One part of this thesis deals with the preparation and optimalization of the experimental setup and increasing the signal/noise ratio.
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Time resolved spectroscopy of SiV centers in diamond
Hamráček, Karol ; Trojánek, František (advisor) ; Žídek, Karel (referee)
Title: Time-resolved spectroscopy of SiV centers in diamond Author: Karol Hamráček Department: Department of chemical physics and optics Supervisor: doc. RNDr. František Trojánek, Ph.D. Abstract: In recent years, the negatively charged silicon center (SiV-) has become a promising competitor to well-established nitrogen (NV-) due to its excellent spectral properties, such as narrow zero phonon line transitions and weak phonon sidebands, and therefore, research of point defects in diamond is currently receiving a lot of attention. These isolated defects, the so- called color centers, can serve as sources of photons and thus, ultimately, can be used to construct SiV Lasers, whose physical nature is stimulated emission. However, it has not yet been generated at SiV centers. For its generation, it is necessary to examine in detail the radiant and non-radiative processes in optical centers and the properties and parameters of stimulated emission during optical excitation. Also part of the research is the optimization of sample preparation in order to obtain samples with a high concentration of centers and at the same time high optical quality. That is why the aim of this diploma thesis will be to perform optical characterization of diamond samples with SiV centers (supplied by the cooperating department of the...
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