159 research outputs found
Combined effect of electron and lattice temperatures on the long intersubband relaxation times of Ge/SixGe1−x quantum wells
No full textUltrafast photoconductivity and charge carrier transport in semiconductor nanostructures: a study by terahertz spectroscopy
No full textTitle: Ultrafast photoconductivity and charge carrier transport in semiconductor nanostructures: a study by terahertz spectroscopy Author: Vladimir Pushkarev Institute: Institute of Physics of the Czech Academy of Sciences Supervisor of the doctoral thesis: doc. RNDr. Petr Kužel, Ph.D. Abstract: The thesis is devoted to a study of charge transport in nanostructured systems by means of time-resolved terahertz spectroscopy. We address various aspects of electron confinement in such systems, and the role of depolarization fields on the effective photoconductive response of samples. Accurate evaluation of localized carrier properties has been achieved using quantum mechanical calculations of the mobility. Theoretical interpretation of the terahertz conductivity spectra is based on the solution of the wave equation for the terahertz wave in inhomogeneous photoexcited media described in terms of VBD effective medium model developed in our laboratory. This approach is employed for investigation of the terahertz photoconductivity in two types of nanomaterials: superlattices of Si nanocrystals prepared by thermal decomposition of SiOx layers and lithographically prepared GaAs nanobars. Experimental spectra were measured at 300 and 20 K and for a wide range of photocarrier densities. A careful analysis of electron..
Germanium quantum fountain structures on silicon substrates
No full textAsymmetric coupled quantum wells in the conduction band of germanium grown on silicon wafers have been investigated by THz pump-THz broadband probe picosecond spectroscopy. The optically-pumped laser gain coefficient is estimated
Ultrarychlá fotovodivost a transport náboje v polovodičových nanostrukturách: studium pomocí terahertzové spektroskopie
Get PDFNázev práce v češtině: Ultrarychlá fotovodivost a transport náboje v polovodičových nanostrukturách: studium pomocí terahertzové spektroskopie Autor: Vladimir Pushkarev Ústav: Fyzikální ústav Akademie věd České republiky Supervisor of the doctoral thesis: doc. RNDr. Petr Kužel, Ph.D. Abstrakt: Tato práce se věnuje studiu transportu náboje v nanostrukturovaných systémech pomocí časově rozlišené terahertzové spektroskopie. Zabývá se různými aspekty lokalizace elektronů v těchto systémech a rovněž vlivem depolarizačních polí na efektivní fotovodivou odezvu vzorků. Korektního vyhodnocení vlastností lokalizovaných nosičů náboje je dosaženo pomocí kvantově mechanických výpočtů jejich pohyblivosti. Teoretická interpretace spekter terahertzové vodivosti je založena na řešení vlnové rovnice pro terahertzové vlny v nehomogenních fotoexcitovaných prostředích popsaných pomocí tzv. VBD modelu efektivního prostředí vyvinutého v naší skupině. Tohoto přístupu jsme využili při studiu terahertzové fotovodivosti ve dvou typech nanomateriálů: v supermřížkách křemíkových nanokrystalů připravených termální dekompozicí vrstev SiOx a v litograficky připravených GaAs nanotyčinkách. Spektra byla měřena při 300 a 20 K a pro široký rozsah hustot opticky excitovaných nosičů náboje. Pečlivá analýza transportu elektronů v křemíkových...Title: Ultrafast photoconductivity and charge carrier transport in semiconductor nanostructures: a study by terahertz spectroscopy Author: Vladimir Pushkarev Institute: Institute of Physics of the Czech Academy of Sciences Supervisor of the doctoral thesis: doc. RNDr. Petr Kužel, Ph.D. Abstract: The thesis is devoted to a study of charge transport in nanostructured systems by means of time-resolved terahertz spectroscopy. We address various aspects of electron confinement in such systems, and the role of depolarization fields on the effective photoconductive response of samples. Accurate evaluation of localized carrier properties has been achieved using quantum mechanical calculations of the mobility. Theoretical interpretation of the terahertz conductivity spectra is based on the solution of the wave equation for the terahertz wave in inhomogeneous photoexcited media described in terms of VBD effective medium model developed in our laboratory. This approach is employed for investigation of the terahertz photoconductivity in two types of nanomaterials: superlattices of Si nanocrystals prepared by thermal decomposition of SiOx layers and lithographically prepared GaAs nanobars. Experimental spectra were measured at 300 and 20 K and for a wide range of photocarrier densities. A careful analysis of electron...Matematicko-fyzikální fakultaFaculty of Mathematics and Physic
Electron Dynamics in Silicon-Germanium Terahertz Quantum Fountain Structures
No full textAsymmetric quantum well systems are excellent candidates to realize semiconductor light emitters at far-infrared wavelengths not covered by other gain media. Group-IV semiconductor heterostructures can be grown on silicon substrates, and their dipole-active intersubband transitions could be used to generate light from devices integrated with silicon electronic circuits. Here, we have realized an optically pumped emitter structure based on a three-level Ge/Si0.18Ge0.82 asymmetric coupled quantum well design. Optical pumping was performed with a tunable free-electron laser emitting at photon energies of 25 and 41 meV, corresponding to the energies of the first two intersubband transitions 0 1 and 0 2 as measured by Fourier-transform spectroscopy. We have studied with a synchronized terahertz time-domain spectroscopy probe the relaxation dynamics after pumping, and we have interpreted the resulting relaxation times (in the range 60 to 110 ps) in the framework of an out-of-equilibrium model of the intersubband electron-phonon dynamics. The spectral changes in the probe pulse transmitted at pump-probe coincidence were monitored in the range 0.7-2.9 THz for different samples and pump intensity and showed indication of both free carrier absorption increase and bleaching of the 1 2 transition. The quantification from data and models of the free carrier losses and of the bleaching efficiency allowed us to predict the conditions for population inversion and to determine a threshold pump power density for lasing around 500 kW/cm2 in our device. The ensemble of our results shows that optical pumping of germanium quantum wells is a promising route toward silicon-integrated far-infrared emitters
Photoluminescence dynamics in few-layer InSe
Get PDFWe study the optical properties of thin flakes of InSe encapsulated in hexagonal boron nitride. More specifically, we investigate the photoluminescence (PL) emission and its dependence on sample thickness and temperature. Through the analysis of the PL line shape, we discuss the relative weights of the exciton and electron-hole contributions. Thereafter we investigate the PL dynamics. Two contributions are distinguishable at low temperature: direct band-gap electron-hole and defect-assisted recombination. The two recombination processes have lifetimes of tau(1)similar to 8 ns and tau(2) similar to 100 ns, respectively. The relative weights of the direct bandgap and defect-assisted contributions show a strong layer dependence due to the direct-to-indirect band-gap crossover. Electron-hole PL lifetime is limited by population transfer to lower-energy states and no dependence on the number of layers was observed. The lifetime of the defect-assisted recombination gets longer for thinner samples. Finally, we show that the PL lifetime decreases at high temperatures as a consequence of more efficient nonradiative recombinations
Tunable cryogenic terahertz cavity for strong light–matter coupling in complex materials
Get PDFWe report here the realization and commissioning of an experiment dedicated to the study of the optical properties of light–matter hybrids constituted of crystalline samples embedded in an optical cavity. The experimental assembly developed offers the unique opportunity to study the weak and strong coupling regimes between a tunable optical cavity in cryogenic environment and low energy degrees of freedom, such as phonons, magnons, or charge fluctuations. We describe here the setup developed that allows for the positioning of crystalline samples in an optical cavity of different quality factors, the tuning of the cavity length at cryogenic temperatures, and its optical characterization with a broadband time domain THz spectrometer (0.2–6 THz). We demonstrate the versatility of the setup by studying the vibrational strong coupling in CuGeO3 single crystal at cryogenic temperatures
Wechselwirkung von Millimeter- und Submillimeter-Strahlung mit Miniband-Elektronen in GaAs/AlAs Übergittern
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