Ultrafast carrier dynamics caused by photoexcitation in quasi-one-dimensional two-leg ladder cuprate Sr_4Ca_{10}Cu_{24}O_{41} was investigated by the femtosecond reflection spectroscopy. After the photoexcitation along the leg direction, the transient reflectivity changes (Δ R/R) in the mid-infrared region showed instant decrease within 150 fs. The suppressed Δ R/R increased so rapidly on the picosecond time scale that the reflectivity value finally became larger than that in the initial state. Such a successive response in Δ R/R, which was also observed in other photon energy regions, is discussed in terms of ultrafast variation of the Drude weight in the ladder system by the photoirradiation.
Positron annihilation lifetime spectroscopy has shown to be a powerful tool to study the nanostructures of porous materials. Positron emission tomograph is a device allowing imaging of metabolic processes e.g. in human bodies. A newly developed device, the Jagiellonian PET will allow positron annihilation lifetime spectroscopy in addition to imaging, thus combining both analyses providing new methods for physics and medicine. In this contribution we present a computer program that is compatible with the Jagiellonian PET software. We compare its performance with the standard program LT 9.0 by using positron annihilation lifetime spectroscopy data from hexane measurements at different temperatures. Our program is based on an iterative procedure, and our fits prove that it performs as good as LT 9.0.
We report on the first principle calculations of photocarriers kinetics in a photoconductive antenna excited by an ultrashort optical laser pulse. The solution of non-equilibrium Boltzmann equation is used to derive the expression for the irradiated electric field. The analysis reveals the important role of non-uniform photocarrier distribution inside the active layer in the formation of the terahertz radiation from the emitter in both collinear and anti-collinear directions.
Terahertz emission from the electron-hole plasma excited by a femtosecond optical pulse in GaAs-based emitters is studied by the Monte Carlo simulations. The THz energy radiated from the n- and p-doped GaAs surface THz emitters, from the contactless p-i-n emitter, and from the photoconductive emitter is evaluated. The obtained results show that the THz energy radiated by the photoconductive emitter exceeds the energy radiated by the surface and p-i-n THz emitters by more than one order of magnitude.
THz pulses were used to investigate carrier dynamics in narrow-gap semiconductors. The measurement of the optically induced THz pulse absorption transients provided important insights into electron energy relaxation in the conduction band. In the second set of experiments, THz generation from the surfaces of various semiconductors was studied and compared. It was found that the most efficient THz emitters are semiconductors with a narrow band gap, large intervalley separation in the conduction band, and low nonparabolicity of the main valley.
We present measurements of the dynamics of photoexcited excitons in an ensemble of InAs/GaAs self-assembled quantum dots using a femtosecond spectral hole burning technique. We use this technique to examine the exciton spin relaxation and the line shape of optical transitions in the dots.
Terahertz emission from laser-generated air plasmas has recently been identified as an interesting source for THz radiation. High intensities and a large bandwidth of the THz pulses can be achieved. We briefly review several mechanisms which were employed to generate the quasi-static dipole moment needed for the optical rectification process. This leads us to a discussion of a specific application of THz emission from an air plasma, namely the investigation of the carrier-envelope phase of few-cycle optical pulses. Such pulses of a duration of less than 10 fs induce a spatial charge asymmetry in the plasma directly via non-linear tunneling ionization. The asymmetry, and with it the emission of the THz radiation from the plasma, depend on the carrier-envelope phase, with the consequence that one can determine the phase by measurement of the amplitude and polarity of the THz pulse.
Time-resolved correlation measurements were performed to examine the statistics of photon emission from a single quantum dot at high excitation levels. The range of strong powers was determined by the saturation of intensity of excitonic lines. The significant contribution of pulsed background luminescence forced us to sophisticated analysis of the photoluminescence spectra. For quantum dots grown in the Stranski-Krastanov mode, one-photon emission was found to a good approximation. Fluctuation type quantum dots exhibit a significant level of multiphoton emission.
New powerful sources and advanced analytical techniques have been considered in the last decade to face up the continuously increasing scientific demands, in particular, in materials science. As an example, nano- science and nanotechnology researches are characterized by ultimate spatial resolution, fast and ultrafast time-resolved analysis, but the complexity of the investigated phenomena requires new analytical capabilities and new experimental techniques were introduced in the research arena. The availability all over the world of brilliant synchrotron radiation sources offers incredible opportunities. Many challenging experiments were made possible by these sources and understanding of many complex dynamical problems was obtained. Nevertheless, a strong demand of new analytical approaches, mainly based on concurrent and possibly simultaneous time-resolved experimental techniques, is emerging. Pioneering time resolved experiments combining X-ray and infrared radiation with a conventional source were performed more than a decade ago. Nowadays, many beamlines at third generation synchrotron radiation facilities are equipped with conventional sources to allow complementary techniques and the strategy of a concurrent analysis is mandatory in the investigation of many phenomena in frontier multidisciplinary researches. Moreover, new opportunities will be available by means of concurrent spectroscopic experiments investigating complex phenomena on a short timescale, from the sub-second to the microsecond time domain. We will present and discuss researches where the combination of IR and X-ray simultaneous experiments may return unique information on complex dynamical processes and phase transitions occurring in materials science. Finally, we will briefly describe the conceptual layout of a synchrotron radiation beamline to perform concurrent IR and X-ray experiments.
Optical properties of CdTe/ZnTe quantum dots are studied as a function of a capping layer thickness by means of time-integrated and time-resolved microphotoluminescence. The samples are grown by MBE and covered with 10 nm and 100 nm capping layer. Despite that the proximity of the surface may result in an enhanced rate of non-radiative processes limiting the quantum dots optical performance, the set of results indicates that reduction of the capping layer thickness down to 10 nm has no effect on the quantum dot emission intensity and decay rate, contrary to the previously reported case of InAs/GaAs quantum dots.
Systematic studies of amplitude and energy changes of excitonic lines in a strong excitation regime were carried out by a pump-probe method. The series of samples containing quantum wells with well width from 80 Åto 140 Åwas investigated. One 80 Åsample was n-doped with iodine, the rest of the quantum wells were intentionally undoped but contained a 2D gas of free holes. Its density could be varied by changing an intensity of additional illumination. The resonant creation of high population of e1hh1 excitons causes the energetic blue shift of the same due to interactions between excitons. The blue shifts did not depend on the concentration of 2D gas of carriers whereas it did depend on the power of excitation beam. Model calculations of absorption show qualitative agreement with the experimental data.
The parameters of radiation sensitivity of the oxygen-doped fluorite crystals were calculated in a one-dimensional model. The limit concentrations of the color centers as a function of the concentration of the oxygen impurity in the fluorite crystal were defined. Fluorite crystals with anti-Frenkel defects in the anion sublattice of the crystal have a specific property: the discolored after irradiation crystal being irradiated repeatedly with ionizing radiation retains the "memory" of the preceding irradiation. Using an ion chain model this paper studies under what conditions the "radiation memory" effect can arise in the MeF₂-O²¯ crystals as well as the extent of its contribution into the overall radiation sensitivity of the crystal.
The spectral and kinetic parameters of electron-pulse-initiated transient absorption of oxygen-doped CaF_{2} crystals were studied using pulsed spectrometry with a nanosecond time resolution. It is shown that the formation of a M_{A}^{+} color centers in CaF_{2}-0.01M%CaO crystals occurs by thermally activated diffusion of the vacancies. Activation energy of M_{A}^{+} color centers formation process of 0.4 eV is established.
Systematic studies of neutral heavy-hole excitonic line energy changes in a strong excitation regime were carried out by means of a pump-probe method for quantum wells containing a 2D gas of free holes. Energy shift of X_{e1hh1} line was analyzed for different excitation energies at fixed delay between pump and probe pulses, also under external magnetic field. It was observed that this shift depends not only on the density of created excitons but also directly on the pump energy. In co-polarization configuration for excitation energy below an absorption resonance the energetic blue shift rises linearly with the elevated exciton density (localized excitons are created). For energies slightly above the resonance, the blue shift diminishes dramatically in spite of high exciton density present (delocalized excitons are created). Model absorption calculations are in qualitative agreement with the experimental data.
Terahertz emission from the freestanding InGaN/GaN heterostructure illuminated by femtosecond optical pulse is considered using Monte Carlo simulations. The results of Monte Carlo simulations show that the power of terahertz emission from InGaN/GaN heterostructure exceeds the power of the emission from InN surface by one order of magnitude.
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