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Advantages and disadvantages of experiments with ultrashort two-color pulses
Matija Stupar, doktorska disertacija

Opis: Advances in the development of lasers have led to a new class of radiation sources generating coherent, tunable, ultrashort light pulses in the spectral region ranging from infrared to soft X-rays. This includes high-order harmonics generation in gas (HHG), on which relies the CITIUS facility at University of Nova Gorica (Slovenia), and free-electron lasers (FELs), such as the facility FERMI at Elettra-Sincrotrone Trieste (Italy). The distinctive structure of HHG and FEL radiation paved the way to time-resolved experiments, which are performed to investigate events occurring on a short, or very short, temporal scale, from picoseconds to femtoseconds. This work focuses on the advantages and disadvantages of some experimental techniques based on using these novel light sources to investigate the microscopic and/or ultrafast dynamics of matter samples, which have been previously driven out of equilibrium. Advantages rely on the implementation of various applications based on two-color schemes and, more specifically, include the possibility of acquiring two-dimensional frequency maps, measuring electrons’ effective masses, or investigating electronic properties decoupled from the influence of the lattice. Particular focus will be put on experimental methods relying on photoelectric effect and photoelectron spectroscopy. In all experiments, we took advantage of one or more specific properties of HHG and FEL sources, such as controllable chirp, to study laser dressed states in helium, variable polarization, to study electronic properties of iron-based pnictides and ultrashort pulses (< 10 fs) to study the purely electronic dynamics in transition metal dichalcogenides. On the other hand, the study of the interface between a molecule and a topological insulator revealed some intrinsic limitations and physical drawbacks of the technique, such as spurious effects originating from the high power pulses, like multiphoton absorption and the space charge effect, or the reduction of experimental resolution when pushing for shorter and shorter pulse durations. Some disadvantages are also connected to the current state-of-the-art in the field of ultrashort laser systems, where a trade-off needs to be found between repetition rate and laser power. Finally, state-of-the-art experiments based on the ability to generate ultrashort pulses carrying orbital angular momentum in visible, near-infrared as well as extreme UV range will be presented. The use of these pulses opens the door to the investigation of new physical phenomena, such as probing magnetic vortices using extreme ultraviolet light from a free-electron laser or imprinting the spatial distribution of an ultrashort infrared pulse carrying orbital angular momentum onto a photoelectron wave packet.
Najdeno v: ključnih besedah
Ključne besede: ultrafast lasers, two-color experiments, photoemission, high-order harmonic generation, free-electron lasers, hot-electrons dynamics, surface science, pump-probe photoemission, ultraviolet photoemission, orbital angular momentum
Objavljeno: 02.12.2020; Ogledov: 152; Prenosov: 4
.pdf Polno besedilo (19,78 MB)

The H.E.S.S. measurement of the cosmic-ray electron spectrum
Serguei Vorobiov, Kathrin Egberts, 2010, objavljeni povzetek znanstvenega prispevka na konferenci

Najdeno v: ključnih besedah
Ključne besede: ground based y-ray telescopes, H.E.S.S. system, Cherenkov telescopes, very high energy, cosmic-ray electrons, energy spectrum
Objavljeno: 25.04.2014; Ogledov: 2215; Prenosov: 15
URL Polno besedilo (0,00 KB)

Ultra-visokovakuumski sistem za naparevanje organskih polprevodnikov
Robert Hudej, Gvido Bratina, 2001, izvirni znanstveni članek

Opis: A new molecular-beam-epitaxy system for the growth of organic semiconductor layers was recently installed in the Laboratory for epitaxy and nanostructures of the Nova Gorica Polytechnic. The system is built around an ultra-high-vacuum chamber. The organic semiconductor material is evaporated from a low-temperature Knudsen cell. The system is equipped with a refraction high-energy electron-difraction system, a quadrupole mass spectrometer and a spectroscopic ellipsometer.
Najdeno v: ključnih besedah
Ključne besede: ultra-high vacuum, molecular-beam epitaxy, Knudsen cell, mass quadrupole spectrometer, spectroscopic ellipsometer, high-energy electrons
Objavljeno: 10.07.2015; Ogledov: 2832; Prenosov: 13
URL Polno besedilo (0,00 KB)

Chemical and structural investigation of the cobalt phthalocyanine
Matija Stupar, 2015, magistrsko delo

Opis: In the last two decades, studies on organic molecules mimicking substances of fundamental importance in nature, like chlorophyll or hemoglobin, have attracted researchers’ attention. These molecules are building blocks for a family of materials also referred to as “organic semiconductors”. Such compounds can be implemented in numerous applications, ranging from data-storage to light harvesting. Some of their fundamental advantages include low cost, light weight, relatively easy engineering and mechanical flexibility, compatible with bending plastic substrates. In this thesis work we investigated the chemical, structural and electronic properties of cobalt phthalocyanines (CoPc). These molecules have promising applications in the field of magnetic data storage and spintronics in general, due to the ferromagnetic properties of the cobalt atom. Several techniques like photoemission core-level spectroscopy and valence band spectroscopy, together with X-ray absorption, have been used in order to determine the CoPc properties in gaseous phase, i.e. in the absence of interaction with the surrounding environment. Another set of experiments was devoted to the commissioning of the CITIUS time-resolved photoemission setup, that will be used in future studies of CoPc molecules on surfaces.
Najdeno v: ključnih besedah
Ključne besede: Cobalt phthalocyanine (CoPc), photoemission spectroscopy (PES), X-ray absorption spectroscopy (XAS), synchrotron radiation, laser, high order harmonic generation (HHG), time resolved spectroscopy
Objavljeno: 29.09.2015; Ogledov: 4153; Prenosov: 195
.pdf Polno besedilo (2,96 MB)

Real-time motor unit identification from high-density surface EMG
Vojko Glaser, Aleš Holobar, Damjan Zazula, 2013, izvirni znanstveni članek

Opis: This study addresses online decomposition of high-density surface electromyograms (EMG) in real-time. The proposed method is based on previouslypublished Convolution Kernel Compensation (CKC) technique and sharesthe same decomposition paradigm, i.e. compensation of motor unit action potentials and direct identification of motor unit (MU) discharges. In contrast to previously published version of CKC, which operates in batch mode and requires ~ 10 s of EMG signal, the real-time implementation begins with batch processing of ~ 3 s of the EMG signal in the initialization stage and continues on with iterative updating of the estimators of MU discharges as blocks of new EMG samples become available. Its detailed comparison to previously validated batch version of CKC and asymptotically Bayesian optimal Linear Minimum Mean Square Error (LMMSE) estimator demonstrates high agreementin identified MU discharges among all three techniques. In the case of synthetic surface EMG with 20 dB signal-to-noise ratio, MU discharges were identified with average sensitivity of 98 %. In the case of experimental EMG, real-time CKC fully converged after initial 5 s of EMG recordings and real-time and batch CKC agreed on 90 % of MU discharges, on average. The real-time CKC identified slightly fewer MUs than its batch version (experimental EMG, 4 MUs versus 5 MUs identified by batch CKC, on average), but required only 0.6 s of processing time on regular personal computer for each second of multichannel surface EMG.
Najdeno v: ključnih besedah
Ključne besede: discharge pattern, high-density EMG, surface EMG, motor unit, real time decomposition
Objavljeno: 05.01.2016; Ogledov: 2428; Prenosov: 0

The flux of ultra-high energy cosmic rays after ten years of operation of the Pierre Auger Observatory
Marko Zavrtanik, Danilo Zavrtanik, Lili Yang, Serguei Vorobiov, Darko Veberič, Marta Trini, Samo Stanič, Ahmed Saleh, Gašper Kukec Mezek, Andrej Filipčič, Inés Valiño, 2015, objavljeni znanstveni prispevek na konferenci

Opis: The flux of ultra-high energy cosmic rays has been measured with unprecedented precision at the Pierre Auger Observatory. We report an update of the all-sky flux of cosmic rays above 3 x 10 [sup] 17 eV obtained by combining four independent data sets. These measurements are based on data from the surface detector arrays (divided into two sets according to the shower zenith angle), from a nested, denser, detector array, and hybrid events measured simultaneously with both the fluorescence detector and the surface detector array. The spectral features are presented in detail and the systematic uncertainties are addressed. The huge amount of data collected to date, with a total exposure exceeding 50,000 km2 sr yr, together with the wide range of sky observed (in declination from -90[deg] to 45[deg]) also allow us to measure the energy spectrum from different regions of the sky. We present the results of the search for a dependence of the measured flux on the declination of the incoming directions.
Najdeno v: ključnih besedah
Ključne besede: ultra-high energy cosmic rays energy spectrum Pierre Auger Observatory
Objavljeno: 02.03.2016; Ogledov: 2817; Prenosov: 152
.pdf Polno besedilo (755,08 KB)

Arrival directions of the highest-energy cosmic rays detected with the Pierre Auger Observatory
Marko Zavrtanik, Danilo Zavrtanik, Lili Yang, Serguei Vorobiov, Darko Veberič, Marta Trini, Samo Stanič, Ahmed Saleh, Gašper Kukec Mezek, Andrej Filipčič, Julien Aublin, 2015, objavljeni znanstveni prispevek na konferenci

Opis: We present the results of a search for small to intermediate scale anisotropies in the distribution of arrival directions of ultra-high energy cosmic rays recorded at the Pierre Auger Observatory. The data set, gathered in ten years of operation, includes arrival directions with zenith angles up to 80◦, and is about three times larger than that used in earlier studies. We update the test based on correlations with active galactic nuclei (AGNs) from the Véron-Cetty and Véron catalog, which does not yield a significant indication of anisotropy with the present data set. We perform a blind search for localized excess fluxes and for self-clustering of arrival directions at angular scales up to 30◦ and for different energy thresholds between 40 EeV and 80 EeV. We also examine the correlation of arrival directions with relatively nearby galaxies in the 2MRS catalog, AGNs detected by Swift-BAT, and a sample of radio galaxies with jets and with the Centaurus A galaxy. None of the searches shows a statistically significant evidence of anisotropy. The two largest departures from isotropy that were found have a post-trial probability ≈ 1.4%. One is for cosmic rays with energy above 58 EeV that arrive within 15◦ of the direction toward Centaurus A. The other is for arrival directions within 18◦ of Swift-BAT AGNs closer than 130 Mpc and brighter than 10[sup]44 erg/s, with the same energy threshold.
Najdeno v: ključnih besedah
Ključne besede: ultra-high energy cosmic rays anisotropy studies active galactic nuclei Pierre Auger Observatory
Objavljeno: 02.03.2016; Ogledov: 2374; Prenosov: 158
.pdf Polno besedilo (1,32 MB)

Indications of anisotropy at large angular scales in the arrival directions of cosmic rays detected at the Pierre Auger Observatory
Marko Zavrtanik, Danilo Zavrtanik, Lili Yang, Serguei Vorobiov, Darko Veberič, Marta Trini, Samo Stanič, Ahmed Saleh, Gašper Kukec Mezek, Andrej Filipčič, Imen Al Samarai, 2015, objavljeni znanstveni prispevek na konferenci

Opis: The large-scale distribution of arrival directions of high-energy cosmic rays carries major clues to understanding their origin. The Pierre Auger Collaboration have implemented different analyses to search for dipolar and quadrupolar anisotropies in different energy ranges spanning four orders of magnitude. A common phase ≈270◦ of the first harmonic modulation in right-ascension was found in adjacent energy intervals below 1 EeV, and another common phase ≈100◦ above 4 EeV. A constancy of phase measurements in ordered energy intervals originating from a genuine anisotropy is expected to appear with a smaller number of events than those needed to achieve significant amplitudes. This led us to design a prescribed test aimed at establishing whether this consistency in phases is real at 99% CL. The test required a total independent exposure of 21,000 km2 sr yr. We report on the status of this prescription. We also report the results of the search for a dipole anisotropy for cosmic rays with energies above 4 EeV using events with zenith angles between 60◦ and 80◦. Compared to previous analyses of events with zenith angles smaller than 60◦, this extension increases the size of the data set by 30%, and enlarges the fraction of exposed sky from 71% to 85%. The largest departure from isotropy is found in the energy range above 8 EeV, with an amplitude for the first harmonic in right ascension r1 = (4.4 ± 1.0) × 10[sup]−2, that has a chance probability P(≥ r1) = 6.4×10[sup]−5, reinforcing the hint previously reported with vertical events alone.
Najdeno v: ključnih besedah
Ključne besede: high-energy cosmic rays large-scale distribution anisotropy studies Pierre Auger Observatory
Objavljeno: 02.03.2016; Ogledov: 2267; Prenosov: 164
.pdf Polno besedilo (862,90 KB)

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