1. Advantages and disadvantages of experiments with ultrashort two-color pulsesMatija Stupar, 2020, doctoral dissertation Abstract: 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.
Found in: ključnih besedah
Summary of found: ...to soft X-rays. This includes high-order harmonics generation in gas (HHG), on which relies the...
Keywords: 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
Published: 02.12.2020; Views: 2581; Downloads: 94
 Fulltext (19,78 MB) |
2. Chemical and structural investigation of the cobalt phthalocyanineMatija Stupar, 2015, master's thesis Abstract: 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.
Found in: ključnih besedah
Summary of found: ...(XAS), synchrotron radiation, laser, high order harmonic generation (HHG), time resolved spectroscopy...
Keywords: Cobalt phthalocyanine (CoPc), photoemission spectroscopy (PES), X-ray absorption spectroscopy (XAS), synchrotron radiation, laser, high order harmonic generation (HHG), time resolved spectroscopy
Published: 29.09.2015; Views: 7632; Downloads: 268
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5. Self-amplified photo-induced gap quenching in a correlated electron materialJurij Urbančič, Eich Steffen, Mathias Stefan, 2016, original scientific article Abstract: Capturing the dynamic electronic band structure of a correlated material presents a powerful capability for uncovering the complex couplings between the electronic and structural degrees of freedom. When combined with ultrafast laser excitation, new phases of matter can result, since far-from-equilibrium excited states are instantaneously populated. Here, we elucidate a general relation between ultrafast non-equilibrium electron dynamics and the size of the characteristic energy gap in a correlated electron material. We show that carrier multiplication via impact ionization can be one of the most important processes in a gapped material, and that the speed of carrier multiplication critically depends on the size of the energy gap. In the case of the charge-density wave material 1T-TiSe 2 , our data indicate that carrier multiplication and gap dynamics mutually amplify each other, which explains—on a microscopic level—the extremely fast response of this material to ultrafast optical excitation.
Self-amplified photo-induced gap quenching in a correlated electron material. Available from: https://www.researchgate.net/publication/308804379_Self-amplified_photo-induced_gap_quenching_in_a_correlated_electron_material [accessed Apr 20, 2017].
Found in: ključnih besedah
Summary of found: ...high harmonic generation, charge-density wave material, 1T-TiSe2, non-equilibrium electron dynamics,...
Keywords: high harmonic generation, charge-density wave material, 1T-TiSe2, non-equilibrium electron dynamics, ultrafast surface science
Published: 20.04.2017; Views: 4553; Downloads: 0
Fulltext (1,10 MB) |
6. Time- and angle-resolved photoemission spectroscopy with optimized high-harmonic pulses using frequency-doubled Ti:Sapphire lasersSteffen Eich, Jurij Urbančič, 2014, original scientific article Abstract: Time- and angle-resolved photoemission spectroscopy (trARPES) using femtosecond extreme ultraviolet high harmonics has recently emerged as a powerful tool for investigating ultrafast quasiparticle dynamics in correlated-electron materials. However, the full potential of this approach has not yet been achieved because, to date, high harmonics generated by 800 nm wavelength Ti:Sapphire lasers required a trade-off between photon flux, energy and time resolution. Photoemission spectroscopy requires a quasi-monochromatic output, but dispersive optical elements that select a single harmonic can significantly reduce the photon flux and time resolution. Here we show that 400 nm driven high harmonic extreme-ultraviolet trARPES is superior to using 800 nm laser drivers since it eliminates the need for any spectral selection, thereby increasing photon flux and energy resolution to <150 meV while preserving excellent time resolution of about 30 fs.
Found in: ključnih besedah
Summary of found: ...angle-resolved photoemission spectroscopy, ARPES, high harmonic generation, ultrafast surface science...
Keywords: angle-resolved photoemission spectroscopy, ARPES, high harmonic generation, ultrafast surface science
Published: 20.04.2017; Views: 4334; Downloads: 0
Fulltext (1,80 MB) |
7. Tunable orbital angular momentum in high-harmonic generationDavid Gauthier, Primoz Rebernik-Ribic, Ganesh Adhikari, A Camper, C Chappuis, LF Dimauro, G Dovillaire, Fabio Frassetto, R Geneaux, Paolo Miotti, Luca Poletto, Barbara Ressel, C. Spezzani, Matija Stupar, T Ruchon, Giovanni De Ninno, 2017, original scientific article Found in: ključnih besedah
Keywords: Angular orbital momentum, High harmonic generation
Published: 05.02.2018; Views: 3353; Downloads: 750
Fulltext (843,67 KB) |
8. Echo-enabled harmonic generation studies for the FERMI free-electron laserPrimož Rebernik Ribič, Eléonore Roussel, Gregory Penn, Giovanni De Ninno, Luca Giannessi, G. Penco, Enrico Allaria, 2017, original scientific article Found in: ključnih besedah
Keywords: free electron laser, harmonic up-conversion, high-harmonic generation, seeding, x-ray, pump-probe, microbunching instability
Published: 13.01.2020; Views: 2221; Downloads: 10
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9. Generation and applications of extreme-ultraviolet vorticesCarlos Hernández-García, Jorge Vieira, Jose T. Mendonça, Laura Rego, Julio San Román, Luis Plaja, Primož Rebernik Ribič, David Gauthier, Antonio Picón, 2017, original scientific article Found in: ključnih besedah
Summary of found: ...XUV/X-ray vortices, orbital angular momentum of photons, high-harmonic generation, free-electron lasers, plasma physics...
Keywords: XUV/X-ray vortices, orbital angular momentum of photons, high-harmonic generation, free-electron lasers, plasma physics
Published: 13.01.2020; Views: 2280; Downloads: 11
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10. Ultrafast Element-Specific Demagnetization in AlloysJurij Urbančič, doctoral dissertation Abstract: The study of magnetism has long been an active area of interest for the scientific research, while also providing vast technological applications such as information storage. It was first observed in the middle of the 90s that irradiating a ferromagnetic material with an intense femtosecond laser pulse can result in a loss of magnetization occurring below one picosecond. Since then, the study of magnetism on these femtosecond timescales has been a field of growing interest, addressing the question of how fast can the magnetization modification occurs and what presents the fundamental limit of this speed. For the understanding of these experimental observations, a number of different models were proposed, although the responsible microscopic mechanisms are still under discussion. According to one group of models, the change in magnetic moment is due to the spin-flip mechanism occurring during scattering events between electrons and (quasi-)particles. An example of this is the so-called Microscopic Three-Temperature Model (M3TM). A different approach is employed by the Superdiffusive Spin Current Model, where the loss of magnetization is attributed to a flow of chargeless spin particles taking place after the laser excitation.
With the purpose to explore the magnetic characteristic of materials, a range of investigation methods has been developed. One of them relies on the exploitation of the Magneto-Optical Kerr Effect (MOKE) and an incorporation of femtosecond laser pulses in pump-probe configuration. In this way, part of the beam from the same laser system is used to optically excite the system (the pump), while another part takes a snapshot of it (the probe), thus allowing us to investigate the magnetization dynamics with a femtosecond temporal resolution. In the experimental part of the thesis, we apply this MOKE technique to explore the demagnetization dynamics of a ferromagnetic alloy consisting of iron and nickel, also called Permalloy. It is observed that the magnetization is suppressed (quenched) in the first few hundred femtoseconds. Part of the experiments were performed using visible light with different fluences of the pump. The most notable observation is that this alloy exhibits different demagnetization dynamics at lower and higher fluence. What we see is a transition from one-step (demagnetization followed by a recovery on a much longer timescale) to a two-step (demagnetization followed by another slower demagnetization step) magnetization process occurring with the increment of pump fluence. This behavior is predicted and reproduced by the M3TM, which is applied for a comparison between experiments and theory.
Found in: ključnih besedah
Summary of found: ...High Harmonic Generation, Magnetism, Ultrafast, Materials...
Keywords: High Harmonic Generation, Magnetism, Ultrafast, Materials
Published: 08.05.2020; Views: 2355; Downloads: 0
Fulltext (12,06 MB) |
