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1.
Advantages and disadvantages of experiments with ultrashort two-color pulses
Matija 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.
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 in RUNG: 02.12.2020; Views: 3535; Downloads: 111
.pdf Full text (19,78 MB)

2.
Observation of short-lived laser-dressed quantum states in the frequency plane
Matjaz Zitnik, Giovanni De Ninno, 2019, original scientific article

Keywords: Laser-dressed quantum states, high-harmonic generation
Published in RUNG: 27.10.2020; Views: 2395; Downloads: 0
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3.
Control of ellipticity in high-order harmonic generation driven by two linearly polarized fields
Benoit Mahieu, Giovanni De Ninno, 2018, original scientific article

Keywords: High-harmonic generation, polarization
Published in RUNG: 27.10.2020; Views: 2209; Downloads: 0
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4.
Coupling of autoionizing states by a chirped laser pulse
Matjaž Žitnik, Andrej Mihelič, Klemen Bučar, Mateja Hrast, Žiga Barba, Špela Krušič, Primož Rebernik Ribič, Jurij Urbančič, Barbara Ressel, Matija Stupar, David Gauthier, Giovanni De Ninno, 2020, published scientific conference contribution abstract

Abstract: We have observed the autoionization of the laser-coupled 2s2p 1Po and 2p2 1Se resonances in helium. The ions were collected while varying the frequency and delay of the extreme-ultraviolet (EUV) excitation pulse with respect to the linearly chirped visible (VIS) laser pulse. From the measured frequency-delay map the Autler- Townes splitting, the EUV-VIS cross-correlation and the linear chirp parameter were extracted.
Keywords: High Harmonic Generation
Published in RUNG: 29.06.2020; Views: 2978; Downloads: 0
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5.
Element-resolved study on the influence of a magnetic dopant on the exchange interaction in FePt on ultrashort timescales
Jurij Urbančič, 2017, published scientific conference contribution abstract

Abstract: In this study, we investigate the optically induced ultrafast demagnetization dynamics of a FePt alloy doped with Mn atoms on the characteristic timescale of the exchange interaction of a few femtoseconds. We use high harmonic generation with photon energies in the XUV to gain elemental resolution in a femtosecond magneto-optical Kerr-effect experiment [1] in transversal geometry. Following up on earlier experiments, which showed the importance of the exchange interaction in the demagnetization process in alloys [2,3], we see clear differences in the dynamical responses of the three elements Fe, Pt, and Mn. Our results suggest, that the exchange coupling between the magnetic moments of the FePt alloy and the Mn dopant changes on ultrashort timescales.
Keywords: FePt, High Harmonic Generation, Laser, Magnetic Materials
Published in RUNG: 22.05.2020; Views: 2879; Downloads: 0

6.
The role of non-equilibrium dynamics in photo-induced phase transitions of correlated materials
Jurij Urbančič, 2015, published scientific conference contribution abstract

Abstract: We use femtosecond extreme-ultraviolet time- and angle-resolved photoelectron spectroscopy to study the ultrafast photo-induced suppression of the charge-density wave (CDW) in 1T-TiSe2. In the following non-equilibrium electron dynamics after femtosecond laser excitation, we see that hot-carrier multiplication is the primary driver for the ultrafast CDW suppression. As soon as the optically excited carriers have relaxed to a quasi-equilibrium hot Fermi-distribution electron gas, the CDW suppression stops. Theoretical calculations of the hot-carrier scattering processes and the screening properties further link the carrier multiplication to the observed CDW gap dynamics.
Keywords: Correlated Materials, High Harmonic Generation, ARPES
Published in RUNG: 22.05.2020; Views: 2775; Downloads: 0

7.
Towards ultrafast X-ray condensed matter physics with MHz repetition rate HHG sources
Jurij Urbančič, 2015, published scientific conference contribution abstract

Abstract: In recent years, an increasing number of ultrafast material science experiments are based on the use of table-top high-harmonic generation (HHG) lightsources [1,2]. Despite the huge success of these first experiments, a major drawback has been the limited repetition rate of these lightsources, in particular with respect to all types of photoemission experiments. Here, we combine newly developed high-repetition rate HHG sources [3,4] with state-of-the-art element-specific magneto-optical Kerr experiments. The chances and prospects for future ultrafast materials science experiments with these lightsources will be discussed.
Keywords: High Harmonic Generation, X-Rays, Condensed Matter
Published in RUNG: 22.05.2020; Views: 3043; Downloads: 0

8.
Ultrafast Element-Specific Demagnetization in Alloys
Jurij 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.
Keywords: High Harmonic Generation, Magnetism, Ultrafast, Materials
Published in RUNG: 08.05.2020; Views: 3100; Downloads: 0
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