Iskanje po repozitoriju

A+ | A- | | SLO | ENG

Opis: 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.
Ključne besede: Correlated Materials, High Harmonic Generation, ARPES
Objavljeno v RUNG: 22.05.2020; Ogledov: 3158; Prenosov: 0

Opis: 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.
Ključne besede: High Harmonic Generation, Magnetism, Ultrafast, Materials
Objavljeno v RUNG: 08.05.2020; Ogledov: 3533; Prenosov: 0
Gradivo ima več datotek! Več...

Ključne besede: free-electron lasers, harmonic generation
Objavljeno v RUNG: 15.01.2020; Ogledov: 3342; Prenosov: 85
Povezava na celotno besedilo
Gradivo ima več datotek! Več...

Ključne besede: free-electron lasers, harmonic generation, seeding
Objavljeno v RUNG: 15.01.2020; Ogledov: 2900; Prenosov: 16
Povezava na celotno besedilo
Gradivo ima več datotek! Več...

Ključne besede: XUV/X-ray vortices, orbital angular momentum of photons, high-harmonic generation, free-electron lasers, plasma physics
Objavljeno v RUNG: 13.01.2020; Ogledov: 3494; Prenosov: 14
Povezava na celotno besedilo