dissertation
Matija Stupar (Author), Giovanni De Ninno (Mentor), Barbara Ressel (Mentor)

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;dissertations;

Data

Language: English
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UNG FPŠ - Graduate School
Publisher: [M. Stupar]
UDC: 535.374:621.375.82(043.3)
COBISS: 40537603 Link will open in a new window
Views: 2752
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Other data

Secondary language: Slovenian
Secondary title: Prednosti in slabosti experimentov z ultrakratkimi dvobarvnimi bliski
Secondary abstract: Napredki v razvoju laserjev so pripeljali do nove vrste svetlobnih virov, ki lahko generirajo koherentne in po valovni dolžini nastavljive ultra kratke svetlobne sunke v spektralnem območju vse od infrardeče svetlobe pa do mehkih rentgenskih žarkov. V to spektralno območje spadajo tudi harmoniki visokega reda generirani v plinu (ang. high-harmonic generation [HHG]), na katerih temelji svetlobni vir CITIUS na Univerzi v Novi Gorici, ter laserji na proste elektrone (ang., free-electron laser [FEL]), kot je svetlobni vir FERMI v laboratoriju Elettra v Trstu. HHG in FEL izvori svetlobe so omogočili izvajanje časovno ločljivih poskusov na časovnih skalah reda velikosti nekaj pikosekund pa vse do nekaj femtosekund. Raziskovalno delo predstavljeno v tej nalogi se osredotoča na prednosti in slabosti nekaterih eksperimentalnih tehnik, ki slonijo na uporabi omenjenih svetlobnih virov, z namenom raziskav ultrahitre dinamike materialov izven termodinamičnega ravnovesja. Prednosti HHG in FEL svetlobnih izvorov so predvsem v možnosti implementacije t.i. dvobarvnih poskusov, ki se lahko uporabljajo npr. za zajemanje dvodimenzionalnih frekvenčnih map ali za merjenje efektivnih mas elektronov ločeno od vplivov kristalne mreže. V ospredju bodo eksperimenti, ki temeljijo na fotoefektu in fotoelektronski spektroskopiji. Pri vseh poskusih je bila uporabljena vsaj ena od posebnih lastnosti HHG ali FEL svetlobnih virov, kot so npr. nadzorovan “chirp” (spreminjanje frekvence svetlobnega sunka s časom) za preučevanje lasersko pogojenih stanj v atomih helija, spremenljiva polarizacija za preučevanje lastnosti železovih pniktidov, ter zelo kratki pulzi (< 10 fs) za preiskovanje izolirane elektronske dinamike v dihalkogenidih prehodnih kovin. Po drugi strani pa smo pri izvajanju dvobarvnih študij na vmesnih plasteh med molekulami in topološkim izolatorjem naleteli na nekaj omejitev ter fizikalnih slabosti, npr. na nezaželene pojave, ki izvirajo iz uporabe svetlobnih sunkov visoke moči, kot so večfotonska absorbcija, prostorski naboj, ter zmanjšanje energijske ločljivosti pri uporabi zelo kratkih svetlobnih sunkov. Nekatere omejitve so povezane tudi s trenutnim stanjem tehnike na področju ultrahitrih laserskih sistemov, kjer je potrebno najti pravi kompromis v ravnovesju med frekvenco svetlobnih sunkov, ter močjo laserja. Na koncu bodo predstavljeni novi napredni poskusi, ki temeljijo na generiranju ultrahitrih svetlobnih bliskov v bližnjem infrardečem ter ekstremnem ultravijoličnem (UV) spektralnem območju, ki nosijo tirno vrtilno količino (ang. orbital angular momentum [OAM]). Uporaba takšnih bliskov bo omogočila preučevanje novih fizikalnih pojavov, kot sta npr. opazovanje magnetnih vrtincev s pomočjo XUV sunkov, ter prenos tirne vrtilne količine s svetlobe na proste elektrone.
Secondary keywords: ultra-hitri laserji;dvobarvni eksperimenti;fotoemisija;generiranje harmonikov visokih redov;laserji na proste elektrone;vroči elektroni;fizika površin;ultraviolična fotoemisija;časovno odvisni eksperimenti;tirna vrtilna količina;disertacije;
URN: URN:SI:UNG
Type (COBISS): Doctoral dissertation
Thesis comment: Univ. v Novi Gorici, Fak. za podiplomski študij
Pages: VI, 146 str.
ID: 12169727