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Picosecond X-ray Diffraction Studies of Bulk and Nanostructure Materials

Jurgilaitis, Andrius LU (2012)
Abstract (Swedish)
Popular Abstract in Swedish

Snabba fenomen, som inträffar efter laserexcitation, har studerats med tidsupplöst röntgendiffraktion (TXRD). I de flesta experimenten har en ljuspuls från en femtosekundlaser använts för att excitera provet, och dynamiken har undersökts med röntgenljus. Tidsupplösta röntgendiffraktionsmätningar har utförts för att studera fasta halvledarmaterial, vätskor, ferro-elektrisk domänswitchning i kaliumdivätefosfat (KDP) och tryckvågsutbredning i grafit och halvledande nanotrådar.

När en laserpuls absorberas av ett fast material kan en mängd olika fasövergångar och andra fenomen induceras. Om laserpulsen innehåller tillräckligt mycket energi för att smälta materialet, kan repetitiv belysning... (More)
Popular Abstract in Swedish

Snabba fenomen, som inträffar efter laserexcitation, har studerats med tidsupplöst röntgendiffraktion (TXRD). I de flesta experimenten har en ljuspuls från en femtosekundlaser använts för att excitera provet, och dynamiken har undersökts med röntgenljus. Tidsupplösta röntgendiffraktionsmätningar har utförts för att studera fasta halvledarmaterial, vätskor, ferro-elektrisk domänswitchning i kaliumdivätefosfat (KDP) och tryckvågsutbredning i grafit och halvledande nanotrådar.

När en laserpuls absorberas av ett fast material kan en mängd olika fasövergångar och andra fenomen induceras. Om laserpulsen innehåller tillräckligt mycket energi för att smälta materialet, kan repetitiv belysning skapa periodiska strukturer på provets yta. Denna effekt har studerats med statisk röntgendiffraktion, och har visat sig vara viktig då vätskespridningsexperiment utförs på repetitivt smälta prover. När energin i laserpulsen inte är tillräcklig för att inducera smältning kan koherenta fononer exciteras. Denna effekt har studerats i halvledande nanotrådar.

Tidsupplösningen för en synkrotronljuskälla bestäms av längden av en elektronpuls i lagringsringen, och är typiskt 50 - 300 ps. Bättre tidsupplösning kan uppnås genom att använda kortare röntgenpulser, som de som produceras vid Swiss Light Source (SLS), eller snabba detektorer, som t.ex. streak-kameran som används vid MAX-lab.

Röntgendiffraktion är en mycket känslig teknik för att studera strukturer, eftersom röntgenfotoner sprids från alla elektroner i provet. Spridd röntgenstrålning kan användas för att rekonstruera provets atomära struktur. I TXRD exciteras provet och belyses därefter med röntgenljus efter en given tidsfördröjning. Detta ger en ögonblicksbild av strukturen vid en tidpunkt. Flera bilder kan sammanställas för att skapa en film som visar de strukturella ändringarna i realtid. Detta har uförts med nanosekund-upplösning vid MAX-lab när en laser-genererad vätska studerades. Utvecklingen av en streak-kamera med tidsupplösning bättre än en pikosekund har varit en förutsättning för flera av studierna som presenteras i denna avhandling. Denna detektor har använts för att studera akustiska vibrationer i nanotrådar av indiumantimonid (InSb). Oscillationer med en period av 30-70 ps har detekterats, och har härletts till akustiska fononer i nanotråden. En dramatisk sänkning av ljudhastigheten har också observerats i dessa strukturer. (Less)
Abstract
Fast phenomena occurring after laser excitation were studied using time-resolved X-ray diffraction (TXRD). In most experiments, a femtosecond laser pulse was used to excite the sample, and X-rays were used as a probe. The X-ray diffraction technique was used to study bulk semiconductor samples, molten liquids, ferro-electric domain switching in potassium dihydrogen phosphate (KDP), and strain propagation in graphite and semiconductor nanowires.

When a laser pulse is absorbed by a solid, a wide range of phase transitions and phenomena can be induced. If the laser fluence is high enough to melt the material, repetitive illumination will create periodic structures on the surface of the sample. This effect was studied using static... (More)
Fast phenomena occurring after laser excitation were studied using time-resolved X-ray diffraction (TXRD). In most experiments, a femtosecond laser pulse was used to excite the sample, and X-rays were used as a probe. The X-ray diffraction technique was used to study bulk semiconductor samples, molten liquids, ferro-electric domain switching in potassium dihydrogen phosphate (KDP), and strain propagation in graphite and semiconductor nanowires.

When a laser pulse is absorbed by a solid, a wide range of phase transitions and phenomena can be induced. If the laser fluence is high enough to melt the material, repetitive illumination will create periodic structures on the surface of the sample. This effect was studied using static X-ray diffraction, and it was shown that the effect is important if liquid scattering experiments are carried out on molten samples using the laser in repetitive mode. When the laser fluence is too low to cause sample melting, coherent acoustic phonons can be excited, and this effect was studied in semiconductor nanowires.

The time resolution of the synchrotron light source is defined by the length of the electron bunch in the storage ring, and is typically 50-300 ps. In order to achieve higher time resolution, short X-ray pulses, such as those at the SLS, or fast detectors, such as the streak cameras available at MAX-lab can be used.

X-ray diffraction is a very sensitive technique for the study of structures, since X-ray photons scatter from all the electrons in the sample. Scattered X-rays can be used to recreate the atomic structure in the sample. In TXRD the sample is perturbed and subsequently probed after a certain delay, giving a snapshot of the structure at a given time. Several images can be merged providing a real-time movie of the structural changes. This was achieved with nanosecond time resolution at MAX-lab, when a laser-created liquid was studied. The development of a sub-picosecond, hard X-ray streak camera was one of the prerequisites for many of the studies presented in this thesis. This detector was used to study the acoustic vibrations in InSb nanowires. Oscillations with a period of 30-70 ps were recorded, and were attributed to acoustic phonons in the semiconductor nanowire. A dramatic decrease in the velocity of acoustic waves was also observed in these structures. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Balling, Peter, Department of Physics and Astronomy, Aarhus University, Denmark
organization
publishing date
type
Thesis
publication status
published
subject
keywords
solid-liquid transitions, X-ray diffraction and scattering, nanowires, laser beam impact phenomena, phonons in crystal lattices, Fysicumarkivet A:2012:Jurgilaitis
pages
118 pages
defense location
Lecture hall B, Department of Physics, Sölvegatan 14, Lund University Faculty of Engineering
defense date
2012-03-02 10:15
ISBN
978-91-7473-225-2
language
English
LU publication?
yes
id
de8ad5f7-83c1-471f-8a6d-9d7183471c31 (old id 2335095)
date added to LUP
2012-02-01 16:19:48
date last changed
2016-09-19 08:45:18
@misc{de8ad5f7-83c1-471f-8a6d-9d7183471c31,
  abstract     = {Fast phenomena occurring after laser excitation were studied using time-resolved X-ray diffraction (TXRD). In most experiments, a femtosecond laser pulse was used to excite the sample, and X-rays were used as a probe. The X-ray diffraction technique was used to study bulk semiconductor samples, molten liquids, ferro-electric domain switching in potassium dihydrogen phosphate (KDP), and strain propagation in graphite and semiconductor nanowires. <br/><br>
When a laser pulse is absorbed by a solid, a wide range of phase transitions and phenomena can be induced. If the laser fluence is high enough to melt the material, repetitive illumination will create periodic structures on the surface of the sample. This effect was studied using static X-ray diffraction, and it was shown that the effect is important if liquid scattering experiments are carried out on molten samples using the laser in repetitive mode. When the laser fluence is too low to cause sample melting, coherent acoustic phonons can be excited, and this effect was studied in semiconductor nanowires.<br/><br>
The time resolution of the synchrotron light source is defined by the length of the electron bunch in the storage ring, and is typically 50-300 ps. In order to achieve higher time resolution, short X-ray pulses, such as those at the SLS, or fast detectors, such as the streak cameras available at MAX-lab can be used. <br/><br>
X-ray diffraction is a very sensitive technique for the study of structures, since X-ray photons scatter from all the electrons in the sample. Scattered X-rays can be used to recreate the atomic structure in the sample. In TXRD the sample is perturbed and subsequently probed after a certain delay, giving a snapshot of the structure at a given time. Several images can be merged providing a real-time movie of the structural changes. This was achieved with nanosecond time resolution at MAX-lab, when a laser-created liquid was studied. The development of a sub-picosecond, hard X-ray streak camera was one of the prerequisites for many of the studies presented in this thesis. This detector was used to study the acoustic vibrations in InSb nanowires. Oscillations with a period of 30-70 ps were recorded, and were attributed to acoustic phonons in the semiconductor nanowire. A dramatic decrease in the velocity of acoustic waves was also observed in these structures.},
  author       = {Jurgilaitis, Andrius},
  isbn         = {978-91-7473-225-2},
  keyword      = {solid-liquid transitions,X-ray diffraction and scattering,nanowires,laser beam impact phenomena,phonons in crystal lattices,Fysicumarkivet A:2012:Jurgilaitis},
  language     = {eng},
  pages        = {118},
  title        = {Picosecond X-ray Diffraction Studies of Bulk and Nanostructure Materials},
  year         = {2012},
}