Doctoral theses
Current Record: 19 of 171
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Identifier |
000440901 |
Title |
Study of electronic phenomena in strongly correlated systems by use of ultrashort laser pulses |
Alternative Title |
Μελέτη ηλεκτρονιακών φαινομένων σε ισχυρώς συσχετιζόμενα συστήματα με τη χρήση υπερβραχέων παλμών λέιζερ |
Author
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Καρανικολόπουλος, Δημήτρης Θ.
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Thesis advisor
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Χαραλαμπίδης, Δημήτριος
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Reviewer
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Λουκάκος, Παναγιώτης
Λάππας, Αλέξανδρος
Ρακιτζής, Πέτρος
Κομίνης, Ιωάννης
Ηλιόπουλος, Ελευθέριος
Κουδουμάς, Εμμανουήλ
|
Abstract |
The main focus of this thesis is the study of vanadium dioxide (VO2), a
strongly correlated transition metal oxide and how its optical properties are
affected by doping. The particular material has attracted much attention because of its near room temperature critical transition threshold of ∼68, which
is very promising for use in applications. The transition can also be induced by
photoexcitation We want to assess the possibility of use as an optical switch
in the visible regime. Two films are studied, one of VO2 and a magnesium
doped variant, in order to examine changes induced by the dopants in the optical response of the system. The dynamical optical response of the materials
was accessed by time-resolved optical spectroscopy using ultrashort, fs, laser
pulses.
Magnesium substitution at the sites distorts the unit cell of both phases, diminishing their differences and thus facilitating the transition. We study the
dynamical response in reflection and transmission of the VO2 films using excitation fluence values of 0.3, 1.0 and 2.6mJ/cm2
, in an effort to capture the
material response in both phases. The films show major differences in their
response. The high fluence measurements show a decrease in reflectivity for
both films, but, a plateau value is reached within the probed time window
of 5ps in the case of the doped film. This hints at the doped system having
completed the phase transition faster. This behavior is more pronounced in
transmittivity of the films where the doped film appears to quickly evolve towards a plateau. This fast, quicker than 2ps, response, in conjunction with a
non-linear change in transmittivity, appeals to applications necessitating for
ultrafast optical switching.
In addition, studies were performed on 3D photonic crystals of the woodpile
architecture, enhanced with cadmium sulfide quantum dots, in an effort to
investigate how additives affect their photonic-band-gap. These hybrid materials have photonic stop bands in the visible spectral region. Time-resolved
transmittance measurements were recorded. Upon low pump excitation a drop
in optical density is observed for a wide wavelength range below the band gap
edge. The transmittance from the woodpile structure increases. This response
is attributed to nonlinear absorption. On the other hand, excitation with high
pump intensities shows a positive change in optical density denoting a drop in
transmittance. A reduction in transmittance, implies fundamental changes in
the system’s response due to the pump. These changes could possibly originate
from a pump-induced shift of the photonic-band-gap. This ultrafast dynamical
response also hold promise for optical switching in the visible regime.
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Language |
English |
Subject |
Laser spectroscopy |
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Solid state |
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Vanadium dioxide |
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Διοξείδιο του Βαναδίου |
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Στερεά κατάσταση |
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Φασματοσκοπία λέιζερ |
Issue date |
2021-07-12 |
Collection
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School/Department--School of Sciences and Engineering--Department of Physics--Doctoral theses
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Type of Work--Doctoral theses
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Permanent Link |
https://elocus.lib.uoc.gr//dlib/0/c/4/metadata-dlib-1624872995-730944-5962.tkl
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Views |
660 |