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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

Καρανικολόπουλος, Δημήτρης Θ.

Thesis advisor

Χαραλαμπίδης, Δημήτριος

Reviewer

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.

Language

English

Subject

Laser spectroscopy

Solid state

Vanadium dioxide

Διοξείδιο του Βαναδίου

Στερεά κατάσταση

Φασματοσκοπία λέιζερ

Issue date

2021-07-12

Collection