ORCID ID

https://orcid.org/0000-0003-0924-0169

Date Awarded

2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Physics

Advisor

Mumtaz Qazilbash

Committee Member

Henry Krakauer

Committee Member

Dennis Manos

Committee Member

Eugeniy Mikhailov

Committee Member

Robert Pike

Abstract

Emergent phenomena that occur at length scales smaller than approximately half the wavelength of light cannot be resolved by conventional optical techniques due to the Abbe diffraction limit. Scattering-type scanning near-field infrared microscopy (S-SNIM) can circumvent this diffraction limit allowing infrared spectroscopy at nano-scale dimensions independent of the wavelength. Additionally, there is enhanced surface sensitivity resulting from this nanoconfinement of infrared light. S-SNIM is uniquely suitable to study a diverse range of material properties inaccessible by far-field optics in the infrared such as the optical properties of ultrathin films as well as hybrid light matter surface waves called polaritons. Initially, this work describes a broadband infrared plasma light source that has been developed and implemented in our S-SNIM setup to realize broadband S-SNIM in the far- and mid-infrared. This system is then utilized to investigate propagating surface phonon polaritons (SPhPs) in bulk strontium titanate (STO). STO is a perovskite polar dielectric that has a uniquely broad range of the far-infrared in which it can support SPhPs while already having a diverse range of technologically advantageous properties. This work opens the door to envisage STO as a platform for perovskite-based broadband far-infrared and terahertz nano-photonics.Finally, the insulator to metal transition (IMT) in ultrathin vanadium dioxide (VO2) films is investigated. An IMT is an emergent characteristic of quantum materials. When the IMT occurs in materials with interacting electronic and lattice degrees of freedom, it is often difficult to determine if the energy gap in the insulating state is formed by Mott electron-electron correlations or by Peierls charge-density wave (CDW) ordering. To solve this problem, we investigate a representative material, VO2, which exhibits strong electron-electron interactions as well as CDW (Peierls) ordering. Ultrathin VO2 films on rutile (001) TiO2 substrates have been fabricated. These VO2 films undergo the IMT without the CDW (Peierls) ordering. Infrared and optical measurements discover the Mott-Hubbard semiconductor gap of 0.6 eV in the rutile phase below Tc ≈ 306 K. Above Tc, a Drude feature along with an increase in the optical conductivity due to a Mott IMT is observed. These results establish the route to a purely electronic IMT with profound implications for fundamental and applied studies of this phenomenon. Near-field infrared nano-imaging on this VO2 film exhibits a percolative phase transition in the vicinity of Tc, a uniform fully metallic phase above Tc, and a uniformly insulating phase below Tc. Near-field infrared spectroscopy demonstrates an SPhP feature stemming primarily from the TiO2 substrate. This SPhP feature is sensitive to the IMT in the ultrathin VO2 film.

DOI

https://doi.org/10.21220/a58y-kr49

Rights

© The Author

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