Date of Graduation

9-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Bellaiche, Laurent

Committee Member

Charles Paillard

Second Committee Member

Hiro Nakamura

Third Committee Member

Surendra Singh

Fourth Committee Member

William Oliver III

Abstract

This dissertation presents a comprehensive first-principles investigation into the interaction of light with ferroelectric materials, focusing on light-induced structural responses (photostriction) and phase transitions in Ferroelectric superlattices, epitaxially strained BiFeO3 thin films, and twodimensional NbOX2 (X = Cl, Br, I) ferroelectrics. The work aims to provide microscopic insights into how light can control the lattice structure and even the crystallographic phase in these systems, with the broader goal of informing the design of light-responsive functional materials for practical technological applications. The first part explores light-induced effects in PbTiO3/SrTiO3 superlattices with different polarization orientations. Our simulations reveal that photoexcitation modifies the internal electric fields and structural distortions, enabling light-driven control of the superlattice properties and inducing a polarization rotation in the superlattice. These results offer a theoretical foundation that supports and complements existing experimental observations. The second and third parts examine how photostriction in PbTiO3/SrTiO3 depends on structural parameters—specifically, thickness and chemical composition. In ultrathin superlattices, free carriers are delocalized and suppress electric dipoles, resulting in contraction. In thicker superlattices, free carriers are strongly localized at the interfaces, inducing internal fields that suppress existing depolarizing/polarizing fields in PbTiO3 and SrTiO3 layers and produce opposite strain responses in PbTiO3 and SrTiO3 layers, leading to net expansion or contraction. The balance between these competing responses can be engineered by tuning the PbTiO3 fraction, enabling thickness- and composition-dependent control of photostrictive behavior. The fourth part investigates the enhancement of photostriction in BiFeO3 thin films under varying epitaxial strains. We demonstrate that strained BiFeO3 phases near phase boundaries exhibit large light-induced strains, far exceeding those in bulk BiFeO3. This enhancement arises from the coupling between epitaxial strain and the photoexcitation-driven lattice response. The final part focuses on two-dimensional NbOX2 (X = Cl, Br, I) materials. We discover that light can induce phase transitions from ferroelectric or antiferroelectric to paraelectric states, along with significant photostrictive effects. These results highlight the potential of 2D ferroelectrics for next-generation optoelectronic devices. Altogether, this dissertation advances the understanding of light–matter interaction in ferroic systems and establishes practical guidelines for tailoring photostrictive responses through structural and chemical design.

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