Domain Engineering via Ion-Exchange in a Ferroelectric Ionic Conductor

QC 2024-11-13

Abstract

Rb-doped KTiOPO₄ (RKTP), a ferroelectric ionic conductor is a promising nonlinear optical material for engineering periodic ferroelectric domains for nonlinear optical frequency conversion via quasi-phase-matching (QPM) technique. The performance of the QPM device relies on the quality of the engineered periodic domains. One of the most reliable methods to achieve high aspect-ratio domains is based on using ion-exchange (IE) to engineer the coercive field (E_c) in RKTP. Recently, this technique has allowed demonstration of periods in the sub-µm range, a challenging feat to achieve with conventional metal electrode periodic poling. Despite its promising potential, the study of E_c engineering is still in its infancy stage. A deeper understanding of the interaction between IE and polarization switching dynamics is crucial for fully exploiting this technique. This thesis explores the engineering of E_c in RKTP through IE processes for ferroelectric domain engineering. It focuses on two primary IE methods in RKTP using: monovalent rubidium (Rb⁺) and divalent barium (Ba²⁺) ions. These two ion species, with different charge numbers and ionic radii, play distinctive roles in enhancing the E_c in RKTP through different mechanisms. 

By comparing the switching time of Rb-exchanged and Ba-exchanged samples — where a higher switching time corresponds to a higher E_c — the induced E_c from these two types of IE can be compared systematically. Their distinct difference in polarization switching properties calls for the use of different techniques to effectively probe and analyze these two types of exchanges. Raman spectroscopy is employed to examine the microstructural changes induced by Rb-exchange in RKTP crystals, revealing that in diffused Rb⁺ has impact on the octahedral TiO₆ and tetrahedral PO₄ groups. Additionally, time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements are conducted under in-situ heating conditions to understand the dynamics of IE process. Although ToF-SIMS measurements show some of the fundamental differences between the two IE processes, in order to further understand the behavior of Ba-exchange, band excitation piezoresponse force microscopy (BE-PFM) is employed to probe polarization switching properties in Ba-exchanged samples at the nanoscale. 

Using Rb-exchange, we achieve periodic poling with 3.43 μm QPM period in a 3 mm thick periodically-poled RKTP crystal, marking the shortest period attained for an aperture larger than the standard 1 mm thick crystal. Additionally, by integrating Ba-exchange for periodic poling with a post-poling Rb-exchange process, we have preliminary developed waveguide device through independent engineering of both the coercive field and refractive index on a single platform. These devices showcase the versatility of these ion-exchange techniques and push the boundaries of current QPM devices.

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