Coercive field (Ec) engineering based upon the monovalent Rb+ ion (Rb/K/Ba-exchange) and the divalent Ba2+ ion (Ba/K-exchange) has enabled the reliable periodic poling of RbKTiOPO4 (RKTP) for quasi-phase-matching (QPM). Previously, there have been no systematic studies to understand and compare the changes in polarization-switching properties induced by these two families of exchanges. In this paper, we compare different compositions of Rb/K/Ba- and Ba/K-exchanges in terms of how they affect the polarization-switching time, ts, and ionic conductivity in RKTP. We discuss the change in switching time, ts, that is ascribed to the interplay between the monovalent and divalent cations in the exchange. Moreover, we propose exchange-induced strain as the cause of bulk phase-matching shift and show that exchanges containing lower amounts of Rb induce less strain. This is corroborated by strain-deformation measurements on the exchanged samples. Finally, we demonstrate highly efficient periodically poled RKTP crystals free of bulk changes, using two different high ts - low strain exchanges.

For ferroelectric ionic-conductors, polarization switching is complicated by the interplay between ion mobility and charge screening effects. When the ionic charge carriers also play a key role in the domain reversal, such as in Rb-doped KTiOPO4 (RKTP), a higher level of complexity is introduced. RKTP provides an ideal platform for investigating the relationship between ionic conductivity and polarization reversal because its highly anisotropic crystal properties allow selective modification of material characteristics through diffusive cation doping. Here, we use indiffused Ba/K doping to create a significant increase in the ionic conductivity. Time-of-flight secondary ion mass spectrometry is employed to map Ba/K doping within the RKTP crystal and correlate it to changes in ionic mobility and polarization switching characteristics under an external field applied to the nonpolar face. Using band-excitation piezoresponse force microscopy, we demonstrate a selective switching-inhibition mechanism driven by the enhanced charge screening.

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.

Rb-dopad KTiOPO₄, ett ferroelektriskt jonledare är ett lovande ickelinjärt optiskt material för att konstruera periodiska ferroelektriska domäner för frekvenskonvertering via kvasi-fasmatchningsteknik (QPM). Prestandan hos QPM-komponenten är beroende av kvaliteten på de konstruerade periodiska domänerna och nyligen rapporterades det som en av de mest pålitliga metoderna för att uppnå domäner med hög bildförhållande. Det är baserat på att använda jonutbyte för att modifiera koercitivfältet för RKTP. Denna nya metod har tillåtit demonstration av periodiska domäner i sub-µm-området, vilket möjliggör många intressanta ickelinjära optiska fenomen som ännu inte har demonstrerats med andra ferroelektriska material. I början av detta arbete var den underliggande fysiken för koercitivfält dåligt förstådd.  En djupare förståelse för interaktionen mellan jonbyte och polningsdynamik är avgörande för att fullt ut kunna utnyttja denna teknik för konstruktion av nya QPM-komponenter. Denna avhandling undersöker tekniker för kontroll av koercitivfältet genom jonbytesprocesser för ferroelektrisk domänteknik. Den studerar två primära jonbytesmetoder i RKTP: monovalent rubidium (Rb⁺) och tvåvärt barium (Ba²⁺) joner. Dessa två jonarter, med olika laddningsnummer och jonradier, spelar distinkta roller för att förstärka koercitivfältet i RKTP genom olika mekanismer. Olika tekniker används för att utföra dessa studier.

Växlingen av polarisationen karaktäriseras i bulk hos Rb- och Ba-utbytt RKTP. Genom att undersöka switchtiden kan det inducerade koercitivfältet från dessa två typer av jonutbyte jämföras systematiskt, där en högre switchtid motsvarar ett högre koercitivfält. Ramanspektroskopi används för att undersöka de mikrostrukturella förändringarna som induceras av Rb-utbytet i RKTP-kristaller. Denna teknik avslöjar att indiffuserat Rb⁺ har inverkan på de oktaedriska TiO₆- och tetraedriska PO₄-grupperna. Dessutom utförs time-of-flight sekundär jonmasspektrometri (ToF-SIMS) mätningar under in-situ uppvärmningsförhållanden för att förstå dynamiken i jonbytesprocessen.  För att ytterligare förstå beteendet hos Ba-exchange, används bandexcitation piezoresponse force microscopy (BE-PFM) för att undersöka egenskaper hos växlingen av polarisationen i Ba-utbytta prover på nanoskala. Mätningarna avslöjar att Ba-utbytet ökar den lokala jonkonduktiviteten, vilket effektivt screenar polarisationsväxling och gör domänkärnbildning mer energiskt kostsam. Från jämförelsen och observationerna från Raman och BE-PFM blir det uppenbart att mekanismerna för att inducera koercitivfältet av dessa två typer av joner är mycket olika.

Båda teknikerna används för att demonstrera högeffektiva QPM-komponenter. Genom att kombinera Ba-utbyte för periodisk polning, och en post-poling Rb-utbytesprocess, visar vi vågledare med oberoende kontroll av koercitivfält och brytningsindex. 

We investigate a new method of coercive field engineering for periodic poling of RbKTiOPO4 (RKTP). By ion exchanging RKTP in a molten salt containing 7 mol% Ba(NO3)2 and 93 mol% KNO3 we achieve more than an order of magnitude difference in polarization switching time between the exchanged and non-exchanged regions. This method is used to fabricate periodic gratings of 2.92 µm in 1 mm thick bulk RKTP for second harmonic generation at 779 nm with a normalized conversion efficiency of 2%/Wcm. We show that the poled domain structures are stable at 300 °C, and that there is no bulk refractive index modification associated with the periodic ion exchange.

Recently, ion exchange (IE) has been used to periodically modify the coercive field (Ec) of the crystal prior to periodic poling, to fabricate fine-pitch domain structures in Rb-doped KTiOPO4 (RKTP). Here, we use micro- Raman spectroscopy to understand the impact of IE on the vibrational modes related to the Rb/K lattice sites, TiO6 octahedra, and PO4 tetrahedra, which all form the basis of the RKTP crystal structure. We analyze the Raman spectra of three different RKTP samples: (1) a RKTP sample that shows a poled domain grating only, (2) a RKTP sample that has an E c grating only, and (3) a RKTP sample that has both an E c and a domain grating of the nominally same spacing. This allows us to determine the impact of IE on the vibrational modes of RKTP. We characterize the changes in the lower Raman peaks related to the alkali-metal ions, as well as observe lattice modifications induced by the incorporation of Rb+ that extend further into the crystal bulk than the expected IE depth. Moreover, the influence of IE on the domain walls is also manifested in their Raman peak shift. We discuss our results in terms of the deformation of the PO4 and TiO6 groups. Our results highlight the intricate impact of IE on the crystal structure and how it facilitates periodic poling, paving the way for further development of the E c-engineering technique.

We demonstrate a new method to fabricate waveguides in KTP. It allows for independently fabrication of the periodically poled grating via coercive field engineering and post-poling waveguide inscription via ion exchange.

We demonstrate a new method to fabricate waveguides in KTP. It allows for independently fabrication of the periodically poled grating via coercive field engineering and post-poling waveguide inscription via ion exchange.

Rapid manufacturing of high purity fused silica glass micro-optics using a filament-based glass 3D printer has been demonstrated. A multilayer 5 x 5 microlens array was printed and subsequently characterized, showing fully dense lenses with uniform focal lengths and good imaging performance. A surface roughness on the order of R-a = 0.12 nm was achieved. Printing time for each lens was <10 s. Creating arrays with multifocal imaging capabilities was possible by individually varying the number of printed layers and radius for each lens, effectively changing the lens height and curvature. Glass 3D printing is shown in this study to be a versatile approach for fabricating silica micro-optics suitable for rapid prototyping or manufacturing.

We demonstrate periodic poling of large aperture Rb-doped KTP (RKTP) crystals with a QPM period of 3.43 µm via coercive field engineering. The periodically-poled RKTP has excellent pattern-fidelity, with a second harmonic generation (SHG) conversion efficiency of 1.4%/Wcm at 405 nm. We use ion-exchange (IE) to fabricate grating of high-and-low coercive fields and establish that the depth of the in-diffused Rb+ is the main parameter to achieve sufficiently large coercive field contrast that enables uniform periodic poling in the thick crystal. Furthermore, we demonstrate that our robust coercive field grating allows multiple cycles of polarization-switching which can be employed to decrease the refractive-index change along the polar axis, induced during the IE process.