Domain and Ultraviolet Engineered Device Development of Potassium Tantalate Niobate Single Crystal Electro Optic Modulators and Deflectors
Open Access
- Author:
- Shang, Annan
- Graduate Program:
- Electrical Engineering (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 08, 2023
- Committee Members:
- Madhavan Swaminathan, Program Head/Chair
Susan Trolier-McKinstry, Major Field Member
Jian hsu, Outside Unit & Field Member
Victor Pasko, Major Field Member
Shizhuo Yin, Chair & Dissertation Advisor - Keywords:
- KTN
PNR
polar nano regions
permittivity
interferometry
electrostriction
potassium tantalate niobate
electro optic
polarization
domain
beam deflector
beam modulation
niobate
ferroelectrics
relaxor ferroelectrics
dielectrics - Abstract:
- Electro optic (EO) based systems are commonly used in the areas of high-speed sensing, 3D printing, and imaging; and lead to the development of devices such as high-speed beam deflectors and modulators. This material characteristic also has cross-dependent terms, like permittivity and electrostriction (ES), that make them proportional to other crystal properties and determine the macroscopic state of many crystals. Detailed analyses of these cross-dependent term can both improve our material comprehension and expand our approach towards the advancement of these systems. This dissertation examines the experimental utilization of an externally applied strain, thermal condition and light illumination to optimize or improve the relative permittivity within potassium tantalate niobate ( , KTN) based devices. The KTN based devices within this dissertation focus on those whose composition result in a paraelectric (PE) phase transition temperature near room temperature, a composition, xcomp, of 0.395-0.404). Their strong electro optic properties near this composition are the major motivator for many advancements in KTN based and related devices. However, its temperature dependency, frequency dependency and perovskite relaxor ferroelectric nature often inhibit the functionality of their applications. Investigations conducted within this dissertation confirm that rapid cooling can create both a large EO and ES effect that affects its properties above and below its phase transition temperature. It is observed that a high cooling rate of 0.75 ⁰C/s results in a 2X increase in the relative permittivity of the KTN crystal. Rapid cooling creates quenched and reoriented polar nano-regions (PNRs) due to the nano-disordered property of KTN crystals; and increasing this cooling rate promotes the enhancement of this nano-disordered property which results in enhanced EO and ES properties. As the operation temperature of KTN based devices are above and near its phase transition temperature and the applied electric field is large, the crystal undergoes an electric field induced phase transition that inhibits its operating speed and deflection range. To nullify this property and resolve the functional complexity of an electric field induced transparent ferroelectric state, a thermally controlled, domain engineering (DE) recipe is developed to enable a transparent ferroelectric state whose functionality is not limited by an electric field induced phase transition and displays a 5X increase in its linear EO coefficient compared to conventional (non-DE) KTN crystals of similar composition with no loss in modulation speed. The transparent ferroelectric state (DE-KTN) is confirmed by observing the Raman spectra, electrical hysteresis, polarized light microscopy and beam profile data of both the DE and non-DE ferroelectric KTN crystal. The result of this state may be due to the two-step thermal annealing process that combines the creation of PNRs upon the rapid cooling first step, and the irregular, diffuse boundaries and high anisotropic traits of these PNRs that enable an abnormal domain growth-like process under the slow cooling second step. The paraelectric to ferroelectric phase transition of KTN crystals decrease the permittivity from 15,000 to 3,000. Although the modulation and deflection speed should increase due to the decrease in permittivity, it is unclear how this will affect the deflection characteristics of the crystal at temperatures below its phase transition. In response, the deflection properties of transparent ferroelectric KTN crystals are analyzed to explore their potential as a megahertz EO deflector. These have shown a 10X increase in deflection speed and a 2X increase in deflection angle in comparison to its paraelectric equivalent. The physical mechanism behind this may involve both the optimization of permittivity and injected space charge, as well as the influence of piezoelectricity under a transparent ferroelectric state, but this may require further investigation. Even with these qualities, beam profile deformation can arise due to the deflection angle mid-point position, the deflection angle amplitude and laser pulse fluence. In particular, the laser pulse fluence has made high quality beam deflection of ultrashort laser pulses impractical. To solve this, it is demonstrated that UV illumination can eliminate the beam deformation affects that arise from fs pulsed laser deflection and improve the modulation switching speed and ratio of a pulsed energy source. This is revealed to be due to the UV light ability to increase conductivity within KTN crystal. This results in an increased flow of free charge within the crystal which decreases the depolarization field due to a pulsed energy source and limits the amount of trapped charges. This can result in the use of UV to reveal other traits and applications for KTN based devices that leads to the discussion of future works that define the next steps toward optimizing KTN based devices, such as sub-nano- to pico-second switching in ultrathin KTN devices using a UV-fs pulsed lasing source.