The Emergent Monoclinic Phase in KNbO3 Discovered Using Optical Second Harmonic Generation
Open Access
- Author:
- Leung, Jessica
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- December 07, 2012
- Committee Members:
- Venkatraman Gopalan, Thesis Advisor/Co-Advisor
- Keywords:
- potassium niobate
second harmonic generation
monoclinic
orthorhombic
ferroelectric
SHG
KNbO3
nonlinear optical - Abstract:
- Potassium Niobate (KNbO3) is a ferroelectric material with a perovskite structure that exhibits the same types of phase transitions as BaTiO3. At room temperature, the structure is orthorhombic, with the spontaneous polarization along the direction. Optical Second Harmonic Generation (SHG) microscopy is used to study the domain structures of KNbO3 single crystals at room temperature. In this experiment, a monoclinic phase of KNbO3, which emits a higher SHG signal than that of the orthorhombic phases, was also detected. The monoclinic phase, which results from a small structural distortion of the orthorhombic phase, has enhanced piezoelectric properties that can be useful in the applications of lead-free ferroelectric materials. In this study, SHG polarimetry was used to study the second harmonic responses of orthorhombic and monoclinic domains in KNbO3, therefore the SHG experiment is described in detail. Through the theoretical modeling of these responses, one can determine the types of domains that were probed. The ratios of the nonlinear optical coefficients were extracted from the theoretical fits for the orthorhombic and monoclinic domains. In addition, the existence of the monoclinic phase was verified with nano-scale scanning X-ray diffraction microscopy, where the relative 2θ center-or-masses of the orthorhombic and monoclinic regions were determined to be ~0.30 and ~0.65 mrad, respectively.