Ultrafast polar vortex dynamics on PTO/STO superlattices: instrument and method development for high-resolution UEM

Abstract

The phase dynamics of topological structures are of interest from both fundamental and applied perspectives. The layer-by-layer superlattice structure of the ferroelectric PbTiO3 (PTO) and the paraelectric SrTiO3 (STO), upon which the precise control over the electrostatic and elastic boundary conditions can be imposed, has been a major model system for the creation and investigation of a variety of novel topological phases. Polar vortices, which exhibit a continuously rotating local polarization state in the PTO layer, are a good example of the delicate balance between the electrostatic, elastic, and gradient energies. However, little is known about the dynamics underlying the functionality of such complex extended nanostructures.In this dissertation, photoexcited phase transition dynamics of the topological polar vortices in the PTO/STO superlattice have been directly imaged on the nanometer-picosecond spatiotemporal scales via ultrafast electron microscopy (UEM). Triggered by the in situ fs laser beam with cross-band photon energy, the nm-scale contrast modulations characteristic of the in-plane propagating vortex-antivortex pairs were spatially resolved to undergo a rapid and substantial intensity decrease, indicating a possible structural response that reverses the vorticity. Compared to results obtained through other ensemble-averaging methods such as MeV-UED and time-resolved XRD, the real-space phase transformation captured by UEM exhibits an asymmetric character at low pump laser fluence, with non-uniform melting of the vortex topology observed among domains of various orientations and sizes. Moreover, as the optical fluence increased, the asymmetric melting behavior was recovered back to a nearly symmetric fashion, potentially indicating a local variation in the chemical potential related to the in-plane nano-scale domain structures. This finding implies a degree of freedom that has not been well utilized in the creation and stabilization of novel topological phases. The direct observation of polar vortices’ structural features via the pulsed photoelectron packets of a relatively low repetition rate was facilitated by the instrumentation and methodology development implemented in our system. Such effort is also demonstrated in this thesis. An optimization on the pump-probe laser system was performed to enhance its controllability, stability, and efficiency. A quantitative pump beam metrology was established for an in situ and accurate determination of the laser propagation parameters at the specimen position. A systematic investigation was also performed on the time-zero shift originating from the Wehnelt emission, an alternative photoelectron source that has been discovered to possess much better stability in terms of the signal intensity and, thus, be more ideal for experiments requesting extra-long exposure time, e.g., the high-resolution UEM imaging of polar vortices. These development efforts are believed not only to fertilize the breakthrough in capturing the local phase transformation, but also to pave the way for surveying the atomic-scale structural dynamics via the laser-based UEM.