Probing Ultrafast Dynamics of Polarization Clusters in BaTiO₃ by Pulsed Soft X-Ray Laser Speckle Technique

Abstract

The technical development of ultrashort laser pulses covering infrared to extreme ultraviolet has opened a door to study the photo-induced dynamic physical and chemical processes. By using an optical excitation at a particular wavelength, exotic states can be trigged coherently in atoms, molecules, clusters as well as complex condensed systems. The evolution of excited states then can be imaged in the real-time domain by subsequent single or trains of pulse. This laser pump-probe technique features an unprecedented spatial and temporal resolution, thus not only allows us a fundamental insight into the microscopic ultrafast dynamics, but also brings about a potential of selective controlling on the microstructures at atomic scale (Krausz & Ivanov, 2009). In this chapter, a recent advance of soft x-ray laser speckle pump-probe measurement on barium titanate (BaTiO3) is reviewed, with primary concerns on the theoretical description of the photo-matter interaction and photo-induced relaxation dynamics in the crystal. Here, the observed time-dependent speckle pattern is theoretically investigated as a correlated optical response to the pump and probe pulses. The scattering probability is calculated based on a model with coupled soft x-ray photon and ferroelectric phonon mode of BaTiO3. It is found that the speckle variation is related with the relaxation dynamics of ferroelectric clusters created by the pump pulse. Additionally, a critical slowing down of cluster relaxation arises on decreasing temperature towards the paraelectric-ferroelectric transition temperature. Relation between the critical slowing down, local dipole fluctuation and crystal structure are revealed by a quantum Monte Carlo simulation. This chapter is organized as follows. In Section 1, the properties ferroelectric material and experimental techniques are introduced. The theoretical model and methods are elaborated in Section 2. In Section 3, the numerical results on speckle correlation, relaxation dynamics