Unraveling Charge/Orbital Ordering, Magnesium and Lattice Dynamics with Ultrafast Electron Microscopy
The transient response of charge carriers to femtosecond photoexcitation in materials has been vigorously studied, the results of which have provided insights into fundamental ultrafast photoresponse and electronic relaxation dynamics. In contrast, little attention has been paid to the coinciding structural (i.e., atomic and morphological) response, and essentially no experimental work has been aimed at probing how individual defects affect the observed nanoscale dynamics, especially in relation to the large spot sizes typically employed in pump-probe experiments. Here, I will share some of our recent results on femtosecond electron imaging of defect-modulated structural dynamics. I will begin by describing our experimental setup, which consists of an ultrafast transmission electron microscope capable of reaching sub-picosecond timescales and with which ultrafast electron imaging and diffraction can be performed. I will then describe our results on imaging the lattice response of materials to femtosecond photoexcitation; especially the generation, launch, and dispersion of in-plane and c-axis acoustic phonon modes. Additionally, I will discuss our efforts to understand in detail how energy deposited in the lattice via photoexcitation evolves in space and time via multi-mode excitation, coupling, and decay. Finally, I will conclude by sharing some of our recent observations on photoinduced oscillations of individual metal nanocrystals, with apparent oscillatory frequencies matching those measured with transient absorption through plasmon-frequency modulation.
Arnold O. Beckman exemplifies the meaning of the word humanitarian. Combined with his unwavering enthusiasm for life, his keen sense of humor and his strong moral and ethical principles, he is a national icon.