Ultrafast Electron Crystallography: Principles and Applications

Abstract

During the last 20 to 30 years, the development and application of time-resolved experimental techniques with a femtosecond temporal resolution have brought to us much knowledge about the fundamental processes in physics, chemistry and biology. Nevertheless, standard spectroscopic methods have their limitation in the determination of the transient structures during ultrafast dynamics at the atomic level, because the spatial resolution is restricted by the wavelength of the probe pulse used. In contract, with the scheme of femtosecond optical initiation and electron probing and through the diffraction phenomenon, ultrafast electron crystallography (UEC) was recently developed as a time-resolved structure-probing technique for condensed-matter studies. The short wavelength and small pulse duration of the highly accelerated electrons used provide the atomic-scale spatiotemporal resolution. In addition, the large electron–matter interaction enables the detection of small transient changes as well as the investigation of surface and interfacial phenomena. This thesis describes the principles of UEC and its applications to a variety of systems, ranging from nanometer-scale structures to highly correlated materials and to interfacial assemblies. By using a prototype semiconducting material, we elucidated the fundamental processes at work in different parts of the femtosecond-to-nanosecond time range; this investigation led to a conceptual change from the consideration of laser-induced heating to the examination of nonequilibrium structural modifications as a result of the transient dynamical changes in, e.g., carriers, the crystal potential, and phonons. On the basis of such an understanding, we observed and understood the colossal unidirectional expansion induced by the photoexcitation of nanostructures to be a potential-driven result rather than a thermal one. For highly correlated materials, we showed the effectiveness of UEC in resolving the transient intermediate structures during phase transformations as well as identifying new phases in the nonequilibrium state. An important breakthrough made by UEC was the confirmation of the anisotropic involvement of lattice in the electron pairing mechanism for high-temperature superconductors. In interfacial assemblies, we also found a nonequilibrium phase transformation in water and the phenomenon of ultrafast annealing for a better order in a self-assembled monolayer. With these successful experiences, we expect more condensed-matter studies by UEC to come.