Ultrafast Electron Crystallography. 1. Nonequilibrium Dynamics of Nanometer-Scale Structures
In this paper, the first in a series, we give a full account of the development of ultrafast electron crystallography (UEC) and its applications in the study of nanometer-scale semiconductors and their quantum-well heterostructures. With the atomic-scale spatial and ultrafast temporal resolution of UEC, we examine four features of diffraction: Bragg spot movements, intensity changes, width changes, and the degree of inhomogeneity, as demonstrated here for gallium arsenide. Following the ultrafast heating made using femtosecond pulses, we observe a universal behavior of diffraction changes, with amplitudes far exceeding those of thermal heating (in a time of ∼10 ps), and we can monitor such lattice change with an accuracy of ∼0.001 Å. This nonequilibrium state of structures at ultrashort times, and the subsequent restructuring at longer times, is generally observed in other materials. The physical picture for such structural dynamics is described here with emphasis on the motion of atoms under the influence of optical and acoustic vibrations, the potential anisotropy of the lattice, which results in weakening of bonding, and the diffusion at much longer times. The experimental results are given in details for the different conditions of fluence, energy, and electron probing angle, and we also provide the theoretical foundation needed. The ability to probe structures and dynamics of such nanometer-scale materials should be applicable to other studies of surfaces and interfaces, and in the accompanying paper, we detail such studies for adsorbates of fatty acids and phospholipids.