Implications for Ultrafast Reflection Electron Diffraction from Temporal and Spatial Evolution of Transient Electric Fie
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Implications for Ultrafast Reflection Electron Diffraction from Temporal and Spatial Evolution of Transient Electric Fields Hyuk Park1,2 and Jian-Min Zuo1,2 Department of Materials Science and Engineering, University of Illinois, Urbana-Champaign, Urbana, IL 61801 USA 2 Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana-Champaign, Urbana, IL 61801 USA
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ABSTRACT Understanding interaction of ultrafast pulsed laser with matter is critical for probing ultrafast processes in materials science, understanding the physics of laser ablation and the laser induced non-equilibrium carrier dynamics in metals and semiconductors, including plasmonics. When an intense laser pulse of femtoseconds (fs) in duration hits the surface of a targeted matter, it excites a hot electron gas. Part of the hot electrons is emitted from the surface in a way similar to thermionic emission. Electrons can also be emitted through multiphoton photoemission (MPPE) or thermally assisted MPPE. The emitted electrons travel at speeds that create transient electric fields (TEFs). To detect TEFs and study the dynamics of emitted electrons, we have developed a time resolved electron beam imaging technique that allows us to measure TEFs above a sample surface at picoseconds time resolution. We have also developed a model of the TEFs based on the propagation of emitted electrons and the percentage of electrons escaping from the surface. We examine the significance of TEFs for ultrafast reflection electron diffraction by examining anomalous effects in ultrafast reflection high energy electron diffraction (RHEED) of silicon surfaces. INTRODUCTION Reflection high-energy electron diffraction (RHEED) is a powerful technique for examining the surface structures of a substrate and a supported crystal since it is sensitive to surface structure to the monolayer level[1]. Separate developments in ultrafast lasers and photocathode have led to the development of time-resolved electron diffraction with temporal resolution approaching tenths of picoseconds (ps)[2-9]. Combining RHEED with ultrafast, time resolved, electron diffraction techniques enables us to study the surface dynamics at the atomic time and length scales[10]. Recently, a number of publications, including in high profile journals, reported large lattice contraction and expansion and phase transitions in several materials on the time scale of few picoseconds (ps) to tens or hundreds of ps (see Table 1 for a summary) using ultrafast RHEED[11-13]. For example, Yang, Lao and Zewail reported colossal expansions of Zinc Oxide nanowires at two orders of magnitude higher than the expected thermal expansion by electron diffraction [14]. The change in the crystal lattice in the c-axis direction was measured from the Bragg peak positions of (00l) reflections, which should be consistent with the reciprocal lattice of ZnO: r r ∆c ( t ) r r 1 1 (1) G (t ) − G (0) = l [ c *(t ) − c * (0) ] cˆ = l − ≈ −l cˆ c + ∆ c t c c ( )
A comparison of the difference intensity m
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