Femtosecond electron diffraction: Direct probe of ultrafast structural dynamics in metal films

Abstract

Femtosecond electron diffraction is a rapidly advancing technique that holds a great promise for studying ultrafast structural dynamics in phase transitions, chemical reactions, and function of biological molecules at the atomic time and length scales. In this paper, we summarize our development of a tabletop femtosecond electron diffractometer. Using a delicate instrument design and careful experimental configurations, we demonstrate the unprecedented capability of detecting submilli-å ngström lattice spacing change on a subpicosecond timescale with this new technique. We have conducted an in-depth investigation of ultrafast coherent phonon dynamics induced by an impulsive optical excitation in thin-film metals. By probing both coherent acoustic and random thermal lattice motions simultaneously in real time, we have provided the first and unambiguous experimental evidence that the pressure of hot electrons contributes significantly to the generation of coherent acoustic phonons under nonequilibrium conditions when electrons and phonons are not thermalized. Based on these observations, we also propose an innovative approach to measure the electronic Grü neisen parameter in magnetic materials at and above room temperature, which provides a way to gain new insights into electronic thermal expansion in ferromagnetic transition metals. Microsc. Res. Tech. 72:131-143, 2009. V

Spatio-temporal resolution studies on a highly compact ultrafast electron diffractometer

“…After sample transmission, a liquid cooled magnetic lens with iron pole-pieces separated by a 1 mm vacuum gap maps the diffraction pattern on the detector system. Compared to dc electron sources with a condenser solenoid in front of the sample [15,[24][25][26][27][28][29], we place the magnetic lens afterwards as an object lens and focus the full diffraction pattern on the detector [17]. This results in an extraordinary short propagation distance from the electron source to the sample of 8.5 mm, a trifold improvement to the so far most compact source design [26,48].…”

“…In UED experiments, ultrashort electron pulses are generally created by laser-induced emission. Mainly three different source concepts can be distinguished [5]: Photoelectric emission from planar metal surfaces [7,15,[24][25][26][27][28][29][30][31][32], emission from sharp metal tips [33][34][35][36][37] and photo-ionization of ultracold gases [38][39][40][41][42]. Emitted electrons are accelerated either by electrostatic fields to lower energies of 0.2-1.5 keV [33][34][35][36] as well as high energies of 30-95 keV [15, 16, 24-32, 37, 42, 43] or with RF photo-injectors up to [3][4][5] MeV [44][45][46][47].…”

“…However, transient diffraction data with an adequate signal-to-noise ratio can only be obtained by detecting 10 6 -10 8 electrons at each pump-probe delay within a maintainable measurement time [2][3][4][5]43]. Depending on the source concept, different approaches are thus pursued; ranging from the single-electron regime in combination with high repetition rates [5,[35][36][37]51], over the application of multi-electron pulses at a lower number of pump-probe cycles [15,16,[24][25][26][27][28][29]42] to single-shot experiments with compressed dense electron bunches [22,30,32,[43][44][45][46][47]50]. In this connection the irreversible or reversible nature of the investigated dynamics additionally determines the method of choice [2,3,5].…”

Direct and real time probe of photoinduced structure transition in colossal magnetoresistive material

“…Before being put into the ultrahigh vacuum chamber to study the photoinduced phase transition, the sample was carefully examined by TEM to ensure high quality and uniform microscopic structure across a large area of a few hundred microns. The overall FED setup is similar to our earlier experiments 21 Author to whom correspondence should be addressed. Electronic mail: jcao@magnet.fsu.edu in Fig.…”

Bandgap modulation in photoexcited topological insulator Bi2Te3 via atomic displacements

“…5(a) and 5(b) are the products of three contributions: Debye-Waller effects, acoustic phonons, and atomic displacements within the unit cell. The intensity of the electron diffraction (I) can be expressed via the following equation: [40][41][42] …”