Hyuk Park

Direct measurement of transient electric fields induced by ultrafast pulsed laser irradiation of silicon

We use 30 kV electron pulses to probe transient electric fields above silicon surfaces by pump-probe. Electron beam deflection at 0.29 mm away from the sample surface on the order of 10−2 degrees is measured as a function of time delay and used to measure the local electric fields. The measured field strength and direction change with time; at the pump laser fluence of 67.7 mJ/cm2, the maximum field reaches 34 kV/m. We model the transient electric fields based on the propagation of electrons emitted from the Si surface and the percentage of electrons escaping from the surface.

Measurement of femtosecond electron pulse length and the temporal broadening due to space charge

The temporal width of ultrashort electron pulses as a function of beam intensity was measured on the femtosecond time scale with a customized streak camera. The results show that the temporal profile of an electron pulse is Gaussian at low beam intensity and progressively evolves to a top-hat shape due to space charge broadening as the beam intensity increases. The strong correlation between the pulse width and beam intensity observed in our streaking measurements agrees very well with the mean-field calculation and supports the main conclusion of previous theoretical studies that the space charge broadening plays a determinant role.

Comment on "Structural preablation dynamics of graphite observed by ultrafast electron crystallography".

A Comment on the Letter by Fabrizio Carbone et al., Phys. Rev. Lett. 100, 035501 (2008). The authors of the Letter offer a Reply.

In Situ Observation of Pt Icosahedral Nanoparticles Transformation into FCC Single Crystal

Significant progress has been made in spatial resolution using environmental transmission electron microscopes (ETEM), which now enables atomic resolution visualization of structural transformation under variable temperature and gas environments close to materials’ real operational conditions (for a review, see ref [1]). Here we report the observation of transformation of Pt icosahedral nanoparticles into FCC single crystals upon heating in oxygen, at 0.11 nm spatial resolution and 2.5 ms time resolution.

Materials Processes Observed using Dynamical Environmental TEM at University of Illinois

The progress in the development of dynamical environmental TEM (DETEM) at University of Illinois is reported here together with preliminary results that have been obtained in the study of nanostructure transformation and metal oxidation. The instrument development project was motivated by the need to improve time resolution in in-situ TEM study of various materials processes. TEM in general provides outstanding spatial resolution for atomic structure determination and small probes for electronic structure and chemical analysis. The use of imaging aberration corrector in an ETEM now enables atomic resolution visualization of structural transformations under variable temperatures and gas environments close to materials’ real operational conditions (for a review, see ref [1]). However, the types of structural transformations that can be observed are limited by the sample stability and acquisition rate of electron images or diffraction patterns [2]. The development of MEMS based heating holders has improved the sample stability issue. For time resolution, the use of laser driven photocathodes provides superior time resolution by taking snap shots using ultrafast electron pulses [3]. While time resolution at 15 ns has been reported for recording a sequence of irreversible processes using pulsed electron beams [3], the time interval that can be recorded by this technique is currently very limited.