Shouhua Nie

Synchronization of femtosecond laser and electron pulses with subpicosecond precision

The temporal evolution of electron shadow images, formed by the projection of primary femtosecond electron pulses (probe) over a metal target and perturbed by the transient space-charge field near the target surface induced by the excitation of femtosecond optical pulses (pump), is recorded in real time. By quantitatively analyzing the evolution of these shadow images as a function of pump-probe delay times, we are able to synchronize the femtosecond laser and electron pulses with sub-ps precision. This approach is independent of the structural dynamics under investigation and can be applied to a variety of diffraction setups and target materials using laser pulses of different wavelengths.

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.

Electronic Grüneisen parameter and thermal expansion in ferromagnetic transition metal

We report the measurement of the electronic Gruneisen parameter γe of the ferromagnetic transition metal nickel. In this measurement, the electronic thermal expansion was differentiated from other thermal contributions by simultaneously monitoring the laser-induced ultrafast stress and structural dynamics in the time domain using femtosecond electron diffraction. This method overcomes the restriction of traditional low temperature methods and offers a unique path to study electronic thermal expansion in magnetic metals. The result indicates that the local magnetic moment, which persists in the paramagnetic state of nickel, does not significantly contribute to the thermal expansion.We report the measurement of the electronic Gruneisen parameter γe of the ferromagnetic transition metal nickel. In this measurement, the electronic thermal expansion was differentiated from other thermal contributions by simultaneously monitoring the laser-induced ultrafast stress and structural dynamics in the time domain using femtosecond electron diffraction. This method overcomes the restriction of traditional low temperature methods and offers a unique path to study electronic thermal expansion in magnetic metals. The result indicates that the local magnetic moment, which persists in the paramagnetic state of nickel, does not significantly contribute to the thermal expansion.

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

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‐ångström 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 Grüneisen 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. 2009.

Comment on "Nanoscale heat transfer in a thin aluminum film and femtosecond time-resolved electron diffraction" ( Appl. Phys. Lett. 92, 011901 (2008) )

In a recent letter [Appl. Phys. Lett. 92, 011901 (2008)], Tang reported a simulation of structural dynamics in metal films induced by ultrafast laser heating using the two-temperature model [P. B. Allen, Phys. Rev. Lett. 59, 1460 (1987) and R. W. Schoenlein et al., Phys. Rev. Lett. 58, 1680 (1987)] and one-dimensional anharmonic chain model [E. Fermi J. Pasta S. Ulam, No. LA-1940 (1955)]. In this comment, we would like to point out several issues in the physical concepts and formulations in the simulation which we strongly disagree with the author. Consequently, we believe that the main conclusion of Tangs paper that the interpretation of ultrafast diffraction data requires both nonlocal collective atomic motion and the conventional linear thermal expansion lacks physical justification and is questionable.

Electronic Thermal Expansion and the Coherent Acoustic Phonons Generation

We have investigated the thermal expansion dynamics of metal films using femtosecond electron diffraction. We show that the electronic thermal expansion from the transient heating of conduction electrons contributes significantly in driving coherent acoustic phonons.

Femtosecond Electron Diffraction: Probe and Control of Ultrafast Structural Dynamics

Coherent phonon control utilizes classical or quantum interference to manipulate structural responses in crystals. It provides a unique way of driving the lattice into novel and non-equilibrium states not accessible by the conventional means. Previously, this technique has been mostly used with fs optical spectroscopy as an indirect probe of lattice motions. Here we report a direct and real-time probe and control of coherent lattice motions with fs electron diffraction [1].