Ultrafast Electron Diffraction with Solid-State Samples
Thesis Defense
1:00 pm –
3:00 pm
Jorgensen Hall Room: 207
Target Audiences:
Contact:
Physics Department, (402) 472-2770, paoffice2@unl.edu
Yibo Wang will present his defense topic, “Ultrafast Electron Diffraction with Solid-State Samples” in person.
Abstract: We report the modification of an ultrafast electron diffraction (UED) instrument that enables experiments with both gas and condensed matter targets. The instrument relies on a hybrid DC-RF gun to deliver femtosecond electron pulses on the target, which are synchronized with femtosecond laser pulses. The laser pulses and electron pulses are used to excite the sample and to probe the structural dynamics, respectively. The new system is added with capabilities to perform transmission ultrafast electron diffraction on thin solid samples. It allows cooling samples to cryogenic temperatures and performs time-resolved measurements. Apart from the hardware improvements, the methodology and techniques required for a successful UED experiment, such as sample preparation, alignment determination, and data analysis, are fully explored.
We apply the adapted instrument to investigate the ultrafast response of ferroelectric NbOI2 to the optical stimuli. NbOI2 is a low-symmetry 2D van der Waals ferroelectric material with large intrinsic polarization and piezoelectric coefficients. In this study, the NbOI2 samples were excited with a femtosecond UV pulse, and the transient non-thermal states of the samples were probed by a femtosecond electron pulse. Here, the polarization and lattice dynamics in the ferroelectric crystal are simultaneously probed. After the laser excitation, the evolution of the polarization of the sample is represented by a transient deflection of the entire diffraction pattern along the polar direction. We see a transient suppression of the polarization in the first 2 picoseconds followed by a recovery that leads to an enhanced polarization in the next 50 picoseconds. We also observe large oscillations in the height and width of diffraction peaks, which we attribute to the generation of coherent acoustic phonons due to thermal stress. The interplay between the polarization of the sample and its temperature and lattice strain is evaluated. These findings provide a temporally resolved characterization of the thermal response of ferroelectrics and offer a new paradigm for sensing the change of polarization in ferroelectrics on a sub-picosecond time scale.
Abstract: We report the modification of an ultrafast electron diffraction (UED) instrument that enables experiments with both gas and condensed matter targets. The instrument relies on a hybrid DC-RF gun to deliver femtosecond electron pulses on the target, which are synchronized with femtosecond laser pulses. The laser pulses and electron pulses are used to excite the sample and to probe the structural dynamics, respectively. The new system is added with capabilities to perform transmission ultrafast electron diffraction on thin solid samples. It allows cooling samples to cryogenic temperatures and performs time-resolved measurements. Apart from the hardware improvements, the methodology and techniques required for a successful UED experiment, such as sample preparation, alignment determination, and data analysis, are fully explored.
We apply the adapted instrument to investigate the ultrafast response of ferroelectric NbOI2 to the optical stimuli. NbOI2 is a low-symmetry 2D van der Waals ferroelectric material with large intrinsic polarization and piezoelectric coefficients. In this study, the NbOI2 samples were excited with a femtosecond UV pulse, and the transient non-thermal states of the samples were probed by a femtosecond electron pulse. Here, the polarization and lattice dynamics in the ferroelectric crystal are simultaneously probed. After the laser excitation, the evolution of the polarization of the sample is represented by a transient deflection of the entire diffraction pattern along the polar direction. We see a transient suppression of the polarization in the first 2 picoseconds followed by a recovery that leads to an enhanced polarization in the next 50 picoseconds. We also observe large oscillations in the height and width of diffraction peaks, which we attribute to the generation of coherent acoustic phonons due to thermal stress. The interplay between the polarization of the sample and its temperature and lattice strain is evaluated. These findings provide a temporally resolved characterization of the thermal response of ferroelectrics and offer a new paradigm for sensing the change of polarization in ferroelectrics on a sub-picosecond time scale.