Nanoscale diffractive probing of strain dynamics in ultrafast transmission electron microscopy

The control of optically driven high-frequency strain waves in nanostructured systems is an essential ingredient for the further development of nanophononics. However, broadly applicable experimental means to quantitatively map such structural distortion on their intrinsic ultrafast time and nanometer length scales are still lacking. Here, we introduce ultrafast convergent beam electron diffraction (U-CBED) with a nanoscale probe beam for the quantitative retrieval of the time-dependent local distortion tensor. We demonstrate its capabilities by investigating the ultrafast acoustic deformations close to the edge of a single-crystalline graphite membrane. Tracking the structural distortion with a 28-nm/700-fs spatio-temporal resolution, we observe an acoustic membrane breathing mode with spatially modulated amplitude, governed by the optical near field structure at the membrane edge. Furthermore, an in-plane polarized acoustic shock wave is launched at the membrane edge, which triggers secondary acoustic shear waves with a pronounced spatio-temporal dependency. The experimental findings are compared to numerical acoustic wave simulations in the continuous medium limit, highlighting the importance of microscopic dissipation mechanisms and ballistic transport channels

Polarization-dependent nonlinear optical microscopy methods for the analysis of crystals and biological tissues

The ability to solve a high-resolution protein structure is largely dependent on the successful generation and identification of protein crystals prior to X-ray diffraction (XRD). For novel protein targets, high-throughput crystallography often involves generation of multiple targets and thousands of crystallization trials per target to generate diffraction-quality crystals. Second harmonic generation (SHG) imaging has been developed as a fast, non-destructive and sensitive method for the selective identification of protein crystals, even in highly scattering environments. Polarization-dependent SHG microscopy methods were developed to assess the presence of multidomain crystals to provide a handle on crystal quality. In addition, polarization-dependent two-photon excited fluorescence (TPEF) microscopy was developed as a complementary method to SHG, providing selectivity based on the presence of protein and crystalline order, thereby reducing the potential for false negatives and positives that can arise with SHG and conventional TPEF imaging. Novel instrumentation, data acquisition methods, and data analysis techniques were developed for quantitative polarization-modulated SHG microscopy at imaging speeds up to video rate, offering significantly greater signal to noise ratios compared to polarization modulation through the manual rotation of wave plates. Quantitative polarization-dependent SHG imaging was extended to the analysis of collagen structures in biological tissues, where local-frame second order susceptibility tensors were solved for every pixel within an image of collagenous tissue and combined with ab initio modeling to assess internal ordering of collagen fibers in different tissue types

Enhanced spin-orbit coupling in core/shell nanowires

The spin-orbit coupling (SOC) in semiconductors is strongly influenced by structural asymmetries, as prominently observed in bulk crystal structures that lack inversion symmetry. Here, we study an additional effect on the SOC: the asymmetry induced by the large interface area between a nanowire core and its surrounding shell. Our experiments on purely wurtzite GaAs/AlGaAs core/shell nanowires demonstrate optical spin injection into a single free-standing nanowire and determine the effective electron g-factor of the hexagonal GaAs wurtzite phase. The spin relaxation is highly anisotropic in time-resolved micro-photoluminescence measurements on single nanowires, showing a significant increase of spin relaxation in external magnetic fields. This behavior is counterintuitive compared to bulk wurtzite crystals. We present a model for the observed electron spin dynamics highlighting the dominant role of the interface-induced SOC in these core/shell nanowires. This enhanced SOC may represent an interesting tuning parameter for the implementation of spin-orbitronic concepts in semiconductor-based structures

In-situ electron microscopy investigation of ferroelectric domain switching kinetics

Due to their ultra-high piezoelectricity, pyroelectric properties, mechanical/electrical hysteresis properties and their possessing of non-volatile polarization states, ferroelectric materials have been used in various electronic devices, including various sensors, actuators, transducers, micromotors, and non-volatile memories. The mechanical, electrical, electromechanical, and thermoelectric properties are crucial factors for device applications of ferroelectric materials. These properties are particularly sensitive to the change of the embedded microscopic structures. Therefore, the mechanical and electrical characterisation of ferroelectric materials and the observation of their microstructural evolution under external stimuli are necessary for understanding their unique properties. However, this is not an easy task because of the difficulty of mechanical and electrical testing of nano/microscale materials. Various techniques have been used to investigate the mechanical and electrical behaviours of ferroelectric materials, among which the in-situ transmission electron microscopy is one of the most effective techniques. This thesis aims to combine state-of-the-art in-situ transmission electron microscopy techniques, the scanning transmission electron microscopy high-angle annular dark-field imaging technique, and phase-field modelling to investigate microstructural evolution in ferroelectric materials under different external stimuli. One of the ultimate goals of this research is to improve the performance of non-volatile ferroelectric memory devices