Zhexin Zhao

Research Abstract:

My research includes studies of physical principles, design, and applications of dielectric laser accelerators, quantum features of free electrons, and phenomenological theory and design principles of photonic devices. Dielectric Laser Accelerators – Recent progress in ultrafast lasers and nanophotonics enables the development of dielectric laser accelerators (DLAs) where laser pulses interact with electron beams in the vicinity of dielectric nanostructures. This new application of light has the potential to revolutionize particle accelerators, since DLAs have high acceleration gradients (about one to two orders of magnitude higher than conventional microwave-cavity accelerators) and compact size. I contributed to the design of DLAs and the on-chip power delivery waveguide system. To continuously match the velocity of the accelerating electrons, I designed a tapered slot waveguide DLA. To increase the electron throughput limited by the sub-micron electron channel, I designed a multi-channel DLA based on the photonic crystal. I also proposed an all-optical electron pulse compression with optical beat note, which can be realized with DLAs and is crucial for the high temporal resolution in electron microscopy. Quantum Features of Free Electrons – Electron pulses, which can provide better spatial resolution than light pulses and potentially high temporal resolution, are excellent probes for optical/atomic excitations. The advancements in electron pulse control enable the quantum engineering of the free-electron wave function. It is crucial to investigate new phenomenon and reveal quantum features in the interaction among photons, electrons, and atoms, where the electron wavefunction can be modulated. I studied this problem fully quantum mechanically and found the enhancement of interaction from the modulation of the free-electron wave function, which can be used to probe the atomic coherence. Photonic Theory and Design – To obtain an efficient representation of the photonic system, I studied temporal coupled-mode theory (TCMT), which intuitively describes the dynamics of a resonant system. Further, optical systems are typically constrained by symmetries like time-reversal symmetry, energy conservation, and Lorentz reciprocity. Thus, I studied the constraints on TCMT for systems where only one of the three symmetries is present. We also utilized TCMT to study nonreciprocal thermal emitters, polarization-conversion metasurfaces, and the resonant lineshape of optical force spectra. To optimize the performance of a photonic system, I studied the general design principles of apodized grating couplers, by considering the constraints on the upper and lower bounds of the scattering strength as determined by fabrication technology. At Meta Reality Labs, I worked on the photonic design of waveguide combiners in the near-eye display for augmented reality, by exploring waveguide architectures and key components.

Bio:

Dr. Zhexin Zhao received the bachelor degree in electronic engineering from Tsinghua University, Beijing, China, in 2015, and the M.S., and Ph.D. degrees in electrical engineering (with Ph.D. minor in physics) from Stanford University, U.S., in 2018 and 2021, respectively. Dr. Zhao did her PhD study on nanophotonics under the supervision of Professor Shanhui Fan. From 2021 to 2023, Dr. Zhao was a research scientist in Reality Labs Research at Meta, working on photonic design for the augmented reality near-eye display. Dr. Zhao is currently a postdoctoral researcher under the supervision of Prof. Dr. Peter Hommelhoff at FAU Erlangen-Nürnberg, Germany. Her research focuses on photonics, including electromagnetic theory, light-matter interaction, laser-based electron acceleration and modulation, and optical design for augmented reality.