PHYSICS PHD DISSERTATION DEFENSE: Neil Sapra
Ph.D. Candidate: Neil Sapra
Research Advisor: Jelena Vuckovic
Date: Wednesday, May 6, 2020
Time: 2pm
Zoom Meeting Link: https://stanford.zoom.us/j/98067696334
Zoom Meeting Password: Email mariaf67 [at] stanford.edu (mariaf67[at]stanford[dot]edu) for password
Title: Design and demonstration of on-chip integrated laser-driven particle accelerators
Abstract:
Particle accelerators represent an indispensable tool in science, healthcare, and industry. However, the size and cost of conventional radio-frequency accelerators limits the utility and reach of this technology. Dielectric laser accelerators (DLAs) provide a compact and cost-effective solution to this problem by driving accelerator nanostructures with visible or near-infrared (NIR) pulsed lasers, resulting in a factor of 10,000x reduction in scale. Furthermore, as their name implies, DLAs do not use metals as their material of construction, as metals are highly absorptive at optical and NIR wavelengths, but rather dielectric materials such as silicon, silicon dioxide, silicon nitride. The large damage threshold of these materials allows for the potential of DLAs to operate at acceleration gradients upwards of 1 GV/m, as compared to RF-accelerators which are limited to gradients around 30-100 MV/m due to the breakdown of copper. Additionally, DLAs benefit from the precise nanofabrication techniques that have been developed over the past 60 years in the semiconductor industry to allow for high-yield and low-cost. Lastly, DLAs can be pumped by commercially available femtosecond pulsed lasers that are capable of peak fields easily exceeding GV/m. For the above reasons, dielectric laser accelerators are well poised to become an impactful accelerator technology. Current implementations of DLAs rely on free-space lasers directly incident on the accelerating structures, limiting the scalability of this technology due to the need of bulky optics and precise mechanical alignment. Therefore, integration with an inherently scalable architecture, such as photonic integrated circuits, is paramount to the development of an MeV-scale DLA for applications.
In this talk, I will present the first demonstration of a waveguide-integrated DLA, designed using a photonic inverse design approach. I will first review the operation of DLAs and describe how one can formulate a figure-of-merit for the optimization of these structures. I will then briefly introduce the inverse design framework that allows for efficient free-form optimization of these structures, enabling search of a design-space that goes far beyond that of the tuning of a few geometric parameters. With an integrated accelerator design obtained, we then turn our attention to on-chip coupling methods for DLA applications. Here again, the inverse design framework is employed to produce broadband grating couplers. We then show experimental results of our single-stage on-chip integrated accelerator, from which we infer a maximum energy gain of 0.915 keV over 30 um, corresponding to an acceleration gradient of 30.5 MeV/m. Lastly, we explore new directions to reach higher on-chip acceleration gradients and larger energy-gain, including utilizing foundry fabrication for multi-stage accelerators.