Title

Nanopatterning of Phase Change Material Ge2SbTe5 towards Novel and Improved Reconfigurable Photonic Devices

Date of Award

2021

Degree Name

Ph.D. in Electro-Optics and Photonics

Department

Department of Electro-Optics and Photonics

Advisor/Chair

Imad Agha

Abstract

Reconfigurable photonics has been at the forefront of modern optics research, especially as optics and electronics merge into proper single-device integration. Within that umbrella, we can identify a series of critical devices commonly used in free-space applications adaptive optical elements, such as liquid crystal on silicon spatial light modulators (SLMs) and micromechanical adaptive beam steering mirrors. Light modulating devices play an essential role in spatio-temporal beam shaping, image processing, and display technologies for their role in converting intensity patterns into phase or amplitude light modulation. At the core, the physical idea remains the same: locally controlling the refractive index of the constituent material to affect the amplitude, phase, and polarization of incident light. Critical issues common to many reconfigurable devices are bulkiness, low speeds, and large voltage requirements or power consumption. However, phase change materials (PCMs) such as Ge2Sb2Te5 (GST) offer an alternative path for high-speed light modulation circumventing each of these issues. In general, PCMs alter their atomic structure via a thermal stimulus which yields large electrical, thermal, and optical contrast. Moreover, the nanosecond transition between the amorphous and metastable rock salt phase state leads to a substantial difference in the complex refractive index. In this work, an investigation is conducted on GST as a solid-state material for light modulators in the visible and infrared regimes. A holistic approach is taken to investigate the design, fabrication, and performance of each device. This work addresses critical issues in each design such as mitigating the electrical contact resistance, maximizing amplitude modulation, and improving phase transition speeds. Additionally, this work investigates the optical properties of nanopatterned GST using both top-down and bottom-up fabrication approaches that incorporate the necessary thermal management of the devices while adhering to scalability requirements for industrial applications. Broadly speaking, the first thrust of the work investigates the electrical and optical switching properties of tungsten alloyed GST in a pixel configuration designed for amplitude modulation at telecom wavelengths. Second, we explore tunable metasurfaces designed for amplitude and polarization control the SWIR regime by exciting magnetic dipole and other higher-order eigenmodes in various sub-wavelength configurations. Finally, we study the optical properties in the visible regime of an effective medium comprised of chiral GST nanomaterials. We present angular resolved Mueller Matrix measurements that indicate strong and tunable optical activity in chiral medium in the visible regime. Nanopatterned PCMs provide additional degrees of freedom for designing optical devices when compared to many other platforms, and so this work is pivotal in advancing the optics, photonics, physics, and engineering communities by designing and demonstrating on-demand reconfigurable devices to improve applications in polarization control sensing, detection, autonomous vehicles, and beam shaping. The research outlined in this work centers on the study of chalcogenide glass material from an experimental and growth perspective where numerical methods are established to provide quantitative analysis for the design of nanopatterned chalcogenides for applications in high-speed reconfigurable photonics.

Keywords

Optics, Physics, Phase change materials, nanophotonics, glancing angle deposition, chiral media, metasurfaces

Rights Statement

Copyright 2021, author.

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