High-confinement alumina waveguides for nonlinear optics in the visible and ultraviolet

McKay, Elissa Grace (2024) High-confinement alumina waveguides for nonlinear optics in the visible and ultraviolet. PhD thesis, University of Glasgow.

Abstract

There is increasing interest in photonic integrated circuits (PICs) operating in the blue and ultraviolet (UV), driven by applications such as sensing and optical timing. Novel material platforms are required to operate at these wavelengths; amorphous alumina has an exceptionally wide bandgap, and is therefore a promising candidate. High-confinement waveguide geometries allow for high component integration densities and are essential for nonlinear frequency conversion; however, they can be challenging to fabricate and are sensitive to material absorption losses. This work establishes a methodology for fabrication of high-confinement alumina waveguides, then demonstrates that fabricated devices are suitable for low-loss visible waveguiding and integrated nonlinear optics.

Challenges in developing a high-confinement alumina platform are twofold. Atomic layer deposition (ALD) is a deposition method which can produce pure and homogenous films with low absorption losses; however, depositing amorphous alumina using a conventional thermal ALD process is extremely slow, placing a practical limit on the maximum attainable film thickness. Using an oxygen plasma as the oxidant considerably reduces deposition time; in this work, I establish that alumina films deposited using a dioxygen plasma-enhanced atomic layer deposition (PEALD) process can be used to produce low-loss optical waveguides. Furthermore, alumina is difficult to etch selectively, especially whilst maintaining the low sidewall roughness required for operation in the UV-visible region. I address this by developing a waveguide definition process using an hydrogen silsesquioxane (HSQ) mask exposed using electron beam lithography (EBL) and a BCl3 etch plasma, which allows etching of waveguides up to 800nm thick.

Using this process, I fabricate 400nm-thick amorphous alumina waveguides with confinement factors over 0.9 at a wavelength of 450nm. With propagation losses of 0.8dB/cm, these are the lowest-loss high-confinement waveguides reported for blue light. I present dispersion engineering calculations which demonstrate the necessity of high-confinement, unclad waveguides for stimulation of spectral broadening through nonlinear optical processes, and demonstrate the first supercontinuum generated in alumina waveguides, extending from 384nm to 951nm at a level of −50dB, in 700nm-thick waveguides. These results establish the viability of amorphous alumina deposited using dioxygen PEALD as a wide-bandgap material platform for UV-visible linear and nonlinear optics.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QC Physics; T Technology > T Technology (General)
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Funder's Name: and Technology Laboratory (DSTL)
• Supervisor's Name: Sorel, Professor Marc
• Date of Award: 2024
• Depositing User: Theses Team
• Unique ID: glathesis:2024-84493
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 21 Aug 2024 14:32
• Last Modified: 23 Aug 2024 13:27
• Thesis DOI: 10.5525/gla.thesis.84493
• URI: https://theses.gla.ac.uk/id/eprint/84493

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