Abstract

Multi-mode interferometer (MMI) spectrometers are a type of reconstructive micro-spectrometer based on imaging light propagation patterns in MMI waveguides. This dissertation describes how the MMI spectrometer was implemented for the first time using a silicon dioxide core. This approach showed a 6 dB improvement in SNR over SU-8 core devices. To implement the silicon dioxide MMI spectrometer, a fabrication technique for selectively roughening the surface of the waveguide needed to be developed. SU-8, a photosensitive polymer, was etched with oxygen plasma to create grass-like nanostructures. SU-8 grass can then be used as a transfer mask to plasma etch a surface. The resulting nano-scale silicon dioxide features were larger than the SU-8 grass but were uniform across the sample surface. Scattering features were first created on top of the SiO2 using SU-8 nanograss as an etch mask in a reactive ion etch (RIE). With optimized etch parameters, the SiO2 core MMI spectrometer proved capable of spectroscopy at light levels as low as 3.15 pW, which was a sensitivity of almost 6 dB lower than designs with an SU-8 core. Next this paper describes a fabrication technique in which the scattering features were etched into the cladding of the device prior to core deposition. Having roughened features in the cladding instead of on top of the core layer improved spin coating for the lithography steps. Devices fabricated this way were butt-coupled to an optical fiber and secured with UV-curable adhesive for increased mechanical stability. These spectrometers also had an enhanced sensitivity comparable to the roughening in cladding SiO2 MMI spectrometers. Finally, a method for increasing the roughness of the scattering features using a nickel etch mask was developed. SU-8 grass on SiO2 was coated with varying thicknesses of nickel from 0 to 21 nm. The etched samples were characterized with an optical 3D profilometer, scanning electron microscope (including energy dispersive X-ray spectroscopy to ensure removal of the nickel in the etch), and for optical reflectance. Nine nanometers of nickel resulted in a root-mean squared surface roughness of 0.38 µm, 30 times rougher than with an SU-8 mask alone. Optical testing showed samples etched with nickel had similar reflection characteristics despite their roughness differences.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering; Electrical and Computer Engineering

Rights

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