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Title:Design and fabrication of a single-layer guided mode resonance grating transmission filter
Author(s):Ganjoo, Maanav S
Advisor(s):Dallesasse, John M
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
guided mode resonance
anodic bonding
Abstract:To improve upon a multi-layer linear variable filter used as a spectral analyzer in a previous microfluidic lab-on-a-chip viral diagnosis assay, a single-layer spectral analyzer is envisioned. The device uses the guided-mode resonance effect, which has been used in the past to make optical stopband filters that only reflect a very narrow range of wavelengths. It has been shown that these filters can be designed to reflect an arbitrarily narrow range of wavelengths, if designed correctly. In this work, the same phenomenon is exploited to design an optical passband filter, using a simple, single-layer design, which only transmits a narrow range of wavelengths. To accomplish this, a discussion of the guided mode resonance effect and its more general form (the Wood’s anomaly) is included. This discussion throws light on the nature of the optical phenomenon which relies on the excitation of leaky modes on the grating surface, with the anomalous transmission or reflection resulting from the coupling of incident light to these modes. The envisioned design requires the preparation of a high index, optically transmissive thin film on a low index substrate. The semiconductor gallium phosphide (GaP) is chosen for its high index and transmissivity over its bandgap (549 nm). Since depositing GaP on glass using CVD would be difficult, the thin film is prepared by ion implanting a GaP wafer with a high dose of helium and bonding to borosilicate glass. It is seen that the surface of the implanted GaP adheres strongly to the glass and separates from the substrate, leaving a thin film with a consistent thickness and an rms roughness of around 10 nm. Gratings are defined using an electron beam lithography technique. Three etch chemistries were tried to etch the thin film, starting with Cl2 and BCl3 only. The etch was found to be too fast and isotropic, so a O2 was added to dilute the etch gasses. However, this resulted in undesirable micro masking, so a BCl3, Cl2, H2 and CH4 mixture was used, which was found to have the slowest etch rate and the least micro masking, at the cost of a low selectivity against PECVD SiO2 and SiN masks. The finished gratings are tested using visible spectrophotometry and the results are shown to be far inferior to the simulations. This is likely due to the sensitivity of the simulated design, resulting from instability due to the low reflection coefficient of the GaP-glass interface. An alternate design using a metal-on-waveguide design is suggested for its higher background reflectivity.
Issue Date:2021-04-28
Rights Information:Copyright 2021 Maanav Ganjoo
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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