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Recent Submissions

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    Inverse Design and Lithographic Pattern Transfer of a Thin Near-Infrared All-Silicon Absorber
    (University of Waterloo, 2026-07-09) Roy, Lucas
    All-silicon photodetectors are seldom used for Near-Infrared (NIR) Light Detection and Ranging (LiDAR) due to the low absorption coefficient of silicon in the NIR wavelength range. The absorptance of the absorption region is typically increased by increasing the depth of the absorption region, however, this approach adds timing jitter to a photode- tector that can severely limit the ranging resolution of the LiDAR system. This work maximized the absorptance of a silicon absorber in a fixed-depth 2.475 μm absorption re- gion from a theoretical baseline value of 2.58% to a simulated value of 8.33% for 950 nm wavelength incident light, a more than 3-fold improvement. The simulated absorptance enhancement compared to the baseline value was 3.2, whereas perfect black silicon only displays an absorptance enhancement of less than 1.5 at the 950 nm wavelength, giving the structure designed and simulated in this work a more than 2-fold absorptance enhancement improvement over perfect black silicon. This work applied topological optimization and inverse design methodologies to gen- erate the unique all-silicon 2 μm by 2 μm meta-atom absorber with vertical sidewalls. A pattern transfer using electron-beam lithography on a planar silicon sample with 340 nm of ZEP520A resist was conducted as a proof of concept that the pattern can be transferred faithfully into a real silicon sample. At the time that this work was conducted, the author was unaware of any such studies that applied topological optimization to maximize the absorptance of a thin all-silicon NIR absorber. This work showed that topological opti- mization can be effectively utilized to automatically design photonic devices where classical device architectures and structures designed by humans have poor performance. This work also proposes an initialization procedure for generating initial parameters for the optimization procedure. The structure optimized in this work was seeded using the initialization procedure showcasing its validity and effectiveness.
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    Emerging Contaminants in Groundwater and Surface Water at a Mine Reclamation Site: Occurrence, Fate and Remediation
    (University of Waterloo, 2026-07-09) Yuan, Yizhi
    Mine operations can generate large volumes of sulfide-rich waste rock and tailings. When exposed to atmospheric O2 and water, sulfide minerals can oxidize, producing low-quality drainage that contains high concentrations of dissolved metal(loid)s, sulfate, and acid. To limit the release of these toxic substances and promote landscape revegetation, municipal biosolids are increasingly utilized in mine rehabilitation programs. A multilayer cover system, consisting of a 0.5 m biosolids amended organic carbon (OC) layer and a 2 m desulfurized tailings (DST) layer, was applied on the high-sulfide tailings area in northern Ontario to retain moisture in the underlying tailings at high degrees of saturation, and promote revegetation of the landscape. Water samples from the vadose and saturated zones in the covered tailings system were collected versus depth using discrete sampling, whereas adjacent surface water was monitored using both discrete sampling and two passive samplers (diffusive gradients in thin films (DGT) and polar organic chemical integrative samplers (POCIS)) to assess spatial and temporal variations in major ions, trace elements, pharmaceutical and personal care products (PPCPs), artificial sweeteners, and per- and polyfluoroalkyl substances (PFASs) over a two-year period. Field investigations showed improved geochemical conditions below the cover system. In the vadose zone, the composite cover maintained circumneutral porewater pH through O2 consumption by OC, the enhanced neutralization capacity provided by added CaO, and the restriction of O2 infiltration in the high-moisture content DST layer, which effectively limited sulfide mineral oxidation compared to the acidic conditions (pH 3–4) observed in uncovered areas. While elevated concentrations of SO₄²⁻ and trace metals (e.g., Ni, Co, Mn, Cu, Zn) were observed in the subsurface groundwater due to historical oxidation, lower Fe concentrations beneath the biosolids layer are attributed to precipitation of goethite favoured under the circumneutral pH conditions. Concentrations of major ions and trace elements peaked adjacent to the tailings area (Ca: 152–298 mg L-1; SO42-: 347–745 mg L-1, Fe: 122–715 µg L-1; Ni: 810–1960 µg L-1) then decreased downstream within the alkaline treatment system decreasing critical trace element concentrations to well below regulatory limits (e.g., 6.0–426 µg L⁻¹ for Fe; 15.5–663 µg L⁻¹ for Ni). Porewater and groundwater monitoring indicated leaching of five pharmaceutical and personal care products (PPCPs) (95% within 480 hours), which followed a double first-order in parallel (DFOP) kinetic mechanism. The defluorination process approached 44.7±0.5% at 40 minutes and was accompanied by the formation of various shorter-chained PFASs and possible generation of organic acids (e.g., formic and acetic acid), indicating concurrent decarboxylation and defluorination pathways. Fluoride recovery, as the sum of the total inorganic and organic fluorine, reached a maximum of 67.4±1.5% at 40 min and decreased to 37.6±1.1% at 240 h. Given the near-complete removal of both PFOA and PFOS and low masses of generated short-chained PFASs (C4-C6), it is likely that PFOA and PFOS were transformed into PFBA, PFPeS and/or ultrashort-chain PFASs (C<4) that were not analyzed in the study. Alternatively, geochemical analysis indicated a potential fluoride sink via precipitation of fluorapatite associated with Ca and phosphate release from biochar. These outcomes demonstrate the feasibility of nZnNi-BC as an effective material for PFAS degradation and highlights the importance of geochemical sinks in controlling fluoride mass balance.
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    Optimal timing of hazardous waste clean-up under an environmental bond and a strict liability rule
    (University of Waterloo, 2018) Aghakazemjourabbaf, Sara; Insley, Margaret
    Inadequate site clean-up and restoration by resource extraction firms leave a toxic legacy which must be dealt with by governments. This study compares the impacts of an environmental bond and a strict liability rule on a firm's incentives for cleaning up hazardous waste during resource extraction and upon termination. The firm's problem is modelled as a stochastic optimal control problem that results in a system of Hamilton Jacobi Bellman equations. The model is applied to a typical copper mine in Canada. The resource price is modelled as a stochastic differential equation, which is calibrated to copper futures prices using a Kalman filtering approach. A numerical solution is implemented to determine the optimal abatement and extraction rates as well as the critical levels of copper prices that would motivate a firm to clean up the accumulated waste under each policy. The paper demonstrates that an environmental bond provides strong waste abatement incentives, implying that the waste is more likely to be cleaned up under the bond than the liability. The strict liability rule imposes sunk costs on a firm upon termination which would motivate it to remain inactive as a way to escape clean-up costs. However, the environmental bond raises funds ex ante for future clean-up costs and thus encourages site restoration.
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    Spousal labour supply adjustments
    (University of Waterloo, 2018-10) Lluis, Stephanie; McCall, Brian
    In this paper, we study the impact of increased generosity in the unemployment insurance system on labour supply adjustments of a spouse following the job loss of his/her partner. We exploit the longitudinal household format of the Canadian Labour Force Survey following labour force transitions of each spouse over time and estimate spousal labour supply responses arising from an added worker effect, whereby spousal labour supply increases following the partner's job loss. We study whether the additional weeks of benefits offered by the Extended Weeks (EW) pilot, an initiative of the Employment Insurance program implemented in a subset of regions, had a differential impact on spousal labour supply. We use a difference-in-difference (DiD) approach to identify (separately from the added worker effect) a crowding-out effect of EI on the spousal labour supply resulting from the greater generosity of the added benefits weeks. Our fixed-effect estimation results show a statistically significant and substantial added worker effect for married women. Our DiD results show evidence of EI crowing-out the labour supply of wives whose spouse's job loss qualifies for EI benefits. The crowding-out effect of EI diminishes about 55% of the added worker effect.
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    Marital property laws and women's labour supply
    (University of Waterloo, 2018) Lluis, Stephanie; Pan, Yazhuo (Annie)
    In this paper we study whether and if so how changes in the marital property law following the amendment of the Civil Code of Quebec to improve economic equality between spouses impacted household labour supply and individuals' marital decisions. We exploit detailed information on individual labour market and marital status from the Labour Force Survey to analyze short-term changes in labour supply and marital decisions before and after the reforms in Quebec relative to other provinces that did not experience the changes in the marital property law over that time period. Investigating the labour supply and marital decisions' responses to a policy changing the distribution of resources between men and women may assist welfare agencies in the design of family reforms and more generally, help further reduce women's entry into poverty. Furthermore, analyzing whether the Quebec marital property law changed the demographic mix of couples may further inform policymaker about possible ways to improve gender equality.