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Microsphere photolithography (MPL) is a promising technique for cost-effective fabrication of large-scale metasurfaces. This approach generates an array of photonic jets by the collimated illumination of self-assembled microspheres. The photonic jets can be precisely steered within the unit cell defined by each microsphere by changing the angle of incidence. This allows for the creation of complex metasurface element geometries. Computer controlled articulation of the substrate relative to a static UV source allows the direct-write of different metasurface elements. However, this is time-consuming and requires registration between each exposure for complex features. This paper investigates a single exposure method with the dynamic continuous angle of incidence control provided by a Digital Micromirror Device (DMD) in the front Fourier plane of the projection system. The grayscale values of the DMD pixels can be adjusted to provide optical proximity correction. Larger patterns can be achieved by scanning the substrate relative to the exposure beam. This approach is demonstrated with the creation of hierarchical patterns. This work greatly simplifies the MPL exposure process for complex resonators and provides potential for full light field control.

 

Rare earth elements (REE) are critical elements found in monazite, xenotime, and hydrated REE phosphates which typically form in hydrothermal mineral deposits. Accurate predictions of the solubility of these REE phosphates and the speciation of REE in aqueous fluids are both key to understanding the controls on the transport, fractionation, and deposition of REE in natural systems. Previous monazite and xenotime solubility experiments indicate the presence of large discrepancies between experimentally derived solubility constants versus calculated solubilities by combining different data sources for the thermodynamic properties of minerals and aqueous species at hydrothermal conditions. In this study, these discrepancies were resolved by using the program GEMSFITS to optimize the standard partial molal Gibbs energy of formation (ΔfG°298) of REE aqueous species (REE3+ and REE hydroxyl complexes) at 298.15 K and 1 bar while keeping the thermodynamic properties fixed for the REE phosphates. A comprehensive experimental database was compiled using solubility data available between 25 and 300 °C. The latter permits conducting thermodynamic parameter optimization of ΔfG°298 for REE aqueous species. Optimal matching of the rhabdophane solubility data between 25 and 100 °C requires modifying the ΔfG°298 values of REE3+ by 1–6 kJ/mol, whereas matching of the monazite solubility data between 100 and 300 °C requires modifying the ΔfG°298 values of both REE3+ and REEOH2+ by ∼ 2–10 kJ/mol and ∼ 15–31 kJ/mol, respectively. For xenotime, adjustments of ΔfG°298 values by 1–26 kJ/mol are only necessary for the REE3+ species. The optimizations indicate that the solubility of monazite in acidic solutions is controlled by the light (L)REE3+ species at <150 °C and the LREEOH2+ species at >150 °C, whereas the solubility of xenotime is controlled by the heavy (H)REE3+ species between 25 and 300 °C. Based on the optimization results, we conclude that the revised Helgeson-Kirkham-Flowers equation of state does not reliably predict the thermodynamic properties of REE3+, REEOH2+, and likely other REE hydroxyl species at hydrothermal conditions. We therefore provide an experimental database (ThermoExp_REE) as a basic framework for future updates, extensions with other ligands, and optimizations as new experimental REE data become available. The optimized thermodynamic properties of aqueous species and minerals are available open access to accurately predict the solubility of REE phosphates in fluid-rock systems. 

Microsphere photolithography (MPL) is an alternative low-cost technique for the large-scale fabrication of periodic structures, such as metasurfaces. This technique utilizes the photonic nanojet generated in the photoresist (PR), by microspheres in near proximity, which are exposed to collimated ultraviolet (UV) flood illumination. In the basic approach, a microsphere array is self-assembled on, or transferred to, the substrate prior to exposure. After exposure, the microspheres are washed away in the development step. The process to recover and clean these microspheres for reuse is complicated. This paper investigates the use of reusable microsphere masks created by fixing the microspheres on a UV transparent support. This is then brought into contact with the photoresist with controlled pressure. There is a trade-off between the quality of the fabricated samples and the wear of the mask determined by the contact pressure. The system is demonstrated using a digital micromirror device (DMD)-based direct-write exposure system to fabricate infrared (IR) metasurfaces. These metasurfaces are characterized and compared to simulation models. Finally, a series of 50 hierarchically patterned IR metasurfaces was fabricated using a single reusable mask. These samples had a <3% coefficient of variance when viewed with a thermal camera. This work shows the potential of mask-based MPL and other contact microlens array-based photolithography techniques for low-cost large-scale fabrication. 

Chemical and isotopic processes occur in every segment of the hydrological cycle. Hydrogeochemistry—the subdiscipline that studies these processes—has seen a transformation from “witch's brew” to credible science since 2000. Going forward, hydrogeochemical research and applications are critical to meeting urgent societal needs of climate change mitigation and clean energy, such as (1) removing CO₂from the atmosphere and storing gigatons of CO₂in soils and aquifers to achieve net‐zero emissions, (2) securing critical minerals in support of the transition from fossil fuels to renewable energies, and (3) protecting water resources by adapting to a warming climate. In the last two decades, we have seen extensive activity and progress in four research areas of hydrogeochemistry related to water‐rock interactions: arsenic contamination of groundwater; the use of isotopic and chemical tracers to quantify groundwater recharge and submarine groundwater discharge; the kinetics of chemical reactions and the mineral‐water interface's control of contaminant fate and transport; and the transformation of geochemical modeling from an expert‐only exercise to a widely accessible tool. In the future, embracing technological advances in machine learning, cyberinfrastructure, and isotope analytical tools will allow breakthrough research and expand the role of hydrogeochemistry in meeting society's needs for climate change mitigation and the transition from fossil fuels to renewable energies.

 

Microsphere photolithography (MPL) has shown promise for the low-cost large-scale manufacturing of infrared (IR) metasurfaces. One challenge of the technique is that the microsphere array needs to be in immediate proximity to the photoresist because of the near-filed effect of the photonic jet. This is typically accomplished by directly transferring the microsphere array onto the photoresist layer. The microspheres are then washed away during the development of the photoresist. While there may be a possibility of recovering, cleaning, and reusing the microspheres, this is not typically done. This work studies the self-assembly of the microspheres on a superstrate which can be reused as a contact mask. The microspheres are fixed to this superstrate to minimize debonding when they are brought into contact with the substrate. IR metasurfaces are fabricated and spectrally characterized. The resonant wavelength of IR metasurfaces is shown to be a good statistical metric for the variation of the patterned surface. The results indicate pressure between the substrate and superstrate is a critical factor in maintaining a minimum gap between the microspheres and photoresist. This work shows a way forward for mask-based microsphere photolithography and provides guidance for future microlens array-based photolithographic techniques.