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Ultraviolet and visible integrated photonics enable applications in quantum information, sensing, and spectroscopy, among others. Few materials support low-loss photonics into the UV, and the relatively low refractive index of known depositable materials limits the achievable functionality. Here, we present a high-index integrated photonics platform based on HfO₂and Al₂O₃composites deposited via atomic layer deposition (ALD) with low loss in the visible and near UV. We show that Al₂O₃incorporation dramatically decreases bulk loss compared to pure HfO₂, consistent with inhibited crystallization due to the admixture of Al₂O₃. Composites exhibit refractive indexnfollowing the average of that of HfO₂and Al₂O₃, weighted by the HfO₂fractional compositionx. Atλ = 375 nm, composites withx = 0.67 exhibitn = 2.01, preserving most of HfO₂’s significantly higher index, and 3.8(7) dB/cm material loss. We further present fully etched and cladded waveguides, grating couplers, and ring resonators, realizing a single-mode waveguide loss of 0.25(2) dB/cm inferred from resonators of 2.6 million intrinsic quality factor atλ = 729 nm, 2.6(2) dB/cm atλ = 405 nm, and 7.7(6) dB/cm atλ = 375 nm. We measure the composite’s thermo-optic coefficient (TOC) to be 2.44(3) × 10⁻⁵RIU/°C nearλ = 397 nm. This work establishes (HfO₂)_x(Al₂O₃)_1−xcomposites as a platform amenable to integration for low-loss, high-index photonics spanning the UV to NIR. 

Fluorescence collection from individual emitters plays a key role in state detection and remote entanglement generation, fundamental functionalities in many quantum platforms. Planar photonics have been demonstrated for robust and scalable addressing of trapped-ion systems, motivating consideration of similar elements for the complementary challenge of photon collection. Here, using an argument from the reciprocity principle, we show that far-field photon collection efficiency can be simply expressed in terms of the fields associated with the collection optic at the emitter position alone. We calculate collection efficiencies into ideal paraxial and fully vectorial focused Gaussian modes parameterized in terms of focal waist, and further quantify the modest enhancements possible with more general beam profiles, establishing design requirements for efficient collection. Toward practical implementation, we design, fabricate, and characterize two diffractive collection elements operating atλ = 397 nm; a forward emitting design is predicted to offer 0.25% collection efficiency into a single waveguide mode, while a more efficient reverse-emitting design offers 1.14% collection efficiency, albeit with more demanding fabrication requirements. Close agreement between simulated and measured emission for both designs indicates practicality of these collection efficiencies, and we indicate avenues to improved devices approaching the limits predicted for ideal beams. We point out a particularly simple integrated waveguide configuration for polarization-based remote entanglement generation enabled by integrated collection. 

Geodesign is a participatory planning approach in which stakeholders use geographic information systems to develop and vet alternative design scenarios in a collaborative and iterative process. This study is based on a 2019 geodesign workshop in which 17 participants from industry, government, university, and non-profit sectors worked together to design an initial network of hydrogen refueling stations in the Hartford, Connecticut, metropolitan area. The workshop involved identifying relevant location factors, rapid prototyping of station network designs, and developing consensus on a final design. The geodesign platform, which was designed specifically for facility location problems, enables breakout groups to add or delete stations with a simple point-and-click operation, view and overlay different map layers, compute performance metrics, and compare their designs to those of other groups. By using these sources of information and their own expert local knowledge, participants recommended six locations for hydrogen refueling stations over two distinct phases of station installation. We quantitatively and qualitatively compared workshop recommendations to solutions of three optimal station location models that have been used to recommend station locations, which minimize travel times from stations to population and traffic or maximize trips that can be refueled on origin–destination routes. In a post-workshop survey, participants rated the workshop highly for facilitating mutual understanding and information sharing among stakeholders. To our knowledge, this workshop represents the first application of geodesign for hydrogen refueling station infrastructure planning. 

The recent growth in the California hydrogen fuel cell vehicle (FCV) market offers the opportunity to analyze how refueling stations that drivers use after some experience compare with those they initially intended to use. Online surveys completed by 124 FCV adopters in California in early 2019 were analyzed. Respondents listed stations they initially planned to use, stations that they later used, subjective reasons for using them, and important travel destinations. Network GIS analysis was then used to measure estimated travel times between both available and planned retail hydrogen stations and home, work, and frequently traveled routes, both at the time of adoption and at the time of the survey. Results show that 40% of respondents changed refueling stations over time. Those with stations objectively nearer to home, work, and frequently traveled routes were less likely to change their list. Drivers were more likely to subjectively label stations as near home and less likely to label them as on the way compared with objective measurements of these criteria, though these differences are greater for respondents who changed stations. Regardless of whether the station was available pre-adoption or opened post-adoption, stations that respondents added to their lists were farther from home than those they initially intended to use. For stations available pre-adoption, reliability positively influenced adding them after experience, while stations added by drivers that opened post-adoption tended to require short deviations to reach. These results indicate that a mixture of geographic and station-level characteristics contribute to FCV drivers changing stations over time. 

The market for hydrogen fuel cell vehicles (FCVs) continues to grow worldwide. At present, early adopters rely on a sparse refuelling infrastructure, and there is only limited knowledge about how they evaluate the geographic arrangement of stations when they decide to get an FCV, which is an important consideration for facilitating widespread FCV diffusion. To address this, we conducted several related studies based on surveys and interviews of early FCV adopters in California, USA, and a participatory geodesign workshop with hydrogen infrastructure planning stakeholders in Connecticut, USA. From this mixed-methods research project, we distil 15 high-level findings for planning hydrogen station infrastructure to encourage FCV adoption.