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Numerical modeling of ultrashort pulse propagation is important for designing and understanding the underlying dynamical processes in devices that take advantage of highly nonlinear interactions in dispersion-engineered optical waveguides. Once the spectral bandwidth reaches an octave or more, multiple types of nonlinear polarization terms can drive individual optical frequencies. This issue is particularly prominent inχ⁽²⁾devices where all harmonics of the input pulse are generated and there can be extensive spectral overlap between them. Single-envelope approaches to pulse propagation have been developed to address these complexities; this has led to a significant mismatch between the strategies used to analyze moderate-bandwidth devices (usually involving multi-envelope models) and those used to analyze octave-spanning devices (usually involving models with one envelope per waveguide mode). Here we unify the different strategies by developing a common framework, applicable to any optical bandwidth, that allows for a side-by-side comparison between single- and multi-envelope models. We include bothχ⁽²⁾andχ⁽³⁾interactions in these models, with emphasis onχ⁽²⁾interactions. We show a detailed example based on recent supercontinuum generation experiments in a thin-film LiNbO₃on sapphire quasi-phase-matching waveguide. Our simulations of this device show good agreement between single- and multi-envelope models in terms of the frequency comb properties of the electric field, even for multi-octave-spanning spectra. Building on this finding, we explore how the multi-envelope approach can be used to develop reduced models that help build physical insights about new ultrafast photonics devices enabled by modern dispersion-engineered waveguides, and discuss practical considerations for the choice of such models. More broadly, we give guidelines on the pros and cons of the different modeling strategies in the context of device design, numerical efficiency, and accuracy of the simulations.

 

This phenomenological study explores the mentoring needs of 13 Black and Latinx engineering postdoctoral scholars who aspire to the professoriate. An adaptation of the ideal mentoring model (Zambrana et al., 2015) is employed as the conceptual framework. Moustakas’ (1994) four-stage process of phenomenological data analysis was utilized to examine the interview data: epoché, horizontalization, imaginative variation, and synthesis. The phenomenon’s essence is: Black and Latinx engineering postdoctoral scholars have primary and secondary mentoring needs pertaining to their immediate career acquisition of a tenure-track faculty position. Primary mentoring needs include expanding their professional network and receiving support in being a competitive faculty applicant, as well as coaching on work-life balance. Secondary needs consist of enhancing and promoting their technical skills and acquiring political guidance on racial/ethnic bias in academia. The findings of this study reveal the importance of higher education institutions and postdoctoral advisors assuming greater responsibility for ensuring postdoctoral scholars receive the mentorship and career support they desire, which may require a systematic change in the postdoctoral training environment. 

This phenomenological study explores the mentoring needs of 13 Black and Latinx engineering postdoctoral scholars who aspire to the professoriate. An adaptation of the ideal mentoring model (Zambrana et al., 2015) is employed as the conceptual framework. Moustakas’ (1994) four-stage process of phenomenological data analysis was utilized to examine the interview data: epoché, horizontalization, imaginative variation, and synthesis. The phenomenon’s essence is: Black and Latinx engineering postdoctoral scholars have primary and secondary mentoring needs pertaining to their immediate career acquisition of a tenure-track faculty position. Primary mentoring needs include expanding their professional network and receiving support in being a competitive faculty applicant, as well as coaching on work-life balance. Secondary needs consist of enhancing and promoting their technical skills and acquiring political guidance on racial/ethnic bias in academia. The findings of this study reveal the importance of higher education institutions and postdoctoral advisors assuming greater responsibility for ensuring postdoctoral scholars receive the mentorship and career support they desire, which may require a systematic change in the postdoctoral training environment. 

This research paper describes a study designed to help inform STEM faculty hiring practices at institutions of higher education in the U.S., where over the past two decades, diversity statements have become more popular components of application packages for faculty jobs. The purpose is to explore the ways and extent to which diversity statements are utilized in evaluating faculty applicants. The research questions are: (1) To what extent do universities equip search committees to evaluate applicants’ diversity statements? (2) What are STEM faculty’s perspectives of diversity statements in job applications? This paper is derived from a larger two-phase sequential mixed methods study examining the factors current faculty members and administrators consider important when hiring new STEM faculty. During the first phase, we deployed a nationwide survey to STEM faculty members and administrators who have been involved in faculty searches, with 151 of 216 respondents answering questions specific to diversity statements. About 29% of survey respondents indicated their departments required diversity statements; 59% indicated their institutions did not provide guidance for evaluating them. The second phase was a phenomenological study involving interviews of 25 survey respondents. Preliminary analyses of interview data indicated that a little more than half (52%) of participants’ departments required a diversity statement. Of the departments that required diversity statements, a little more than half used a rubric for evaluation, whether as part of a larger holistic rubric, or as a standalone rubric. For some departments that did not require diversity statements, applicants were required to discuss diversity within their other application materials. Regarding faculty members’ perceptions of diversity statements, some felt that diversity statements were necessary to assess candidates’ beliefs and experiences. Some noted that when diversity is discussed as part of another document and is not required as a stand-alone statement, it feels like the candidate “slaps on a paragraph” about diversity. Others viewed diversity statements as a “bump” that gives candidates “bonus points.” A few faculty felt that diversity statements were “redundant,” and if applicants were passionate about diversity, they would organically discuss it in the other required documents. Many shared frustrations regarding the requirement and evaluation practices. Most participants indicated their postings provided applicants with little to no guidance on what search committees were looking for in submitted diversity statements; they felt it would be beneficial for both the search committee and the applicants to have this guidance. Shared through a traditional lecture, results from this study may be used to help inform strategies for recruiting faculty who are committed to diversity - and ideally, equity and inclusion - and for addressing equity in faculty hiring. 

Guidelines identifying best practices for harvesting tissues that lead to optimal DNA preservation are few but are important curatorial concerns for genetic resource collections. We conducted a temporal study to establish rate of DNA degradation of tissue samples extracted from field-caught museum specimens. Five individuals of Sigmodon hispidus were collected and their liver and muscle tissues were harvested. Each tissue type was sectioned into 15 subsamples, and each was preserved in liquid nitrogen at different time intervals (2, 4, 8, 16 and 32 minutes; 1, 2, 4, 8 and 16 hours; and 1, 2, 4, 8 and 16 days) following death. DNA was extracted using an automated robotic instrument and molecular mass profiles were determined fluorometrically. Post-mortem DNA degradation was continuous and dependent on time, but also was significantly affected by differences among individual cotton rats. DNA fragments of ≥10,000 base pair in length were present in muscle samples across all time intervals, whereas DNA fragments of this size in liver samples were no longer present after 8 to 16 hours post-mortem. DNA molecular mass profiles showed that muscle samples retained 80% of their longest fragments (≥10,000 bp) until 1 day post-mortem, whereas liver samples retained the same percentage only until 8 minutes after death. Although rates of decay were measured from samples in a laboratory (not field) setting, rates of decay presented here can guide field and museum workers in best practices. Results suggest that opportunistic samples, such as those from roadkill specimens, are more likely to be of use for a variety of molecular methods when muscle is preserved. Considerations of differences in rates of degradation may also guide selection of tissue types housed in genetic resource collections, especially under space-limited circumstances. 

Abstract While the ecological significance of hyporheic exchange and fine particle transport in rivers is well established, these processes are generally considered irrelevant to riverbed morphodynamics. We show that coupling between hyporheic exchange, suspended sediment deposition, and sand bedform motion strongly modulates morphodynamics and sorts bed sediments. Hyporheic exchange focuses fine-particle deposition within and below mobile bedforms, which suppresses bed mobility. However, deposited fines are also remobilized by bedform motion, providing a mechanism for segregating coarse and fine particles in the bed. Surprisingly, two distinct end states emerge from the competing interplay of bed stabilization and remobilization: a locked state in which fine particle deposition completely stabilizes the bed, and a dynamic equilibrium in which frequent remobilization sorts the bed and restores mobility. These findings demonstrate the significance of hyporheic exchange to riverbed morphodynamics and clarify how dynamic interactions between coarse and fine particles produce sedimentary patterns commonly found in rivers. 

Abstract Reliable neutron star mass measurements are key to determining the equation of state of cold nuclear matter, but such measurements are rare. Black widows and redbacks are compact binaries consisting of millisecond pulsars and semi-degenerate companion stars. Spectroscopy of the optically bright companions can determine their radial velocities, providing inclination-dependent pulsar mass estimates. Although inclinations can be inferred from subtle features in optical light curves, such estimates may be systematically biased due to incomplete heating models and poorly understood variability. Using data from the Fermi Large Area Telescope, we have searched for gamma-ray eclipses from 49 spider systems, discovering significant eclipses in 7 systems, including the prototypical black widow PSR B1957+20. Gamma-ray eclipses require direct occultation of the pulsar by the companion, and so the detection, or significant exclusion, of a gamma-ray eclipse strictly limits the binary inclination angle, providing new robust, model-independent pulsar mass constraints. For PSR B1957+20, the eclipse implies a much lighter pulsar (1.81 ± 0.07 solar masses) than inferred from optical light curve modelling. 

Given an input stream of size N , a -heavy hiter is an item that occurs at least N times in S. The problem of finding heavy-hitters is extensively studied in the database literature. We study a real-time heavy-hitters variant in which an element must be reported shortly after we see its T = N - th occurrence (and hence becomes a heavy hitter). We call this the Timely Event Detection (TED) Problem. The TED problem models the needs of many real-world monitoring systems, which demand accurate (i.e., no false negatives) and timely reporting of all events from large, high-speed streams, and with a low reporting threshold (high sensitivity). Like the classic heavy-hitters problem, solving the TED problem without false-positives requires large space ((N ) words). Thus in-RAM heavy-hitters algorithms typically sacrfice accuracy (i.e., allow false positives), sensitivity, or timeliness (i.e., use multiple passes). We show how to adapt heavy-hitters algorithms to exter- nal memory to solve the TED problem on large high-speed streams while guaranteeing accuracy, sensitivity, and timeli- ness. Our data structures are limited only by I/O-bandwidth (not latency) and support a tunable trade-off between report- ing delay and I/O overhead. With a small bounded reporting delay, our algorithms incur only a logarithmic I/O overhead. We implement and validate our data structures empirically using the Firehose streaming benchmark. Multi-threaded ver- sions of our structures can scale to process 11M observations per second before becoming CPU bound. In comparison, a naive adaptation of the standard heavy-hitters algorithm to external memory would be limited by the storage device’s random I/O throughput, i.e., approx 100K observations per second.