Publications

We present a modular integration platform, combining active and passive photonic devices with CMOS integrated electronics. Based on this integration platform, an integrated Frequency Modulated Continuous Wave (FMCW) Laser Detection and Ranging (LADAR) source chip is being developed. Such a LADAR source chip can be used in miniaturized 3D imaging systems for defense and consumer electronics applications. In this paper, we discuss the integration approach, the performance of select individual components, and experimental results on a bench top LADAR model system.

We report spectropolarimetric observations of the Type Ia supernova (SN) SN 2021rhu at four epochs: −7, +0, +36, and +79 days relative to its $B$-band maximum luminosity. A wavelength-dependent continuum polarization peaking at $3890 ± 93$ Angstroms and reaching a level of $p_{max} = 1.78$ % was found. The peak of the polarization curve is bluer than is typical in the Milky Way, indicating a larger proportion of small dust grains along the sight line to the SN. After removing the interstellar polarization, we found a pronounced increase of the polarization in the Ca ii near-infrared triplet, from ∼0.3% at day −7 to ∼2.5% at day +79. No temporal evolution in high-resolution flux spectra across the Na i D and Ca ii H and K features was seen from days +39 to +74, indicating that the late-time increase in polarization is intrinsic to the SN as opposed to being caused by scattering of SN photons in circumstellar or interstellar matter. We suggest that an explanation for the late-time rise of the Ca ii near-infrared triplet polarization may be the alignment of calcium atoms in a weak magnetic field through optical excitation/pumping by anisotropic radiation from the SN.

We present a constraint on the abundance of supergiant (SG) stars at redshift z approx. 1, based on recent observations of a strongly lensed arc at this redshift. First we derive a free-form model of MACS J0416.1-2403 using data from the BUFFALO program. The new lens model is based on 72 multiply lensed galaxies that produce 214 multiple images, making it the largest sample of spectroscopically confirmed lensed galaxies on this cluster. The larger coverage in BUFFALO allows us to measure the shear up to the outskirts of the cluster, and extend the range of lensing constraints up to ~ 1 Mpc from the central region, providing a mass estimate up to this radius. As an application, we make predictions for the number of high-redshift multiply-lensed galaxies detected in future observations with JWST. Then we focus on a previously known lensed galaxy at z=1.0054, nicknamed Spock, which contains four previously reported transients. We interpret these transients as microcaustic crossings of SG stars and compute the probability of such events. Based on simplifications regarding the stellar evolution, we find that microlensing (by stars in the intracluster medium) of SG stars at z=1.0054 can fully explain these events. The inferred abundance of SG stars is consistent with either (1) a number density of stars with bolometric luminosities beyond the Humphreys-Davidson (HD) limit (L ~ $6\times10^5 L_{\odot}$) that is below 400 stars per sq. kpc, or (2) the absence of stars beyond the HD limit but with a SG number density of ~ 9000 per sq. kpc for stars with luminosities between $10^5$ and $6\times10^5$. This is equivalent to one SG star per 10x10 pc$^2$. We finally make predictions for future observations with JWST's NIRcam. We find that in observations made with the F200W filter that reach 29 mag AB, if cool red SG stars exist at z~1 beyond the HD limit, they should be easily detected in this arc

Bimetallic 15 nm Pd-core Rh-shell Rh1−x Pd x nanoparticle catalysts have been synthesized and studied in CO oxidation by NO. The catalysts exhibited composition-dependent activity enhancement (synergy) in CO oxidation in high NO pressures. The observed synergetic effect is attributed to the favorable adsorption of CO on Pd in NO-rich conditions. The Pd-rich bimetallic catalysts deactivated after many hours of oxidation of CO by NO. After catalyst deactivation, product formation was proportional to the Rh molar fraction within the bimetallic nanoparticles. The deactivated catalysts were regenerated by heating the sample in UHV. This regeneration suggests that the deactivation was caused by the adsorption of nitrogen atoms on Pd sites. TOF of CO2 as a function of Pd molar fraction for Rh1−x Pd x nanocrystals at 230 °C (before deactivation) and 250 °C (after deactivation) in 80 Torr NO, 8 Torr CO, and 672 Torr He. At 230 °C, prior to deactivation, the Pd-rich catalysts were more active for CO2 production than the Rh-rich catalysts.

Abstract Current explanations of the mass-loss mechanism for stripped-envelope supernovae (SNe) remain divided between single and binary progenitor systems. Here we obtain deep ultraviolet (UV) imaging with the Hubble Space Telescope (HST) of the Type Ic SN 2012fh to search for the presence of a surviving companion star to the progenitor. We synthesize these observations with archival HST imaging, ground-based spectroscopy, and previous analyses from the literature to provide three independent constraints on the progenitor system. We fit the color–magnitude diagram of the surrounding population to constrain the most likely age of the system to be &lt;20 Myr. Analysis of spectra of SN 2012fh provides an estimate of the He core mass of the progenitor star, &gt;5.6 M ⊙ . We analyze deep HST images at the precise location after the SN faded to constrain the luminosity of any remaining main-sequence binary companion to be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>L</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≲</mml:mo> <mml:mn>3.35</mml:mn> </mml:math> . Combining observational constraints with current binary population synthesis models excludes the presence of a faint stellar companion to SN 2012fh at the ≲10% level. The progenitor was therefore either effectively isolated at the time of explosion or orbited by a black-hole companion. The latter scenario dominates if we only consider models that produce successful SNe.

CO₂ reduction using renewable H₂ represents an emerging approach for minimizing dependency on fossil fuels and reducing the carbon footprint while providing chemicals and fuels. In this context, CO₂ hydrogenation using Fe-based oxide, which exhibits outstanding capabilities in both reverse water gas shift (RWGS) and Fischer-Tropsch synthesis (FTS) reactions, integrated with zeolite has been a promising method for heavy hydrocarbon (C₅₊) production. This review investigates the critical roles of promoter, zeolite topology and acidity, and synthesis methods in optimizing product distribution and their contributions to active site proximity. It has been found that the catalyst integration manner and the interaction between the basic sites of Fe-based oxide and the acidic sites of zeolites significantly influence catalytic performance. In addition, the proximity of active sites, a crucial factor in tandem catalysis, can be controlled via different catalyst synthesis methods, dispersion on mesoporous supports, or using encapsulated structures that can provide the confinement effect while guiding the reaction sequence. Furthermore, the choice of alkali promoters (Na vs K) is very important since each can alter electronic properties, reduction behavior, and hydrocarbon distribution due to different electronegativity and ionic radii. While Na could hamper all reduction steps and diffuses into bulk iron oxide, K remains mainly on the surface, increasing electron density and facilitating iron carbide formation. Besides, integrating spectroscopic imaging techniques with proximity metrics will enhance the understanding of active site spatial distribution. To bridge the gap between lab-scale results and industrial applications, advanced computational methods coupled with artificial intelligence (AI) and machine learning (ML) techniques are required to monitor and analyze catalyst behavior and optimize large-scale production. The findings of this study provide a comprehensive understanding of catalyst design principles with emphasis on the importance of the proximity of active sites, offering insights for the next generation of efficient CO₂ hydrogenation catalysts for industrial-scale fuel production.

The new siloxy-germoxy ligand HOGe[OSi(O t Bu) 3 ] 3 generates Mn-, Fe-, and Cr-containing precursors suitable for thermolytic molecular precursor routes to quaternary oxides.

As a step towards clarifying the ionization mechanism of LINERs, we have used the Hubble Space Telescope Faint Object Spectrograph to obtain an ultraviolet spectrum of the nucleus of NGC 6500. Compared with more luminous Seyfert nuclei, the emission-line spectrum of NGC 6500 is remarkable for its low-excitation character: C III], C II], and Mg II are the strongest collisionally excited lines in the UV, and C IV is not detected. The fast shock models of Dopita & Sutherland (1995, 1996) are a poor match to the observed spectrum, as they predict much stronger emission than is observed for high-excitation lines such as C IV. Photoionization by an obscured nonstellar continuum source, or possibly ionization by slower (100 km/s) shocks, are more likely explanations for the emission-line ratios. The origin of the UV continuum is unclear; its overall spectral shape is reasonably well matched by spectral evolution models for single-burst populations with ages in the range 70-100 Myr, but no definite absorption features from young stars are present. Alternately, the UV continuum (or some fraction of it) may be scattered radiation from a hidden active nucleus. We tentatively detect a broad He II 4686 emission feature, which may be due to Wolf-Rayet stars, although the UV spectrum does not show any clear Wolf-Rayet signatures. We also find that NGC 6500 is weakly detected in an archival ROSAT HRI image with a luminosity of 5 x 10^40 erg/s in the 0.1-2.4 keV band, within the normal range for LINERs.