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1. XHEMTs on Ultrawide Bandgap Single‐Crystal AlN Substrates

   https://doi.org/10.1002/aelm.202500393  

   Kim, Eungkyun; Chen, Yu‐Hsin; Pieczulewski, Naomi; Encomendero, Jimy; Muller, David Anthony; Jena, Debdeep; Xing, Huili Grace (February 2026, Advanced Electronic Materials)

   ABSTRACT AlN has the largest bandgap in the wurtzite III‐nitride semiconductor family, making it an ideal barrier for a thin GaN channel to achieve strong carrier confinement in field‐effect transistors, analogous to silicon‐on‐insulator technology. Unlike /Si/, AlN/GaN/AlN can be grown fully epitaxially, enabling high carrier mobilities suitable for high‐frequency applications. However, developing these heterostructures and related devices has been hindered by challenges in strain management, polarization effects, defect control, and charge trapping. Here, the AlN single‐crystal high electron mobility transistor (XHEMT) is introduced, a new nitride transistor technology designed to address these issues. The XHEMT structure features a pseudomorphic GaN channel sandwiched between AlN layers, grown on single‐crystal AlN substrates. XHEMTs demonstrate RF performance on par with the state‐of‐the‐art GaN HEMTs, achieving 5.92 W/mm output power and 65% peak power‐added efficiency at 10 GHz under 17 V drain bias. These devices overcome several limitations present in conventional GaN HEMTs, which are grown on lattice‐mismatched foreign substrates that introduce undesirable dislocations and exacerbated thermal resistance. With the recent availability of 100‐mm AlN substrates and AlN's high thermal conductivity (340 W/), XHEMTs show strong potential for next‐generation RF electronics. 

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2. Exploring the dust grain size and polarization mechanism in the hot and massive Class 0 disk IRAS 16293-2422 B

   https://doi.org/10.1051/0004-6361/202244628  

   Zamponi, Joaquin; Maureira, María José; Liu, Hauyu Baobab; Zhao, Bo; Segura-Cox, Dominique; Ko, Chia-Lin; Caselli, Paola (February 2024, Astronomy & Astrophysics)

   Context. Multiwavelength dust continuum and polarization observations arising from self-scattering have been used to investigate grain sizes in young disks. However, the likelihood of self-scattering being the polarization mechanism in embedded disks decreases for very highly optically thick disks and makes us reconsider some of the size constraints from polarization, particularly for younger and more massive disks. The 1.3 mm polarized emission detected toward the hot (≳400 K) Class 0 disk IRAS 16293-2422 B has been attributed to self-scattering, with predictions of bare grain sizes between 200 and 2000 µm. Aims. We aim to investigate the effects of changing the maximum grain sizes in the resultant continuum and continuum polarization fractions from self-scattering for a hot and massive Class 0 disk extracted from numerical simulations of prestellar core collapse and to compare them with IRAS 16293 B observations. Methods. We compared new and archival dust continuum and polarization observations at high resolution between 1.3 and 18 mm to a set of synthetic models. We developed a new publicly available tool to automate this process called Synthesizer. This tool is an easy-to-use program for generating synthetic observations from numerical simulations. Results. Optical depths are in the range of 130 to 2 from 1.3 to 18 mm, respectively. Predictions of significant grain growth populations, includinga_max= 1000 µm, are comparable to the observations from IRAS 16293 B at all observed wavelengths. The polarization fraction produced by self-scattering reaches a maximum of approximately 0.1% at 1.3 mm for a maximum grain size of 100 µm, which is an order of magnitude lower than the grain size observed toward IRAS 16293 B. Conclusions. From comparison of the StokesIfluxes, we conclude that significant grain growth could be present in the young Class 0 disk IRAS 16293 B, particularly in the inner hot region (< 10 au,T> 300 K) where refractory organics evaporate. The polarization produced by self-scattering in our model is not high enough to explain the observations at 1.3 and 7 mm, and such effects as dichroic extinction and polarization reversal of elongated aligned grains remain other possible but untested scenarios. 

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3. Conservation of longitudinal spin polarization of positrons emitted from a thin Ni(100) foil

   https://doi.org/10.1103/PhysRevA.107.062809  

   Cecchini, G. G.; Greaves, R. G.; Mills, A. P. (June 2023, Physical Review A)

   We have discovered that 5 keV bursts of 5 × 107 positrons with an initial longitudinal spin polarization of (28.8 ± 0.7)%, when implanted into a thin Ni(100) crystal, are emitted with 20% efficiency at thermal energies from its surface with (30.9 ± 0.5)% polarization. We conclude that the positron spin polarization is preserved while interacting with the Ni, despite the 0.61 T average transverse magnetization of the Ni at room temperature. The resulting polarized beam has been focused to a 0.025-mm mean-diameter spot when accelerated to 5 keV and will be uniquely suited for experiments on a neutral spin aligned e+-e − plasma, spin- and angle-resolved positronium emission spectroscopy, and critical for producing a triplet positronium Bose-Einstein condensate. 

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4. Design and characterization of a 60-cm reflective half-wave plate for the CLASS 90 GHz band telescope

   https://doi.org/10.1117/12.3016346  

   Shi, Rui; Brewer, Michael; Chan, Carol; Chuss, David T; Couto, Jullianna D; Eimer, Joseph R; Karakla, John; Shukawa, Koji; Valle, Deniz; Appel, John W; et al (August 2024, SPIE)

   Zmuidzinas, Jonas; Gao, Jian-Rong (Ed.)

   Front-end polarization modulation enables improved polarization measurement stability by modulating the targeted signal above the low-frequency $1/f$ drifts associated with atmospheric and instrumental instabilities and diminishes the impact of instrumental polarization. In this work, we present the design and characterization of a new 60-cm diameter Reflective Half-Wave Plate (RHWP) polarization modulator for the 90 GHz band telescope of the Cosmology Large Angular Scale Surveyor (CLASS) project. The RHWP consists of an array of parallel wires (diameter 50~µm, 175~µm pitch) positioned 0.88~mm from an aluminum mirror. In lab tests, it was confirmed that the wire resonance frequency ($$f_\mathrm{res}$$) profile is consistent with the target, $139$~Hz$$ 

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5. A simple, isotropic depolarized light source

   https://doi.org/10.1063/5.0165103  

   Foreman, K D; Gay, T J (October 2023, Review of Scientific Instruments)

   Unpolarized light can be an important tool in optical experiments. Producing it, however, can prove to be a challenge. Natural sources of light that are commonly thought of as unpolarized are, in fact, either weakly polarized or not practical sources of light in a laboratory setting. Standard, commercially available light depolarizers produce unpolarized light only after the polarization state of the light across the diameter of the output beam has been averaged. Locally, such beams are highly polarized. In this work, we report a simple, low cost light depolarizer capable of producing light with a total polarization of less than 1% for a 15-mm diameter output beam. Based upon diffuse scattering, the light transmitted through the depolarizer discussed here produces only small polarizations locally, with the total polarization for a 1.25-mm diameter area being <6%. The effects of the depolarizer on the transmitted beam’s intensity are also reported. 

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