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1. 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$$<154$$~Hz in the optically active region (diameter smaller than 150~mm), preventing the wire vibration during operation and reducing the RHWP deformation under the wire tension. The mirror tilt relative to the rotating axis was controlled to be $<15''$, corresponding to an increase in beam width due to beam smearing of < $0.6''$, %a beam smearing amplitude of $<0.6''$, negligible compared to the beam's full-width half-maximum of $36'$. The median and 16/84th percentile of the wire--mirror separation residual was $$0.048^{+0.013}_{-0.014}$$~mm in the optically active region, achieving a modulation efficiency $$\epsilon=96.2_{+0.5}^{-0.4}\%$$ with an estimated bandpass of 34~GHz. The angular velocity of the RHWP was maintained to an accuracy of within 0.005\% at the nominal rotation frequency (2.5~Hz). The RHWP has been successfully integrated into the CLASS 90 GHz telescope and started taking data in June 2024, replacing the previous modulator that has been in operation since June 2018. 

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2. Spectroscopic Microtomography in the Short-Wave Infrared Wavelength Range

   https://doi.org/10.3390/s23115164  

   Juntunen, Cory; Abramczyk, Andrew R.; Shea, Peter; Sung, Yongjin (June 2023, Sensors)

   Spectroscopic microtomography provides the ability to perform 4D (3D structural and 1D chemical) imaging of a thick microscopic specimen. Here, we demonstrate spectroscopic microtomography in the short-wave infrared (SWIR) wavelength using digital holographic tomography, which captures both the absorption coefficient and refractive index. A broadband laser in tandem with a tunable optical filter allows us to scan the wavelength from 1100 to 1650 nm. Using the developed system, we measure human hair and sea urchin embryo samples. The resolution estimated with gold nanoparticles is 1.51 μm (transverse) and 1.57 μm (axial) for the field of view of 307 × 246 μm2. The developed technique will enable accurate and efficient analyses of microscopic specimens that have a distinctive absorption or refractive index contrast in the SWIR range. 

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3. Preliminary measurements of phase coupling in the dust acoustic wave

   https://doi.org/10.1063/5.0314609  

   Williams, Jeremiah D (March 2026, Physics of Plasmas)

   Bispectral analysis is a powerful tool for studying nonlinear phenomena and is applied here to the dust acoustic wave. In particular, we examine three situations: (1) the naturally occuring wave mode, (2) the wave mode subject to an external modulation at a single drive frequency, and (3) the wave mode subject to an external modulation at two drive frequencies. Nonlinear interactions are observed in each of these cases. When an external modulation at a single drive frequency is applied, a threshold condition is observed for nonlinear interactions to result in three-wave coupling and the excitation of a third mode. In the case of an external modulation at two-drive frequencies, significant nonlinear wave mixing is observed. Taken together, these measurements demonstrate that the dust acoustic wave is an excellent system for studying nonlinear interactions and three-wave coupling. 

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4. The optical design for Cryoscope: a wide-field NIR telescope with low thermal emission

   https://doi.org/10.1117/12.2630557  

   Fucik, Jason; Smith, Roger (August 2022, SPIE)

   Marshall, Heather K; Spyromilio, Jason; Usuda, Tomonori (Ed.)

   We present the optical design for Cryoscope, a 0.26 m aperture telescope that is a f/2 objective operating over the photometric K band (1.99 to 2.55 μm) with diffraction limited imaging. It has a 16 deg2 FoV with a 7.1′′/pix plate scale on a 2048×2048 18 μm/pixel Teledyne H2RG detector array. The objective is a catadioptric design incorporating two thin fused silica meniscus lenses near the entrance aperture, a spherical primary mirror, and a doublet immediately in front of the detector to flatten the image surface. The design solution is capable of delivering diffraction limited images over a 10° field diameter at f/1.25 in the NIR. The use of fused silica for the first two lens elements allows the design to be used for a broad range of applications from the vacuum ultraviolet to thermal IR with only re-optimization of the field flattening doublet. In the VUV (185 to 300 nm) the design is no longer diffraction limited, but can still be made to be pixel limited with detector arrays having pixels as small as 10 μm. The design provides a compact, wide field, and fast objective that can scale to a 1 m-class telescope and offers several benefits over a classical Schmidt telescope. The convex fused silica meniscus lens is strong enough to serve as a vacuum window allowing the entire optical path to be cryogenically cooled to maintain low thermal emission while delivering two orders of magnitude larger field of view than previous ground-based designs for the thermal infrared. 

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5. Narrow-bandgap titanium sesquioxide with resonant metasurfaces for enhanced infrared absorption

   https://doi.org/10.1063/5.0240125  

   Babicheva, Viktoriia E; Lock, Evgeniya; Kim, Heungsoo (December 2024, Applied Physics Letters)

   We report on the structural, chemical, and optical properties of titanium sesquioxide Ti2O3 thin films on single-crystal sapphire substrates by pulsed laser deposition. The thin film of Ti2O3 on sapphire exhibits light absorption of around 25%–45% in the wavelength range of 2–10 μm. Here, we design an infrared photodetector structure based on Ti2O3, enhanced by a resonant metasurface, to improve its light absorption in mid-wave and long-wave infrared windows. We show that light absorption in the mid-wave infrared window (wavelength 3–5 μm) in the active Ti2O3 layer can be significantly enhanced from 30%–40% to more than 80% utilizing a thin resonant metasurface made of low-loss silicon, facilitating efficient scattering in the active layer. Furthermore, we compare the absorptance of the Ti2O3 layer with that of conventional semiconductors, such as InSb, InAs, and HgCdTe, operating in the infrared range with a wavelength of 2–10 μm and demonstrate that the absorption in the Ti2O3 film is significantly higher than in these conventional semiconductors due to the narrow-bandgap characteristics of Ti2O3. The proposed designs can be used to tailor the wavelengths of photodetection across the near- and mid-infrared ranges. 

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