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1. Electrically driven reprogrammable phase-change metasurface reaching 80% efficiency

   https://doi.org/10.1038/s41467-022-29374-6  

   Abdollahramezani, Sajjad ; Hemmatyar, Omid ; Taghinejad, Mohammad ; Taghinejad, Hossein ; Krasnok, Alex ; Eftekhar, Ali A. ; Teichrib, Christian ; Deshmukh, Sanchit ; El-Sayed, Mostafa A. ; Pop, Eric ; et al ( March 2022 , Nature Communications)

   Phase-change materials (PCMs) offer a compelling platform for active metaoptics, owing to their large index contrast and fast yet stable phase transition attributes. Despite recent advances in phase-change metasurfaces, a fully integrable solution that combines pronounced tuning measures, i.e., efficiency, dynamic range, speed, and power consumption, is still elusive. Here, we demonstrate an in situ electrically driven tunable metasurface by harnessing the full potential of a PCM alloy, Ge₂Sb₂Te₅(GST), to realize non-volatile, reversible, multilevel, fast, and remarkable optical modulation in the near-infrared spectral range. Such a reprogrammable platform presents a record eleven-fold change in the reflectance (absolute reflectance contrast reaching 80%), unprecedented quasi-continuous spectral tuning over 250 nm, and switching speed that can potentially reach a few kHz. Our scalable heterostructure architecture capitalizes on the integration of a robust resistive microheater decoupled from an optically smart metasurface enabling good modal overlap with an ultrathin layer of the largest index contrast PCM to sustain high scattering efficiency even after several reversible phase transitions. We further experimentally demonstrate an electrically reconfigurable phase-change gradient metasurface capable of steering an incident light beam into different diffraction orders. This work represents a critical advance towards the development of fully integrable dynamic metasurfaces and their potentialmore »for beamforming applications.

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2. Topological response of the anomalous Hall effect in MnBi2Te4 due to magnetic canting

   https://doi.org/10.1038/s41535-022-00455-5  

   Bac, S. -K. ; Koller, K. ; Lux, F. ; Wang, J. ; Riney, L. ; Borisiak, K. ; Powers, W. ; Zhukovskyi, M. ; Orlova, T. ; Dobrowolska, M. ; et al ( April 2022 , npj Quantum Materials)

   Three-dimensional (3D) compensated MnBi₂Te₄is antiferromagnetic, but undergoes a spin-flop transition at intermediate fields, resulting in a canted phase before saturation. In this work, we experimentally show that the anomalous Hall effect (AHE) in MnBi₂Te₄originates from a topological response that is sensitive to the perpendicular magnetic moment and to its canting angle. Synthesis by molecular beam epitaxy allows us to obtain a large-area quasi-3D 24-layer MnBi₂Te₄with near-perfect compensation that hosts the phase diagram observed in bulk which we utilize to probe the AHE. This AHE is seen to exhibit an antiferromagnetic response at low magnetic fields, and a clear evolution at intermediate fields through surface and bulk spin-flop transitions into saturation. Throughout this evolution, the AHE is super-linear versus magnetization rather than the expected linear relationship. We reveal that this discrepancy is related to the canting angle, consistent with the symmetry of the crystal. Our findings bring to light a topological anomalous Hall response that can be found in non-collinear ferromagnetic, and antiferromagnetic phases.
3. Rigorous prediction of Raman intensity from multi-layer films

   https://doi.org/10.1364/OE.403705  

   Velson, Nathan Van ; Zobeiri, Hamidreza ; Wang, Xinwei ( November 2020 , Optics Express)

   In the Raman probing of multilayer thin film materials, the intensity of the measured Raman scattered light will be impacted by the thickness of the thin film layers. The Raman signal intensity will vary non-monotonically with thickness due to interference from the multiple reflections of both the incident laser light and the Raman scattered light of thin film interfaces. Here, a method for calculating the Raman signal intensity from a multilayer thin film system based on the transfer matrix method with a rigorous treatment of the Raman signal generation (discontinuity) is presented. This calculation methodology is valid for any thin film stack with an arbitrary number of layers with arbitrary thicknesses. This approach is applied to several thin film material systems, including silicon-on-sapphire thin films, graphene on Si with a SiO₂capping layer, and multilayer MoS₂with the presence of a gap between layers and substrate. Different applications where this method can be used in the Raman probing of thin film material properties are discussed.
4. Are phase change materials ideal for programmable photonics?: opinion

   https://doi.org/10.1364/OME.456895  

   Simpson, Robert E. ; Yang, Joel K. W. ; Hu, Juejun ( June 2022 , Optical Materials Express)

   The objective of this Opinion is to stimulate new research into materials that can meet the needs of tomorrow’s programmable photonics components. Herein, we argue that the inherent property portfolios of the common telluride phase change materials, which have been successfully applied in data storage technologies, are unsuitable for most emerging programmable photonics applications. We believe that newer PCMs with wider bandgaps, such as Sb₂S₃, Sb₂Se₃, and Ge₂Sb₂Se₄Te (GSST), can be optimized to meet the demands of holographic displays, optical neural network memories, and beam steering devices.
5. An Intensity Mapping Constraint on the CO-galaxy Cross-power Spectrum at Redshift ∼3

   https://doi.org/10.3847/1538-4357/ac4888  

   Keenan, Ryan P. ; Keating, Garrett K. ; Marrone, Daniel P. ( March 2022 , The Astrophysical Journal)

   The abundance of cold molecular gas plays a crucial role in models of galaxy evolution. While deep spectroscopic surveys of CO emission lines have been a primary tool for measuring this abundance, the difficulty of these observations has motivated alternative approaches to studying molecular gas content. One technique, line intensity mapping, seeks to constrain the average molecular gas properties of large samples of individually undetectable galaxies through the CO brightness power spectrum. Here we present constraints on the cross-power spectrum between CO intensity maps and optical galaxy catalogs. This cross-measurement allows us to check for systematic problems in CO intensity mapping data, and validate the data analysis used for the auto-power spectrum measurement of the CO Power Spectrum Survey. We place a 2σupper limit on the band-averaged CO-galaxy cross-power ofP_×< 540μK h⁻³Mpc³. Our measurement favors a nonzero 〈T_CO〉 at around 90% confidence and gives an upper limit on the mean molecular gas density atz∼ 2.6 of 7.7 × 10⁸M_⊙Mpc⁻³. We forecast the expected cross-power spectrum by applying a number of literature prescriptions for the CO luminosity–halo mass relation to a suite of mock light cones. Under the most optimistic forecasts, the cross-spectrum could be detected with only moderate extensionsmore »of the data used here, while more conservative models could be detected with a factor of 10 increase in sensitivity. Ongoing CO intensity mapping experiments will target fields allowing for extensive cross-correlation analysis and should reach the sensitivity required to detect the cross-spectrum signal.

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