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1. Ultra compact Bragg grating devices with broadband selectivity

   https://doi.org/10.1364/OL.384688  

   Li, Ang ; Davis, Jordan ; Fainman, Yeshaiahu ( January 2020 , Optics Letters)

   Current silicon waveguide Bragg gratings typically introduce perturbation to the optical mode in the form of modulation of the waveguide width or cladding. However, since such a perturbation approach is limited to weak perturbations to avoid intolerable scattering loss and higher-order modal coupling, it is difficult to produce ultra-wide stopbands. In this Letter, we report an ultra-compact Bragg grating device with strong perturbations by etching nanoholes in the waveguide core to enable an ultra-large stopband with apodization achieved by proper location of the nanoholes. With this approach, a 15 µm long device can generate a stopband as wide as 110 nm that covers the entire$\mathrm{C}+\mathrm{L}$band with a 40 dB extinction ratio and over a 10 dB sidelobe suppression ratio (SSR). Similar structures can be further optimized to achieve higher SSR of$><\#comment/>17\mathrm{d}\mathrm{B}$for a stopband of about 80 nm.

    

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2. Multiphase magnetism in Yb 2 Ti 2 O 7

   https://doi.org/10.1073/pnas.2008791117  

   Scheie, Allen ; Kindervater, Jonas ; Zhang, Shu ; Changlani, Hitesh J. ; Sala, Gabriele ; Ehlers, Georg ; Heinemann, Andre ; Tucker, Gregory S. ; Koohpayeh, Seyed M. ; Broholm, Collin ( October 2020 , Proceedings of the National Academy of Sciences)

   We use neutron scattering to show that ferromagnetism and antiferromagnetism coexist in the low T state of the pyrochlore quantum magnet${\text{Yb}}_2{\text{Ti}}_2{\mathrm{O}}_7$. While magnetic Bragg peaks evidence long-range static ferromagnetic order, inelastic scattering shows that short-range correlated antiferromagnetism is also present. Small-angle neutron scattering provides direct evidence for mesoscale magnetic structure that we associate with metastable antiferromagnetism. Classical Monte Carlo simulations based on exchange interactions inferred from$⟨111⟩$-oriented high-field spin wave measurements confirm that antiferromagnetism is metastable within the otherwise ferromagnetic ground state. The apparent lack of coherent spin wave excitations and strong sensitivity to quenched disorder characterizing${\text{Yb}}_2{\text{Ti}}_2{\mathrm{O}}_7$is a consequence of this multiphase magnetism.

    

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3. Power coupling losses for misaligned and mode-mismatched higher-order Hermite–Gauss modes

   https://doi.org/10.1364/OL.426999  

   Tao, Liu ; Kelley-Derzon, Jessica ; Green, Anna_C ; Fulda, Paul ( May 2021 , Optics Letters)

   This paper analytically and numerically investigates misalignment and mode-mismatch-induced power coupling coefficients and losses as a function of Hermite–Gauss (HG) mode order. We show that higher-order HG modes are more susceptible to beam perturbations when, for example, coupling into optical cavities: the misalignment and mode-mismatch-induced power coupling losses scale linearly and quadratically with respect to the mode indices, respectively. As a result, the mode-mismatch tolerance for the${\mathrm{H}\mathrm{G}}_{3,3}$mode is reduced to a factor of 0.28 relative to the currently used${\mathrm{H}\mathrm{G}}_{0,0}$mode. This is a potential hurdle to using higher-order modes to reduce thermal noise in future gravitational-wave detectors.

    

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4. Shape of particle backscattering in the North Pacific Ocean: the χ factor

   https://doi.org/10.1364/AO.414695  

   Zhang, Xiaodong ; Hu, Lianbo ; Gray, Deric ; Xiong, Yuanheng ( February 2021 , Applied Optics)

   Volume scattering functions were measured using two instruments in waters near the Ocean Station Papa (50°N 145°W) and show consistency in estimating the$\mathrm{\chi <\#comment/>}$factor attributable to particles (${\mathrm{\chi <\#comment/>}}_p$). While${\mathrm{\chi <\#comment/>}}_p$in the study area exhibits a limited variability, it could vary significantly when compared with data obtained in various parts of the global oceans. The global comparison also confirms that the minimal variation of${\mathrm{\chi <\#comment/>}}_p$is at scattering angles near 120°. With an uncertainty of$<<\#comment/>10\mathrm{\%<\#comment/>}$,${\mathrm{\chi <\#comment/>}}_p$can be assumed as spectrally independent. For backscatter sensors with wide field of view (FOV), the averaging of scattering within the FOV reduces the values of${\mathrm{\chi <\#comment/>}}_p$needed to compute the backscattering coefficient by up to 20% at angles$<<\#comment/>{130}^{\circ <\#comment/>}$.

    

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5. Electrically tunable total reflection of light by oblique helicoidal cholesteric

   https://doi.org/10.1557/s43577-024-00723-8  

   Iadlovska, Olena_S ; Thapa, Kamal ; Rajabi, Mojtaba ; Mrukiewicz, Mateusz ; Shiyanovskii, Sergij_V ; Lavrentovich, Oleg_D ( June 2024 , MRS Bulletin)

   An oblique helicoidal state of a cholesteric liquid crystal (Ch_OH) is capable of continuous change of the pitch$$P$$$P$in response to an applied electric field. Such a structure reflects 50% of the unpolarized light incident along the Ch_OHaxis in the electrically tunable band determined by$$P$$$P$/2. Here, we demonstrate that at an oblique incidence of light, Ch_OHreflects 100% of light of any polarization. This singlet band of total reflection is associated with the full pitch$$P$$$P$. We also describe the satellite$$P/2$$$P/2$,$$P/3$$$P/3$, and$$P/4$$$P/4$bands. The$$P/2$$$P/2$and$$P/4$$$P/4$bands are triplets, whereas$$P/3$$$P/3$band is a singlet caused by multiple scatterings at$$P$$$P$and$$P/2$$$P/2$. A single Ch_OHcell acted upon by an electric field tunes all these bands in a very broad spectral range, from ultraviolet to infrared and beyond, thus representing a structural color device with enormous potential for optical and photonic applications.

   Pigments, inks, and dyes produce colors by partially consuming the energy of light. In contrast, structural colors caused by interference and diffraction of light scattered at submicrometer length scales do not involve energy losses, which explains their widespread in Nature and the interest of researchers to develop mimicking materials. The grand challenge is to produce materials in which the structural colors could be dynamically tuned. Among the oldest known materials producing structural colors are cholesteric liquid crystals. Light causes coloration by selective Bragg reflection at the periodic helicoidal structure formed by cholesteric molecules. The cholesteric pitch and thus the color can be altered by chemical composition or by temperature, but, unfortunately, dynamic tuning by electromagnetic field has been elusive. Here, we demonstrate that a cholesteric material in a new oblique helicoidal Ch_OHstate could produce total reflection of an obliquely incident light of any polarization. The material reflects 100% of light within a band that is continuously tunable by the electric field through the entire visible spectrum while preserving its maximum efficiency. Broad electric tunability of total reflection makes the Ch_OHmaterial suitable for applications in energy-saving smart windows, transparent displays, communications, lasers, multispectral imaging, and virtual and augmented reality.

    

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