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1. Polarization-independent photonic Bragg grating filter with cladding asymmetry

   https://doi.org/10.1364/OL.479600  

   Pimbi, Daniel; Hasan, Mehedi; Borhan_Mia, Md; Jaidye, Nafiz; Kim, Sangsik (February 2023, Optics Letters)

   A photonic Bragg grating is a fundamental building block that reflects the direction of wave propagation through spatial phase modulation and can be implemented using sidewall corrugation. However, due to the asymmetric aspect ratio of a waveguide cross section, typical Bragg gratings exhibit a strong polarization sensitivity. Here, we show that photonic Bragg gratings with cladding asymmetry can enable polarization-independent notch filters by rotating input polarizations. Such Bragg gratings strongly couple transverse electric (TE) and transverse magnetic (TM) modes propagating in opposite directions, filtering the input signal and reflecting the rotated mode. We analyzed this polarization-rotating Bragg grating using the coupled-mode theory and experimentally demonstrated it on a silicon-on-insulator platform. Our device concept is simple to implement and compatible with other platforms, readily available as polarization transparent Bragg components. 

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2. High-Efficiency On-Chip Frequency Conversion in the Telecom Ban

   Zhao, Y.; Kim, B.Y.; Ji, X; Okawachi, Y.; Lipson, M.; Gaeta, A.L. (April 2022, CLEO: Science and Innovations 2022)

   Using silicon-nitride microresonators with integrated Moiré-Bragg gratings to suppress parasitic nonlinear processes, we demonstrate on-chip frequency conversion to a single idler tone with a record-high 71% efficiency using Bragg scattering four-wave-mixing. 

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3. Photonic Bragg gratings with cladding asymmetry for polarization independent and rotation filter

   https://doi.org/10.1364/CLEO_AT.2023.JW2A.112  

   Pimbi, Daniel; Mia, Md Borhan; Jaidye, Nafiz; Kim, Sangsik (January 2023, Optica Publishing Group)

   We present a single-etched polarization-independent Bragg filter using cladding asymmetry. An air cladding strongly couples TE and TM modes, rotating its polarization status and achieving polarization-insensitive Bragg reflection at the average of both Bragg wavelengths 

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4. Integrated polarization-free Bragg filters with subwavelength gratings for photonic sensing

   https://doi.org/10.1364/OE.504043  

   Pimbi, Daniel; Mia, Md_Borhan; Jaidye, Nafiz; Ahmed, Ishtiaque; Hasan, Mehedi; Ahmed, Syed_Z; Kim, Sangsik (January 2024, Optics Express)

   We present polarization-free Bragg filters having subwavelength gratings (SWGs) in the lateral cladding region. This Bragg design expands modal fields toward upper cladding, resulting in enhanced light interaction with sensing analytes. Two device configurations are proposed and examined, one with index-matched coupling between transverse electric (TE) and transverse magnetic (TM) modes and the other one with hybrid-mode (HM) coupling. Both configurations introduce a strong coupling between two orthogonal modes (either TE-TM or HM₁-HM₂) and rotate the polarization of the input wave through Bragg reflection. The arrangements of SWGs help to achieve two configurations with different orthogonal modes, while expanding modal profiles toward the upper cladding region. Our proposed SWG-assisted Bragg gratings with polarization independency eliminate the need for a polarization controller and effectively tailor the modal properties, enhancing the potential of integrated photonic sensing applications. 

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5. K-space algorithmic reconstruction (KAREN): a robust statistical methodology to separate Bragg and diffuse scattering

   https://doi.org/10.1107/S1600576719017060  

   Weng, James; Dill, Eric D.; Martin, James D.; Whitfield, Ross; Hoffmann, Christina; Ye, Feng (February 2020, Journal of Applied Crystallography)

   Diffuse scattering occurring in the Bragg diffraction pattern of a long-range-ordered structure represents local deviation from the governing regular lattice. However, interpreting the real-space structure from the diffraction pattern presents a significant challenge because of the dramatic difference in intensity between the Bragg and diffuse components of the total scattering function. In contrast to the sharp Bragg diffraction, the diffuse signal has generally been considered to be a weak expansive or continuous background signal. Herein, using 1D and 2D models, it is demonstrated that diffuse scattering in fact consists of a complex array of high-frequency features that must not be averaged into a low-frequency background signal. To evaluate the actual diffuse scattering effectively, an algorithm has been developed that uses robust statistics and traditional signal processing techniques to identify Bragg peaks as signal outliers which can be removed from the overall scattering data and then replaced by statistically valid fill values. This method, described as a `K-space algorithmic reconstruction' (KAREN), can identify Bragg reflections independent of prior knowledge of a system's unit cell. KAREN does not alter any data other than that in the immediate vicinity of the Bragg reflections, and reconstructs the diffuse component surrounding the Bragg peaks without introducing discontinuities which induce Fourier ripples or artifacts from underfilling `punched' voids. The KAREN algorithm for reconstructing diffuse scattering provides demonstrably better resolution than can be obtained from previously described punch-and-fill methods. The superior structural resolution obtained using the KAREN method is demonstrated by evaluating the complex ordered diffuse scattering observed from the neutron diffraction of a single plastic crystal of CBr 4 using pair distribution function analysis. 

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