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1. On‐Demand Mode Conversion and Wavefront Shaping via On‐Chip Metasurfaces

   https://doi.org/10.1002/adom.202200910  

   Deng, Lin; Xu, Yihao; Jin, Renchao; Cai, Ziqiang; Liu, Yongmin (September 2022, Advanced Optical Materials)

   Abstract In this work, mode conversion and wavefront shaping by integrating a metallic metasurface on top of a planar waveguide are proposed and demonstrated. The metasurface consists of C‐shaped nanoantennas. By controlling the orientation of each nanoantenna, mode conversion and focusing effect for the cross‐polarized electric fields inside the waveguide are achieved. The design and simulation results of 16 scenarios of wideband transverse‐magnetic to transverse‐electric mode converters with the mode purity up to 98%, and on‐chip lenses at the wavelength of 1550 nm are reported. It is worth noting that the dimension of the devices along the propagation direction is only 9.6 µm. This work manifests the potential application of mode division multiplexing systems and on‐chip optical interconnections based on metasurfaces. 

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2. Mode-evolution-based ultra-broadband polarization beam splitter using adiabatically tapered extreme skin-depth waveguide

   https://doi.org/10.1364/OL.434110  

   Mia, Md_Borhan; Ahmed, Syed_Z; Jaidye, Nafiz; Ahmed, Ishtiaque; Kim, Sangsik (September 2021, Optics Letters)

   We present an ultra-broadband silicon photonic polarization beam splitter (PBS) using adiabatically tapered extreme skin-depth (eskid) waveguides. Highly anisotropic metamaterial claddings of the eskid waveguides suppress the crosstalk of transverse-electric (TE) mode, while the large birefringence of the eskid waveguide efficiently cross-couples the transverse-magnetic (TM) mode. Two eskid waveguides are adiabatically tapered to smoothly translate TM mode to the coupled port via mode evolution while keeping the TE mode in the through port. The tapered cross-section of the eskid PBS was designed numerically, achieving a large bandwidth at 1400–1650 nm with extinction ratios $><\#comment/>20\mathrm{d}\mathrm{B}$. We experimentally demonstrated the tapered-eskid PBS and confirmed its broad bandwidth at 1490–1640 nm, limited by laser bandwidth. With its mode evolution, the tapered-eskid PBS is tolerant to fabrication imperfections and should be crucial for controlling polarizations in photonic circuits. 

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3. Octave-spanning microcomb generation in 4H-silicon-carbide-on-insulator photonics platform

   https://doi.org/10.1364/PRJ.449267  

   Cai, Lutong; Li, Jingwei; Wang, Ruixuan; Li, Qing (March 2022, Photonics Research)

   Silicon carbide has recently emerged as a promising photonics material due to its unique properties, including possessing strong second- and third-order nonlinear coefficients and hosting various color centers that can be utilized for a wealth of quantum applications. Here, we report the design and demonstration of octave-spanning microcombs in a 4H-silicon-carbide-on-insulator platform for the first time, to our knowledge. Such broadband operation is enabled by optimized nanofabrication achieving $>1$million intrinsic quality factors in a 36-μm-radius microring resonator, and careful dispersion engineering by investigating the dispersion properties of different mode families. For example, for the fundamental transverse-electric mode whose dispersion can be tailored by simply varying the microring waveguide width, we realized a microcomb spectrum covering the wavelength range from 1100 nm to 2400 nm with an on-chip power near 120 mW. While the observed comb state is verified to be chaotic and not soliton, attaining such a large bandwidth is a crucial step towards realizing $f-2f$self-referencing. In addition, we also observed a coherent soliton-crystal state for the fundamental transverse-magnetic mode, which exhibits stronger dispersion than the fundamental transverse-electric mode and hence a narrower bandwidth. 

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4. 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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5. Wideband Microstrip Patch Antennas with Transverse Electric Modes

   https://doi.org/10.47037/2020.ACES.J.350817  

   Mitha, Tanzeela; Pour, Maria (October 2020, Applied Computational Electromagnetics Society)

   null (Ed.)

   A wideband microstrip patch antenna, exciting the fundamental transverse electric (TE) mode, is investigated. The excitation of the TE mode is facilitated through replacing both of the patch and ground plane of a conventional microstrip antenna with artificial magnetic conductors (AMC), consisting of unipolar compact photonic bandgap (UC-PBG) unit cells. The AMC patch and the ground plane of this antenna behave as magnetic conductors within the bandgap region of the unit cells. Similar to conventional patch antennas, it is shown that by cutting a U-shaped slot in the AMC patch, wideband characteristics are realized. The antenna shows a 40% impedance bandwidth and operates at the TE10 mode. Moreover, the width of the patch is 1.75 times smaller than its length, reducing the overall size of the antenna by about 60%, compared with the conventional U-slot PEC antenna supporting the transverse magnetic (TM) mode. 

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