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1. Degenerate Band Edge Resonances in Air-filled Substrate Integrated Waveguide

   https://doi.org/10.23919/EuCAP48036.2020.9135497  

   Zheng, Tianyu; Casaletti, Massimiliano; Abdelshafy, Ahmed F.; Capolino, Filippo; Ren, Zhuoxiang; Valerio, Guido (March 2020, Proc. of 14th European Conference on Antennas and Propagation (EuCAP))

   The degenerate band edge (DBE) is a special fourth-order degenerate point in a dispersion diagram, where four eigenmodes coalesce to a single degenerate eigenmode. It leads to field enhancement of the Bloch mode and to high quality factors, which are useful for high-Q resonators, oscillators and ultrasensitive sensors. The air-filled substrate integrated waveguide (AFSIW) is a novel form of SIW which is low cost and low loss. We propose a design of an AFSIW supporting a degenerate band edge (DBE). We show the occurrence of the so-called “giant resonance” associated to the DBE and we study how losses influence the DBE. 

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2. General Conditions to Realize Exceptional Points of Degeneracy in Two Uniform Coupled Transmission Lines

   https://doi.org/10.1109/TMTT.2020.2999498  

   Mealy, Tarek; Capolino, Filippo (June 2020, IEEE Transactions on Microwave Theory and Techniques)

   We present the general conditions to realize a fourth-order exceptional point of degeneracy (EPD) in two uniform (i.e., invariant along z) lossless and gainless coupled transmission lines (CTLs), namely, a degenerate band edge (DBE). Until now the DBE has been shown only in periodic structures. In contrast, the CTLs considered here are uniform and subdivided into four cases where the two TLs support combinations of forward propagation, backward propagation, and evanescent modes (when neglecting the mutual coupling). We demonstrate, for the first time, that a DBE is supported in uniform CTLs when there is proper coupling between: 1) propagating modes and evanescent modes, 2) forward and backward propagating modes, or 3) four evanescent modes (two in each direction). We also show that the loaded quality factor of uniform CTLs exhibiting a fourth-order EPD at k=0 is robust to series losses due to the fact that the degenerate modes do not advance in phase. We also provide a microstrip possible implementation of a uniform CTL exhibiting a DBE using periodic series capacitors with very subwavelength unit-cell length. Finally, we show an experimental verification of the existence DBE for a microstrip implementation of a CTL supporting coupled propagating and evanescent modes. 

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3. High-Q Substrate-Integrated-Waveguide Resonator with Degenerate Band Edge

   https://doi.org/10.1109/ICEAA.2019.8879299  

   Zheng, Tianyu; Casaletti, Massimiliano; Ren, Zhuoxiang; Abdelshafy, Ahmed F.; Capolino, Filippo; Valerio, Guido (September 2019, Published in: 2019 International Conference on Electromagnetics in Advanced Applications (ICEAA))

   The novel concept of degenerate band edge (DBE) has been recently proposed by Figotin et al., in the framework of their study of slow wave propagation in photonic crystals [1]. The degenerate band edge is a special dispersion condition near the edge of the Brillouin zone ( kd=π/p, p is the spatial period) where four degenerate Bloch modes coalesce at a same frequency fd (two propagating modes and two evanescent modes). The dispersion relation is locally described by a quartic curve f−fd=−α(k−kd) where fd is the DBE frequency and α is a positive constant that depends on the parameters of the structure. 

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4. Exceptional Points of Degeneracy in Electromagnetic Periodic Waveguides and the Role of Symmetries

   https://doi.org/10.23919/EuCAP53622.2022.9769037  

   Mealy, Tarek; Nada, Mohamed Y.; Abdelshafy, Ahmed F.; Hafezi, Ehsan; Capolino, Filippo (March 2022, Published in: 2022 16th European Conference on Antennas and Propagation (EuCAP))

   We investigate the role of reflection and glide symmetry in periodic lossless waveguides on the dispersion diagram and on the existence of various orders of exceptional points of degeneracy (EPDs). We use an equivalent circuit network to model each unit-cell of the guiding structure. Assuming that a coupled-mode waveguide supports N modes in each direction, we derive the following conclusions. When N is even, we show that a periodic guiding structure with reflection symmetry may exhibit EPDs of maximum order N . To obtain a degenerate band edge (DBE) with only two coupled guiding structures, reflection symmetry must be broken. For odd N,N+1 is the maximum EPD order that may be obtained, and an EPD of order N is not allowed. We present an example of three coupled microstrip transmission lines and show that breaking the reflection symmetry by introducing glide symmetry enables the occurrence of a stationary inflection point (SIP), also called frozen mode, which is an EPD of order three. 

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5. Ultrafast photoinduced band splitting and carrier dynamics in chiral tellurium nanosheets

   https://doi.org/10.1038/s41467-020-17766-5  

   Jnawali, Giriraj; Xiang, Yuan; Linser, Samuel M.; Shojaei, Iraj Abbasian; Wang, Ruoxing; Qiu, Gang; Lian, Chao; Wong, Bryan M.; Wu, Wenzhuo; Ye, Peide D.; et al (August 2020, Nature Communications)

   Abstract Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-splitH₄andH₅and the degenerateH₆valence bands (VB) and the lowest degenerateH₆conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of theH₆CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations, we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along the c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties. 

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