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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. 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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3. The Kondo effect in the quantum XX spin chain

   https://doi.org/10.1088/1751-8121/ad51bb  

   Kattel, Pradip; Tang, Yicheng; Pixley, J H; Andrei, Natan (June 2024, Journal of Physics A: Mathematical and Theoretical)

   Abstract We investigate the boundary phenomena that arise in a finite-sizeXXspin chain interacting through anXXinteraction with a spin $-\frac{1}{2}$impurity located at its edge. Upon Jordan–Wigner transformation, the model is described by a quadratic Fermionic Hamiltonian. Our work displays, within this ostensibly simple model, the emergence of the Kondo effect, a quintessential hallmark of strongly correlated physics. We also show how the Kondo cloud shrinks and turns into a single particle bound state as the impurity coupling increases beyond a critical value. In more detail, using bothBethe Ansatzandexact diagonalizationtechniques, we show that the local moment of the impurity is screened by different mechanisms depending on the ratio of the boundary and bulk coupling $\frac{J_{\mathrm{imp}}}{J}$. When the ratio falls below the critical value $\sqrt{2}$, the impurity is screened via the Kondo effect. However, when the ratio between the coupling exceeds the critical value $\sqrt{2}$an exponentially localized bound mode is formed at the impurity site which screens the spin of the impurity in the ground state. We show that the boundary phase transition is reflected in local ground state properties by calculating the spinon density of states, the magnetization at the impurity site in the presence of a global magnetic field, and the finite temperature susceptibility of the impurity. We find that the spinon density of states in the Kondo phase has the characteristic Lorentzian peak that moves from the Fermi level to the maximum energy of the spinon as the impurity coupling is increased and becomes a localized bound mode in the bound mode phase. Moreover, the impurity magnetization and the finite temperature impurity susceptibility behave differently in the two phases. When the boundary coupling $J_{\mathrm{imp}}$exceeds the critical value $\sqrt{2}J$, the model is no longer boundary conformal invariant as a massive bound mode appears at the impurity site. 

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4. Stabilization of three-dimensional charge order through interplanar orbital hybridization in PrxY1−xBa2Cu3O6+δ

   https://doi.org/10.1038/s41467-022-33607-z  

   Ruiz, Alejandro; Gunn, Brandon; Lu, Yi; Sasmal, Kalyan; Moir, Camilla M.; Basak, Rourav; Huang, Hai; Lee, Jun-Sik; Rodolakis, Fanny; Boyle, Timothy J.; et al (October 2022, Nature Communications)

   Abstract The shape of 3d-orbitals often governs the electronic and magnetic properties of correlated transition metal oxides. In the superconducting cuprates, the planar confinement of the$${d}_{{x}^{2}-{y}^{2}}$$ $d_{x^2-y^2}$orbital dictates the two-dimensional nature of the unconventional superconductivity and a competing charge order. Achieving orbital-specific control of the electronic structure to allow coupling pathways across adjacent planes would enable direct assessment of the role of dimensionality in the intertwined orders. Using CuL₃and PrM₅resonant x-ray scattering and first-principles calculations, we report a highly correlated three-dimensional charge order in Pr-substituted YBa₂Cu₃O₇, where the Prf-electrons create a direct orbital bridge between CuO₂planes. With this we demonstrate that interplanar orbital engineering can be used to surgically control electronic phases in correlated oxides and other layered materials. 

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5. Thomson and collisional regimes of in-phase coherent microwave scattering off gaseous microplasmas

   https://doi.org/10.1038/s41598-021-02500-y  

   Patel, Adam R.; Ranjan, Apoorv; Wang, Xingxing; Slipchenko, Mikhail N.; Shneider, Mikhail N.; Shashurin, Alexey (December 2021, Scientific Reports)

   Abstract The total number of electrons in a classical microplasma can be non-intrusively measured through elastic in-phase coherent microwave scattering (CMS). Here, we establish a theoretical basis for the CMS diagnostic technique with an emphasis on Thomson and collisional scattering in short, thin unmagnetized plasma media. Experimental validation of the diagnostic is subsequently performed via linearly polarized, variable frequency (10.5–12 GHz) microwave scattering off laser induced 1–760 Torr air-based microplasmas (287.5 nm O₂resonant photoionization by ~ 5 ns, < 3 mJ pulses) with diverse ionization and collisional features. Namely, conducted studies include a verification of short-dipole-like radiation behavior, plasma volume imaging via ICCD photography, and measurements of relative phases, total scattering cross-sections, and total number of electrons$$N_{e}$$ $N_e$in the generated plasma filaments following absolute calibration using a dielectric scattering sample. Findings of the paper suggest an ideality of CMS in the Thomson “free-electron” regime—where a detailed knowledge of plasma and collisional properties (which are often difficult to accurately characterize due to the potential influence of inhomogeneities, local temperatures and densities, present species, and so on) is unnecessary to extract$$N_{e}$$ $N_e$from the scattered signal. The Thomson scattering regime of microwaves is further experimentally verified via measurements of the relative phase between the incident electric field and electron displacement. 

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