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1. Broadband subwavelength imaging of flexural elastic waves in flat phononic crystal lenses

   https://doi.org/10.1038/s41598-023-34314-5  

   Danawe, Hrishikesh; Tol, Serife (May 2023, Scientific Reports)

   Abstract Subwavelength imaging of elastic/acoustic waves using phononic crystals (PCs) is limited to a narrow frequency range via the two existing mechanisms that utilize either the intense Bragg scattering in the first phonon band or negative effective properties (left-handed material) in the second (or higher) phonon band. In the first phonon band, the imaging phenomenon can only exist at frequencies closer to the first Bragg band gap where the equal frequency contours (EFCs) are convex. Whereas, for the left-handed materials, the subwavelength imaging is restricted to a narrow frequency region where wave vectors in PC and background material are close to each other, which is essential for single-point image formation. In this work, we propose a PC lens for broadband subwavelength imaging of flexural waves in plates exploiting the second phonon band and the anisotropy of a PC lattice for the first time. Using a square lattice design with square-shaped EFCs, we enable the group velocity vector to always be perpendicular to the lens interface irrespective of the frequency and incidence angle; thus, resulting in a broadband imaging capability. We numerically and experimentally demonstrate subwavelength imaging using this concept over a significantly broadband frequency range. 

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2. Generalized Class-J Theory

   https://doi.org/10.1109/LAMC.2018.8699065  

   Roblin, Patrick; Chang, Hsiu-Chen; Gomez-Perez, Jose-Maria; Martinez-Lopez, Jose I. (December 2018, 2018 IEEE MTT-S Latin America Microwave Conference (LAMC 2018))

   Class J has introduced a new rigorous framework for designing broadband PAs. However, the design of broadband Class-J amplifiers remains challenging, as the fundamental impedance termination is non-Foster when the 2 nd harmonic is Foster. In addition the peak instantaneous voltage varies by up to 50% as the frequency changes. In this paper, a generalized Class-J theory is introduced. Then a DC tunable broadband PA with constant instantaneous peak voltage and Foster fundamental and harmonic terminations is proposed. 

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3. Broadband second harmonic generation of counter-propagating ultrashort pulses

   https://doi.org/10.1364/OE.458570  

   Lytle, Amy_L; Dyke, Eric; Novella, Julia; Branch, Thomas; Gagnon, Etienne (May 2022, Optics Express)

   Counter-propagating ultrafast pulses can disrupt the phase of harmonic generation, and offer a means to achieve quasi-phase matching in processes like high-order harmonic generation. Optimizing this process requires accurate modeling. Using second harmonic generation (SHG) as a simpler and more accessible proxy, we compare the results of two numerical simulations to experimental measurements of SHG with counter-propagating pulses. The first follows previous theoretical work in assuming a quasi-CW pulse and solving the nonlinear wave equation in the time-domain. However, we find that adapting a frequency-domain model to account for the broadband nature of ultrafast pulses better reproduces the salient features we observe in our experimental results. 

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4. Discharge Directionality and Dominance of Right-Handed Modes in Helicon Plasmas due to Radial Electron Density Gradients

   Granetzny, Marcel; Zepp, Michael; Schmitz, Oliver (December 2022, arXivorg)

   Helicon waves are magnetized plasma waves, similar to whistler waves in Earth's ionosphere, that are used to create high-density laboratory plasmas. We demonstrate that the discharge direction can be reversed by changing the antenna helicity or the magnetic field direction. Simulations reproduce these findings if a radial density gradient exists. A helicon wave equation that includes such a density gradient gives rise to a modulating magnetic field that amplifies right-handed but attenuates left-handed helicon modes. This explains for the first time consistently the dominance of right-handed over left-handed modes and the discharge directionality in helicon plasmas. 

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5. On-chip non-Hermitian optical parametric amplifiers with a large bandwidth

   https://doi.org/10.1364/JOSAB.426956  

   Ahmed, Asif; Meng, Xiang; Zhong, Qi; El-Ganainy, Ramy; Osgood, Richard_M; Dadap, Jerry_I (June 2021, Journal of the Optical Society of America B)

   Recently, our groups have introduced the notion of optical parametric amplification based on non-Hermitian phase matching wherein the incorporation of loss can lead to gain in this nonlinear optical process. Previous simulation results using second-order nonlinear optical coupled-mode theory have demonstrated the potential of this technique as an alternative to the stringent phase-matching condition, which is often difficult to achieve in semiconductor platforms. Here we fortify this notion for the case of third-order nonlinearity by considering parametric amplification in silicon nanowires and illustrate the feasibility of these devices by employing rigorous finite-difference time-domain analysis using realistic materials and geometric parameters. Particularly, we demonstrate that by systematic control of the optical loss of the idler in a four-wave mixing process, we can achieve efficient unidirectional energy conversion from the pump to the signal component even when the typical phase-matching condition is violated. Importantly, our simulations show that a signal gain of $\sim <\#comment/>9\mathrm{d}\mathrm{B}$for a waveguide length of a few millimeters is possible over a large bandwidth of several hundreds of nanometers ( $\sim <\#comment/>600\mathrm{n}\mathrm{m}$). This bandwidth is nearly 2 orders of magnitude larger than what can be achieved in the conventional silicon-photonics-based four-wave mixing process. 

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