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1. A Penetrable Reactance Surface for Spherical Invisibility Cloaking

   Lee, Hakjune; Kwon, Do-Hoon (July 2024, 2024 IEEE International Symposium on Antennas and Propagation)

   A penetrable tensorial metasurface on a grounded dielectric shell is presented for 3-D spherical invisibility cloaking. The spatially modulated impedance surface transforms the incident plane wave into a surface wave on the lit side and carries the power to the shadow side. Power is continuously released as a leaky wave with a wavefront consistent with the incident plane wave on the shadow side. A numerical design example for a four-wavelength-diameter conducting sphere is presented, and the cloaking effectiveness is validated by simulation. 

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2. Accurate Modeling and Rapid Synthesis Methods for Beamforming Metasurfaces

   https://doi.org/10.1109/APS/URSI47566.2021.9704077  

   Budhu, Jordan; Szymanski, Luke; Grbic, Anthony (December 2021, 2021 IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting (APS/URSI))

   A general synthesis technique for beamforming metasurfaces is presented which utilizes accurate modeling techniques and rapid optimization methods. The metasurfaces considered consist of patterned metallic claddings supported by finite grounded dielectric substrates. The metasurfaces are modeled using integral equations which accurately account for all mutual coupling and finite dimensions. A beamforming metasurface is designed in three phases: an initial Direct Solve phase involving the solution of the integral equation via the method of moments to obtain a complex-valued initial design satisfying the desired far-field beam specifications, a subsequent Optimization phase to convert the complex-valued metasurface into a purely reactive metasurface, and a final Patterning phase to realize the metasurface as a patterned metallic cladding. The metasurface is optimized using an adjoint optimization method. The method calculates the gradient of the cost function in only two forward problem solutions. An example metasurface designed using this approach is presented. 

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3. Conformal irregular-grid metasurfaces comprising a dense array of free-standing dipoles

   Kwon, Do-Hoon (July 2022, 2022 IEEE International Symposium on Antennas and Propagation)

   For large or infinite metasurfaces, a design tech- nique for a dense array of subwavelength resonators on an irregular grid is presented. For a given incident wave, the desired induced dipole moment distribution determines the local electric field that excites individual meta-atoms. The interaction field that accounts for mutual coupling is evaluated via a combination of discrete coupling from nearby resonators and continuous sheet current coupling from far-separated resonators. Meta-atoms placed on an irregular grid can be treated, greatly enhancing the flexibility in surface profile in practical conformal metasurfaces. 

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4. Quasi-periodic travelling gravity–capillary waves

   https://doi.org/10.1017/jfm.2021.28  

   Wilkening, Jon; Zhao, Xinyu (May 2021, Journal of Fluid Mechanics)

   We present a numerical study of spatially quasi-periodic travelling waves on the surface of an ideal fluid of infinite depth. This is a generalization of the classic Wilton ripple problem to the case when the ratio of wavenumbers satisfying the dispersion relation is irrational. We propose a conformal mapping formulation of the water wave equations that employs a quasi-periodic variant of the Hilbert transform to compute the normal velocity of the fluid from its velocity potential on the free surface. We develop a Fourier pseudo-spectral discretization of the travelling water wave equations in which one-dimensional quasi-periodic functions are represented by two-dimensional periodic functions on the torus. This leads to an overdetermined nonlinear least-squares problem that we solve using a variant of the Levenberg–Marquardt method. We investigate various properties of quasi-periodic travelling waves, including Fourier resonances, time evolution in conformal space on the torus, asymmetric wave crests, capillary wave patterns that change from one gravity wave trough to the next without repeating and the dependence of wave speed and surface tension on the amplitude parameters that describe a two-parameter family of waves. 

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5. Programming nonreciprocity and harmonic beam steering via a digitally space-time-coded metamaterial antenna

   https://doi.org/10.1038/s41598-023-34195-8  

   Vosoughitabar, Shaghayegh; Wu, Chung-Tse Michael (December 2023, Scientific Reports)

   Abstract Recent advancement in digital coding metasurfaces incorporating spatial and temporal modulation has enabled simultaneous control of electromagnetic (EM) waves in both space and frequency domains by manipulating incident EM waves in a transmissive or reflective fashion, resulting in time-reversal asymmetry. Here we show in theory and experiment that a digitally space-time-coded metamaterial (MTM) antenna with spatiotemporal modulation at its unit cell level can be regarded as a radiating counterpart of such digital metasurface, which will enable nonreciprocal EM wave transmission and reception via surface-to-leaky-wave transformation and harmonic frequency generation. Operating in the fast wave (radiation) region, the space-time-coded MTM antenna is tailored in a way such that the propagation constant of each programmable unit cell embedded with varactor diodes can toggle between positive and negative phases, which is done through providing digital sequences by using a field-programmable gate array (FPGA). Owing to the time-varying coding sequence, harmonic frequencies are generated with different main beam directions. Furthermore, the space time modulation of the digitally coded MTM antenna allows for nonreciprocal transmission and reception of EM waves by breaking the time-reversal symmetry, which may enable many applications, such as simultaneous transmitting and receiving, unidirectional transmission, radar sensing, and multiple-input and multiple-output (MIMO) beamformer. 

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