Transverse electric scattering on inhomogeneous objects: spectrum of integral operator and preconditioning

The domain integral equation method with its FFT-based matrix-vector products is a viable alternative to local methods in free-space scattering problems. However, it often suffers from the extremely slow convergence of iterative methods, especially in the transverse electric (TE) case with large or negative permittivity. We identify the nontrivial essential spectrum of the pertaining integral operator as partly responsible for this behavior, and the main reason why a normally efficient deflating preconditioner does not work. We solve this problem by applying an explicit multiplicative regularizing operator, which transforms the system to the form `identity plus compact', yet allows the resulting matrix-vector products to be carried out at the FFT speed. Such a regularized system is then further preconditioned by deflating an apparently stable set of eigenvalues with largest magnitudes, which results in a robust acceleration of the restarted GMRES under constraint memory conditions.Comment: 20 pages, 8 figure

Highly Directional Scattering of Terahertz Radiation by Cylinders using Complex-Frequency Waves

In this study we investigate the directional scattering of terahertz radiation by dielectric cylinders, focusing on the enhancement of directionality using incident radiation of complex-frequency. We explore the optimization of the second Kerker condition, which corresponds to backward scattering. At first, by carefully tailoring the electric and magnetic polarizabilities of the cylinders, we successfully achieve significant backward scattering, and then manage to even further improve it by deploying a decaying incoming wave (\textit{complex}-frequency). Additionally, we present preliminary results on the directional scattering of THz radiation by a magneto-optical cylinder, demonstrating the potential of this approach for advanced control over the propagation of THz waves. Our findings contribute to a deeper understanding of THz directional scattering and pave the way for the development of novel THz devices and applications, such as high-resolution imaging, sensing, and communication systems

Active THz metasurfaces for compact isolation

Metasurfaces constitute an emerging technology, allowing for compact manipulation of all degrees of freedom of an incident lightwave. A key ongoing challenge in the design of these structures is how to allow for energy-efficient dynamic (active) operation, particularly for the polarization of incident light, which other standard devices typically cannot efficiently act upon. Here, we present a quasi-two-dimensional magneto-optic metasurface capable of simultaneously high-contrast on/off operation, as well as rotation of the polarization angle of a linearly polarized wave-that is, without converting the incident linear polarization to elliptical, which is normally particularly challenging. Furthermore, the device's operation is broadband, with a bandwidth of around 5 mu m, and can be conveniently manipulated using an external magnetic bias. Our findings, corroborated using two different full-wave simulation approaches, may allow for functional metasurfaces operating in the terahertz (THz) regime, giving rise to robust, energy-efficient, and high-dynamic-range broadband isolation, to be used for a wealth of optoelectronic and communication applications. (C) 2021 Optical Society of Americ