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Register a new userFinite-Difference Time-Domain simulation of spacetime cloak
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In this work, we present a numerical method that remedies the instabilities of the conventional FDTD approach for solving Maxwells equations in a space-time dependent magneto-electric medium with direct application to the simulation of the recently proposed spacetime cloak. We utilize a dual grid FDTD method overlapped to the time domain to provide a stable approach for the simulation of magneto-electric medium with time and space varying permittivity, permeability and coupling coefficient. The developed method can be applied to explore other new physical possibilities offered by spacetime cloaking, metamaterials, and transformation optics.
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This paper proposes a radial dependent dispersive finite-difference time-domain method for the modelling of electromagnetic cloaking structures. The permittivity and permeability of the cloak are mapped to the Drude dispersion model and taken into account in dispersive FDTD simulations. Numerical simulations demonstrate that under ideal conditions, objects placed inside the cloak are `invisible to external electromagnetic fields. However for the simplified cloak based on linear transformations, the back scattering has a similar level to the case of a PEC cylinder without any cloak, rendering the object still being `visible. It is also demonstrated numerically that the simplified cloak based on high-order transformations can indeed improve the cloaking performance.
This letter is devoted to point out a specific character of the Finite-Difference-Time-Domain method through the study of nano-structures supporting geometrical symmetry-protected modes that can not be excited at certain conditions of illumination. The spatial discretization performed in the FDTD algorithm naturally leads to break this symmetry and allows the excitation of these modes. The quality factor of the corresponding resonances are then directly linked to the degree of the symmetry breaking i.e. the spatial grid dimension even though the convergence criteria of the FDTD are fulfilled. This finding shows that the FDTD must be handled with a great care and, more importantly, that very huge quality-factor resonances could be achieved at the cost of nanometer-scale mastered fabrication processes.
A kind of transformation media, which we shall call the anti-cloak, is proposed to partially defeat the cloaking effect of the invisibility cloak. An object with an outer shell of anti-cloak is visible to the outside if it is coated with the invisible cloak. Fourier-Bessel analysis confirms this finding by showing that external electromagnetic wave can penetrate into the interior of the invisibility cloak with the help of the anti-cloak.
A radial-dependent dispersive finite-difference time-domain (FDTD) method is proposed to simulate electromagnetic cloaking devices. The Drude dispersion model is applied to model the electromagnetic characteristics of the cloaking medium. Both lossless and lossy cloaking materials are examined and their operating bandwidth is also investigated. It is demonstrated that the perfect invisibility from electromagnetic cloaks is only available for lossless metamaterials and within an extremely narrow frequency band.
The stable operation of gas networks is an important optimization target. While for this task commonly finite volume methods are used, we introduce a new finite difference approach. With a summation by part formulation for the spatial discretization, we get well-defined fluxes between the pipes. This allows a simple and explicit formulation of the coupling conditions at the node. From that, we derive the adjoint equations for the network simply and transparently. The resulting direct and adjoint equations are numerically efficient and easy to implement. The approach is demonstrated by the optimization of two sample gas networks.
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