Metasurface virtual absorbers: unveiling operative conditions through equivalent lumped circuit model

Abstract

Virtual absorption concept has been recently introduced as a new phenomenon observed in electromagnetics and optics consisting of theoretically unlimited accumulation of energy within a finite volume of material without dissipation. The anomalous behaviour is achieved by engaging the complex zero scattering eigenmodes of the virtual absorbing system by illuminating it with a proper complex frequency ω = ω r + jω i , whose value is strictly determined by the system characteristics. In this paper, we investigate on the position of the zero-pole scattering pairs in the complex frequency plane as a function of the input impedance of the metasurface-based lossless virtual absorber. We analytically derive the conditions under which a properly modulated monochromatic plane wave can be virtually absorbed by the system and stored within its volume. The analysis is developed by modelling the propagation of a normally impinging plane wave through its equivalent transmission line model terminated in an arbitrary reactive load, which in turn models the input impedance of the metasurface-based system under consideration. The study allows to determine a priori whether the metasurface-based system can support the virtual absorption or not by evaluating the time-constant from its equivalent circuit.

Intelligence Enabled by 2D Metastructures in Antennas and Wireless Propagation Systems

“…During the zero scattering excitation, the signal is completely accumulated inside the free-space cavity and no reflections occur as far as the proper temporally shaped signal excites the structure. We have then demonstrated that virtual perfect absorption can occur under peculiar conditions that are related to values of the metasurface surface reactance and the cavity input reactance [132], [133]. First of all, the two reactances have to show an opposite electric behaviour, that is, .…”

“…First of all, the two reactances have to show an opposite electric behaviour, that is, . Otherwise, scattering zeros will be purely imaginary; this condition cannot lead to free-space propagation, since the signal is static and exponentially growing [132]. This approach, instead, can be used to guided propagation, paving the way to energy storing in circuital purely reactive loads [134]- [136].…”

Perfect Matching of Reactive Loads Through Complex Frequencies: From Circuital Analysis to Experiments

“…A decade ago, the use of temporal profile of incoming signal has been exploited in [36]- [38] for enabling efficient coupling in single atoms and optical resonators. More recently, the same concept has been moved in optics, for enabling energy virtual absorption in lossless systems, making them appear as matched to free space if complex zeros of its scattering eigenmodes are excited [39]- [45], reporting also elastodynamic experimental proof in mechanical systems [46]. In particular, we have investigated in [45] the possibility to enable virtual absorption effects in metasurface-bounded cavities.…”

“…More recently, the same concept has been moved in optics, for enabling energy virtual absorption in lossless systems, making them appear as matched to free space if complex zeros of its scattering eigenmodes are excited [39]- [45], reporting also elastodynamic experimental proof in mechanical systems [46]. In particular, we have investigated in [45] the possibility to enable virtual absorption effects in metasurface-bounded cavities. The analysis of this problem led to the derivation of the complex zeros, and the corresponding limit values of the surface impedances of the metasurface.…”

“…2(c) and (d), respectively. The parallel connection of reactive loads has been already discussed in our recent work [45], where fundamental limits of virtual perfect absorption have been analyzed. Nevertheless, this study included preliminary analytic expressions, and it was related to free-space propagation, focusing on energy accumulation of the structure.…”