Time-Varying Reactive Elements for Extreme Accumulation of Electromagnetic Energy

Accumulation of energy by reactive elements is limited by the amplitude of time-harmonic external sources. In the steady-state regime, all incident power is fully reflected back to the external source, and the stored energy does not increase in time, although the external source continuously supplies energy. Here we show that this limitation can be removed if the reactive element is varying in time, properly modulated by an additional active but lossless device. We show that such time-varying lossless loads of a transmission line or lossless metasurfaces can accumulate electromagnetic energy supplied by a time-harmonic source continuously in time without any theoretical limit. We analytically derive the required time dependence of the load reactance and show that it can, in principle, be realized as a series connection of mixers and filters. Furthermore, we prove that by properly designing time-varying LC circuits, one can arbitrarily engineer the time dependence of the electric current in the circuit fed by a given time-harmonic external source. As an example, we theoretically demonstrate a circuit with a linearly increasing electric current through the inductor. Calculating the energy delivered to the LC circuit and the energy released from it, we show that a modulated LC circuit can accumulate huge energy from both the time-harmonic external source and the modulating device. Finally, we discuss how this stored energy can be released in the form of a time-compressed pulse.