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During the last decade, possibilities to realize new phenomena and create new applications by varying system properties in time have gained increasing attention in many research fields. Although the interest in using time-modulation techniques for engineering electromagnetic (EM) response has become revitalized only in recent years, the field originates from the middle of the previous century, and a multitude of works have been published ever since. In this tutorial article, we provide a historical picture and review the basic concepts in this field. In particular, we introduce the general theory of linear time-varying (LTV) systems and discuss the means to properly account for frequency dispersion of nonstationary systems. Also, we elucidate models of time-varying electrical circuits and materials and discuss some useful effects that can be achieved by time modulation of circuit or material parameters.
Ptitcyn et al. (Mon,) studied this question.