Although photonic time crystals and time-varying metamaterials have been widely studied theoretically and numerically, experimental progress at optical frequencies has so far been limited. In this talk, we show that, despite the often-invoked space-time duality, the characteristics of the temporal dimension, particularly its relation to causality and energy conservation, can lead to various challenges and physical constraints that are unique to time-varying systems. Specifically, we discuss how temporal dispersion can lower the effective temporal modulation strength seen by the probe wave and we show that realizing large variations of the material permittivity in time may require large amounts of energy.
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