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Energy conversion in a physical system requires time-translation invariance breaking according to Noether's theorem. Closely associated with this symmetry-conservation relation, the frequencies of electromagnetic waves are found to be converted as the waves propagate through a temporally varying medium. Thus, effective temporal control of the medium, be it artificial or natural, through which the waves are propagating, lies at the heart of linear frequency conversion. Here, we explain the basic principle of linear frequency conversion in a rapidly time-variant metadevice and show various interesting properties and future prospects of rapidly time-variant metadevices.
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