Not yet reviewed by Pith; the record is open.

Rapid modulation of the electromagnetic response in both time and space creates temporal boundaries in the medium and leads to time-reflection and time-refraction of light and to the eventual formation of the photonic time crystal within the modulated optical material,, offering a new regime of light-matter interactions and a potential for practical applications, from non-resonant light amplification to tunable lasing. However, the conventional approach commonly used for the photonic time crystals and related phenomena that relies on the concept of effective time-dependent refractive index, is fundamentally unsuitable to this domain of ultra-fast modulation at optical time scales. We develop the appropriate theoretical description of the electromagnetic response and the resulting phenomena in this regime, and demonstrate not only quantitative but also qualitative differences from the conclusions obtained using the conventional approach.