Diagrammatic Explanation of the Reverse Doppler Effect in Space-Time   Modulated Photonic Crystals
  An inverse Doppler shift occurs in a photonic crystal (PC) bounded by a moving wall. The interpretation of this result has stirred some controversy. In this paper, we address the problem using a diagrammatic approach. This visual representation provides immediate insight into the phenomenon, and is a powerful tool for the design of time-varying PCs. 
Zo\'e-Lise Deck-L\'eger, Maksim Skorobogatiy, Christophe Caloz
physics.optics