High Power Microwave and Optical Volume Free Electron Lasers (VFELs)
  The use of a non-one-dimensional distributed feedback, arising through Bragg diffraction in spatially periodic systems (natural and artificial (electromagnetic, photonic) crystals), forms the foundation for the development of volume free electron lasers (VFELs). The present review addresses the basic principles of VFEL theory and describes the promising potential of VFELs as the basis for the development of high-power microwave and optical sources. 
Vladimir Baryshevsky
physics.optics physics.acc-ph