Role of photonic crystals used in laser acceleration and in the free electron laser

To investigate the possibility of using photonic crystals for charge acceleration, the intensity of the diffracted evanescent light is calculated for a p-polarized plane-wave light incident on a slab photonic crystal of arrayed spheres. The intensity as a function of the light frequency is shown to have a characteristic resonant enhancement, each time a photonic band is excited by the incident light. As the thickness of the slab increases, the resonance becomes sharper reflecting the improvement of the quality factor of the excited photonic band. As a consequence, the intensity of the evanescent light near the photonic crystal becomes more than ten times larger than that of the incident light. The usefulness of this enhancement is discussed in relation to the laser acceleration, in which a running charge is accelerated by the work done by the diffracted component of the incident laser light. The positive role of photonic crystal in free-electron lasing is also discussed and a low-threshold onset of the lasing is pointed out.