Controlling diffraction of optical beams using photonic crystals

We show that propagation of the envelope of the optical beams inside a periodic structure can be approximately modeled using a linear transformation solely based on its band structure. Using this model a simple description of the diffraction is obtained which relies on the shape of the equal-frequency contours of the photonic crystal band structure. This model provides intuitive insight on how to design these structures for diffraction-based applications. We consider various methods to modify the band structure, or to excite appropriate regions of the band structure to realize different functionalities inside the photonic crystal. Applications like diffraction-free propagation, frequency separation in space (superprism-based devices), and diffraction compensation are discussed in more detail.