Subdiffraction and spatial filtering due to periodic spatial modulation of the gain/loss profile

It is well known, that the materials with the refractive index modulated in space on the wavelength scale, i.e. the photonic crystals (PCs), bring about a significant modification of the propagation properties of waves, both in time and space domains. In time domain the usual (temporal) dispersion is modified, and photonic band gaps appear in the frequency spectra. In the space domain, the spatial dispersion (diffraction) can also be modified, which leads to negative diffraction, also to self-collimation (also called diffractionless or subdiffractive propagation.