Photonic crystals

Photonic crystals are materials characterized by a periodical modulation of the refractive index of the same order of magnitude of the wavelength used to investigate them. The perfect arrangement of the materials that constituted that structures produces, due to multiple internal reflections and refractions, interference phenomena that mould the light propagation in their inside. If the refractive index contrast is big enough and the absorption is negligible, the scattering at the dielectric interface can cause the formation of frequency regions where the propagation of light is inhibited for some particular directions. These frequency regions are called Photonic Band Gaps (PBG) in analogy with respect to the electronic band gap which is responsible of the electrons behavior in semiconductors.
By introducing a defect in the periodicity it is possible to obtain a state inside the PBG region. The light associated to this state is spatially confined in correspondence of the defect due to PBG effect, consequently the defect can be exploited for example as a nano-cavity. By engineering the property of the defect one can modify the behavior of the cavity. The typicalmodal volume of such cavities (V) can be of the order of a cubic wavelength. Consequently when the cavity quality factor Q becomes sufficiently large, the emission coupled to the cavity
mode can be significantly enhanced by a factor of Q(lambda)3/V through the Purcell effect. The ability to modify how and if light can propagate through a material leads to a way to control how embedded emitters can emit light.

Rewritable photonic cirquits
Based on the idea of intentionally introducing crystal defects, a broad range of potentially functional designs,such as integrated micro-cavities, channel drop filters, optical switches, and low-threshold lasers has been proposed. Connecting such devices could essentially enable the photonic version of an integrated electronic circuit. Traditionally, the experimental realization of such structures is limited to simple photonic-crystal design variations, such asmissing pores, pores of different sizes, or pores at different positions, all of which must be incorporated at the growth stage of the photonic crystal. An alternative and much more flexible approach for functionalizing two dimensional photonic crystals consists of locally filling single pores of the crystal with liquids. If the refractive index of the filling material is sufficiently larger than 1, the filled pore behaves the same as a missing pore, except that   the defect can be erased and overwritten. >>

Near-field probing and modification of 2D photonic crystals
Near-field optical microscopy has already proved to be a powerful tool for studying the optical response of photonic structures and in particular two dimensional photonic crystal nano-cavities. Near-field microscopy permits not only to get information about the optical properties of these structures but it also allows to locally modify their optical behavior. 
snom and cavities

The local introduction of a sub-wavelength dielectric tip in the near-field of a two dimensional photonic crystal cavity induces a reversible tuning of the cavity resonancewithout necessarily introducing significant losses. Since the strength of the tip induced tuning is proportional to the electric field stored in the structure, by simply mapping the induced spectral shift one can obtain a high resolution map of the local density of electromagnetic states. A larger tuning efficiency can be obtained also by exploiting the local heating induced by near-field laser excitation at different excitation powers. The temperature gradient due to the optical absorption results in an index of refraction gradient which modifies the dielectric surroundings of the cavity shifting the opticalmodes . >>