Photonic crystals are engineered periodic dielectric structures in which optical waves with wavelengths close to the period of the structure encounter strong dispersion, and in some cases are completely forbidden from propagating within a frequency window (photonic bandgap). The periodicity may be one- two- or three-dimensional, each dimensionality corresponding to photonic crystals with specific properties.
Two-dimensional photonic crystals etched on suspended membranes (see Fig. 1) have recently emerged as a leading platform for the efficient manipulation of photons on a chip, and the realization of functional photonic devices amenable to large-scale integration. The membrane is made of a high-refractive index medium (in our experiments InP), has a thickness of the order of a half-wavelength and is pierced with a two-dimensional array of holes, usually in a triangular or square lattice. This arrangement constitutes a two-dimensional photonic crystal: light is confined in the perpendicular direction by total internal reflection due to the large refractive index difference, while in-plane confinement is ensured by the photonic bandgap effect. Pseudo-3D light control becomes possible as a result.
By engineering the hole arrangement, optical cavities with low optical losses and diffraction- limited size (the ultimate volume allowed in propagative optics) can be formed in a photonic crystal membrane. One type of cavities have been investigated to date by the " Optomechanics with photonic crystals" team :
In a two-dimensional photonic crystal structure displaying an in-plane photonic bandgap, light may be confined in a small region of space by disturbing locally the periodicity of the photonic crystal (in other words, by introducing a “periodicity defect”). For example, by introducing a point defect (1 missing hole) or an extended point defect (for instance, n missing holes in a line, named a Ln cavity – see Fig. 2 and ), light can be confined locally in the region of the missing holes, thereby forming a photonic nanocavity. When a full line of holes is missing (line defect) a waveguide may be engineered channelling the propagation of light in one only dimension.
Figure 2: Scanning Electron Microscope top view of a L3 cavity fabricated at LPN for our experiments on optomehcanics.
 High-Q photonic nanocavity in a two-dimensional photonic crystal,