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.
Two types of cavities have been investigated to date by the
“Quantum Optics in Semiconductors” 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 left 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.Double heterostructure cavities:
The cut-off frequency of a photonic crystal waveguide depends on the periodicity of the surrounding hole lattice, longer periodicities corresponding to lower cut-off frequencies. Thus, by inserting a short portion of low cut-off (long-periodicity) waveguide between two higher cut-off (shorter periodicity) waveguides, one mode may be localized in the low cut-off region of the waveguide. This region is then an optical microcavity of very high quality factor (of the order of 104 to 106) and very small modal volume (of the order of a fraction of a cubic wavelength, the ultimate size allowed by diffraction). As the juxtaposition of lattices of different periodicities is reminiscent of semiconductor heterostructures, such a cavity is usually termed “double heterostructure cavity” (see Fig. 2 right and ).
Figure 2 : Scanning Electron Microscope top view of a L3 cavity (left) and a double heterostructure cavity (right) fabricated at LPN for our quantum optics experiments.
These cavities, that can store photons in very small volumes and long periods of time, are a system of choice for our investigations on nanoscale light-matter interaction, including single photon sources, nanolasers and nanoscale optomechanics.
 High-Q photonic nanocavity in a two-dimensional photonic crystal,