A complete discussion of other light-emitting structures that confine photons is beyond the scope of this chapter. It is useful, however, to discuss the properties of some confined-photon emitters. Photonic crystal or photonic bandgap structures or involve two - or three-dimensional photon confinement achieved by periodic patterning of the light-emitting active region or the material adjoining the active region. Examples of photonic crystal structures were given by Joannopoulos et al. (1995), Baba and Matsuzaki (1996), and Fan et al. (1997). Erchak et al. (2001) reported very encouraging results on photonic crystal LEDs, namely a six-fold enhancement of light extraction along the surface-normal direction.

Photonic crystal structures can consist of a series of rods or holes arranged in a regular pattern, such as a hexagonal close-packed array. The periodicity of the array can create an optical bandgap for lateral emission at certain emission energies and one or both polarizations. By suppressing the lateral emission, a structure consisting of rods will have a large bandgap for TM emission and a smaller bandgap for TE emission, but not at the same emission energies. However, if the emitting region had a dipole oriented mainly along the rods (such as in quantum well electron-to-light-hole recombination), the lateral emission could be efficiently suppressed. A structure consisting of patterned holes will have a smaller bandgap than that of the rod structure, but it has the advantage that it can have a true optical bandgap for both polarizations of light. Photonic crystal structures

either by themselves, or combined with a planar resonant cavity, enable a strong longitudinal emission enhancement.

Another confined-photon emitter is the microdisk laser (McCall et al., 1992), which is fabricated as a thin dielectric disk that couples light out the edges of the disk. Lasing modes can be described by a mode number M, where exp (iMф) is the form of the electric field around the cylindrical disk. Because waves can propagate both ways, M can be positive or negative. The disk can be fabricated with a thickness such that the emission perpendicular to the disk is suppressed. Small disks will only support a few modes, and therefore can have a high spontaneous emission factor в. The Q of these modes are also high enough to achieve lasing. One attractive aspect of such disks is that the lasing emission occurs in the plane of the sample, from a very small device. This could be useful for integration of many photonic devices on a single wafer. However, the output is difficult to efficiently couple into waveguides and fibers, as it only couples evanescently. Advances have been made in improving the longevity, operating temperature range, and active-region passivation of such devices (Mohideen et al., 1993). Room-temperature cw electrical pumping is still a problem, however.