Introducing Defects in 3D Photonic Crystals: State of the Art

Photonic crystals (PhCs) are materials that possess spatial periodicity in their dielectric constant on the order of the wavelength (k) of light. These materials can strongly modulate light and, with sufficient dielectric contrast and an appropriate geometry, may exhibit a photonic bandgap (PBG). This concept was first proposed in 1975 by Bykov but remained relatively unknown until the seminal work of Yablonovitch and John. In a rough analogy to semiconductors, which possess an electronic bandgap, a PBG material prohibits the existence of photons with energies in the PBG. PhCs are naturally classified by the dimensionality of their periodicity, and in order to rigorously prevent the propagation of PBG frequencies in all directions, a 3D PhC with an omnidirectional, or complete PBG (cPBG) is required. cPBG materials have been fabricated and well characterized for operation at microwave and radio frequencies, however, operation in the visible and IR requires the characteristic length scales of these structures to be scaled down by several orders of magnitude, necessitating 3D fabrication techniques capable of defining structures with sub-micrometerto micrometer-scale periodicity and nanometer-scale resolution. Additionally, cPBG structures must be fabricated from optically transparent materials with a high dielectric constant. In the optical regime, there is a limited set of materials satisfying these conditions, largely ruling out organic and metallic materials, most oxides, and many semiconductor-based structures. Because of the materials restrictions and stringent 3D fabrication requirements, there are only a small, but growing, number of cPBG materials that have been constructed in the visible or IR. A review of PBG fabrication methods can be found in the literature. The development of efficient, practical techniques amenable to the fabrication of cPBG materials operating in the visible to IR remains a vibrant area of research, with new approaches regularly being introduced in the literature. R EV EW