Photonic crystals in the optical regime * past, present and future

During the last decade, photonic crystals, also known as photonic microstructures or photonic bandgap structures, have matured from an intellectual curiosity concerning electro-magnetic waves to a "eld with real applications in both the microwave and optical regime. In this review, we shall focus on progress and the prospects for semiconductor structures that mainly involve guided modes interacting with periodic structures, but we also evaluate alterna-tive material systems and fabrication methods, e.g. those based on self-organisation. We shall go from basic concepts, via a discussion of the state of the art, to device applications. Naturally, the discussion of the applications will be more speculative, but we attempt to evaluate the real prospects o!ered by photonic crystals at optical frequencies while considering practical limita-tions. In doing so, we identify a variety of areas such as the combination of quantum dot light emitters with photonic crystals that seem particularly promising. We discuss the prospects for enhanced light}matter interactions in photonic crystals and the related material and design issues. Overall, the aim of this review is to introduce the reader to the concepts of photoni