Photonic crystals and photon gaps - Properties and applications

Controlling electromagnetic waves in photonic circuits in the same way as we control electronic currents in integrated circuits - that's the objective that can be envisaged by exploiting the various "facets" of artificial structures such as photonic crystals, from concepts to applications. The main basic notions, the analogy with solid crystals and the theoretical models of photonic crystals, depending on whether they are assumed to be infinite or finite, with the possible presence of periodicity defects, were discussed in the article "Photonic crystals and photon gaps. Fundamental aspects". This second article looks at the optical properties of photonic crystals, the methods used to produce them, mainly in the field of optics, and the first potential applications of photonic crystals in different wavelength ranges.

Optical properties can be divided into two categories, depending on whether the crystal is "seen" from the inside or the outside. This somewhat simplistic classification should not, however, be dissociated from the concept of optical confinement. We will first consider the photonic crystal mirror for a wave launched from a semi-infinite medium and falling on the crystal. We will then consider the photonic crystal guide for a wave confined in a uniform space between two photonic crystal mirrors. We'll take this opportunity to see how light can be rotated on bends with a small radius of curvature, or how it can be filtered by coupling between guides. We'll then take a look at high-confinement resonators lined in whole or in part with photonic crystal. Finally, we'll show a number of original properties of photonic crystals in the allowed propagation bands instead of the forbidden bands. These include ultra-refraction, the "superprism" effect and the modulation (microwave) or non-linearity (optical) properties of photonic crystals. In passing, we'll introduce the concept of metamaterials with negative refractive index, more specific to metallic photonic crystals.

Exploiting these multiple properties, particularly in the field of optics, requires real prowess on the part of technologists. In this sense, the evolution of photonics is similar to that of electronics, with the development of nanotransistors and quantum dot memories. Here, we describe the main techniques used to create photonic crystals, with 2D and 3D structuring of materials on submicron scales. Progress has already led to a number of applications. Microstructured silica fibers, for example, are well on the way to becoming the first photonic crystal "product" on the market. As for semiconductors, the road from concept to application has been longer than expected, but potential applications are beginning to emerge. In the microwave field, high-impedance surfaces open up the prospect of new circuit...