out at microwave frequencies in scaled structures of 6 millimeter (mm) Al2O3 spheres (Yablonovitch and Gmitter, 1989) or drill/etched Stycast 12 (Yablonovitch et al., 1991), current research on photonic crystals truly embodies the concepts of “nanophotonics,” with spatial index modulation (etched holes or solid rods) at the 100 nanometer (nm) scale, allowing compact, highly integrable waveguides, filters, resonators, and high-efficiency lasers. The original predictions of Yablonovitch and John have been realized: first reports of photonic crystal lasers were made in 1999 (Painter et al., 1999), and localization of photons within photonic crystal “defects” was first observed in 1991 in the microwave regime (Yablonovitch et al., 1991a).

Photonic Band Gap

A key idea for photonic crystal structures is the periodicity of the structure giving rise to the formation of a forbidden gap in the electromagnetic spectrum, thus altering the properties of the light passing through the structure. One-, two-, and three-dimensional photonic crystals, as well as a photonic band structure are described in Figure 2-1.

The photonic band gap defines a set of frequencies for which light cannot propagate in the crystal: the tunability of the band gap, through control of the dimensions and symmetry of the photonic structure, provides exquisite frequency control for multiple wavelength information processing (or wavelength division multiplexing, WDM). Various photonic crystal waveguides have been formed with deliberately engineered stop bands (e.g., Davanco et al., 2006; Fleming and Lin, 1999). Equally interesting, or perhaps more so, is the case in which the perfect translational symmetry of the photonic crystal is disrupted in a controlled manner. John (1987) alluded to these “certain disordered dielectric superlattices” in his 1987 paper, and Yablonovitch et al. (1991b) used the analogy of donor and acceptor modes in semiconductor crystals in defining these “defect” states: the disruption from symmetry providing a photonic state within the photonic band gap, making possible the localization of photons.

FIGURE 2-1 (a) Simple examples of one-, two-, and three-dimensional photonic crystals. The different colors represent materials with different dielectric constants. (b) A notional dispersion diagram for a photonic crystal showing a band gap and regions of anomalous dispersion. SOURCE: Joannopoulos et al. (1995). Reprinted by permission of Princeton University Press.



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