ABSTRACT

Photons can be used as an excellent information carrier. The advantages of photons lie in the large information capacity and the high information transmission speed. To achieve ultrahigh-speed information processing and all-optical computing based on micro/nano-photonic devices is one target of the integrated photonic technology. Proper photonic materials are the essential basis for the realization of micro/nano-photonic devices. Are there photonic materials that can control the propagation states of photons? Photonic crystals are a kind of materials that can perform such functions. The propagation states of photons can be tailored by use of photonic crystals in much the same way as electrons controlled by semiconductor materials. Photonic crystals are artificial photonic materials made of two (or even more) kinds of dielectric materials periodically placed in space. The dielectric distribution of a photonic crystal varies periodically in space, which provides a “periodic potential” for electromagnetic waves entering in the photonic crystal. The electromagnetic waves encounter strong coherent scattering in the interfaces of different dielectric components. Owing to the strong modulation of the electromagnetic wave by the spatially periodic distribution of dielectric constant, photonic bandgaps appear in the dispersion

relation curves of photonic crystal. The incident light whose frequencies drop in the photonic bandgap cannot propagate in the photonic crystal. The propagation states of electromagnetic waves can be tailored at will by use of photonic crystals due to their unique photonic bandgap effect. Just like semiconductor materials, when a structure defect is introduced in a perfect photonic crystal, defect states will appear in the photonic bandgap. Electromagnetic waves with a certain resonant frequency will be strongly confined around the defect structure site. The defect states can possess very high transmittance with elaborately designed defect structures. An incident light whose frequency is resonant with that of the defect mode could transmit through the photonic crystal based on the photon tunneling effect. The role of photonic crystals in the photonic technology is just like semiconductor materials in the microelectronic technology. Various novel integrated photonic devices can be realized based on photonic crystals. 2.1 Configuration of Photonic Crystals

As a kind of artificial photonic materials, photonic crystals are constructed by two (or even more) kinds of dielectric materials arranged periodically in space. Accordingly, the distribution of dielectric function has a spatial periodicity. According to different spatial periodicities of dielectric materials, photonic crystal can be categorized into one-dimensional (1D) photonic crystal, twodimensional (2D) photonic crystal, and three-dimensional (3D) photonic crystal. 2.1.1 One-Dimensional Photonic CrystalOne-dimensional photonic crystals possess the dielectric periodicity in only one direction, for example, in the Z axle direction. One-dimensional photonic crystals can be constructed by alternatively placing a dielectric layer with a high refractive index and a dielectric layer with a low refractive index. The schematic structure of a one-dimensional photonic crystal is shown in Fig. 2.1. One-dimensional photonic crystals possess the configuration of multiple dielectric layers. The photonic bandgap effect of a one-dimensional photonic crystal arises from the strong multiple scattering in the interfaces

between high-refractive-index layers and low-refractive-index layers, and the subsequent destructive interference. When an electromagnetic wave is incident along the direction perpendicular to the plane of the dielectric layers, the propagation states of the electromagnetic wave will be controlled by the photonic bandgap effect. The electromagnetic wave will see a homogeneous distribution of dielectric function when it is incident in the direction parallel to the plane of the dielectric layers. There exists no photonic bandgap effect that can influence the propagation process of the electromagnetic wave in this direction. Therefore, one-dimensional photonic crystals cannot provide the complete confinement and controlling of photons in all three dimensions and all the directions. The major properties of the photonic bandgap of one-dimensional photonic crystals are mainly determined by the following parameters: the thickness of two kinds of dielectric layers, the lattice constant (corres-ponding to the dielectric periodicity), and the refractive index contrast of high and low dielectric layers. The dispersion properties of dielectric materials also have a very important influence on the photonic bandgap effect of one-dimensional photonic crystals.