Unit 9 Optical Ceramics
The word photonics has been coined to describe the collective optical properties of materials and the application of these properties to make useful devices. This section begins with a brief description of some of the more important photonic properties of materials.
Wide bandgap materials, such as ceramics insulator materials, are inherently transparent to light in the range of wavelengths near to or including the visible range, provided that they do not contain internal inhomogeneities which can serve as scattering sites and which will reduce the transparency to translucency or even opacity. Consequently, ceramic single crystals, porefree glasses and even porefree single phase polycrystalline ceramics can be utilized for photonic applications requiring transmission of light beams.
Even though a material is transparent, that does not mean that the material does not interact with light that passes through it. For example, the velocity with which light waves propagate varies from material to material, having its highest possible value in vacuum (this velocity being the universal constant c=3.00×10[8]m/s). In all other media, light travels slower than c, with the ratio of c to the actual velocity in the material being called the index of refraction, a light beam will bend; this principle is used when lenses cause light beams to focus or to diverge. In most materials, the index of refraction varies with the wavelength of the light; this behavior is called dispersion, and it underlies the separation of different wavelengths from a mixed light beam because of differing amounts of bending.
The electromagnetic waves that constitute a beam of light oscillate perpendicular to the direction of propagation of the beam. Under normal conditions, the oscillations are randomly oriented around the direction of propagation. However, some materials are able to modify the passing light beam so that only certain oscillation direction occur. This is called polarization, and many important applications of light require or take advantage of the polarization phenomenon. For example, if a light beam of a given polarization is incident on a material that will only transmit light of a different polarization, then that material will effectively block the passage of the beam.
Stressfree glass and many crystals are optically isotropic, meaning that the index of refraction is the same regardless of the direction of a light beam. However, certain types of crystals will split an incident light beam into two separate beams, each of which is polarized. Glass containing residual stresses will also show this type of behavior. This phenomenon is called birefringence, and materials exhibiting this behavior are said to be double refracting or optically active. This phenomenon is the basis for a number of types of optical devices and also as a means for revealing residual stresses in otherwise optically isotropic materials.
No material is perfectly transparent; some of the light entering a material will be absorbed and converted into heat or other forms of energy. Materials that do not absorb more strongly at one wavelength than at another appear to be colorless, but many transparent materials do show selective absorption and therefore appear to be colored.
Applied electric and magnetic fields can modify the refractive indices of materials to some degree; these effects are called the electrooptic effect and the magnetooptic effect. A particularly interesting ancillary effect of these phenomena is that an applied field can cause a normally optically isotropic material to display birefringence, which disappears when the field is removed. In certain materials, these effects are sufficiently large that optical devices can be built that take advantage of them.
The photonic applications of ceramic materials depend on one or a combination of the properties just described. For example, windows require simple transmission of light beams without alteration. On the other hand, filters are required to be transparent at certain wavelengths and to be strongly absorbing at others. A single material with the appropriate absorption characteristics can serve as a selective filtering window. Lenses are somewhat like windows except that they are made with surfaces that are not parallel to one another.
An especially important photonic application of glass occurs in optical fibers. The function of these fibers is to carry a beam of light from one point to another without appreciable attenuation due either to absorption or to escape from the sides of the fiber. The most frequent application is in communications, where information is encoded in the form of modulations of the light beam, usually using a diode laser. Special glasses are most often used for optical fibers. To insure very low absorption, the glass must be extremely pure and free from inclusions. To prevent escape of the light beam from the sides, the fiber is usually made to have a central core of glass with low index of refraction, surrounded by a cladding of higher index glass. The difference in index will cause perfect reflection of any portions of the beam that encounter the interface, thus insuring that all light launched within the core remains there no matter how the fiber may be curved.
Lasers are devices capable of producing highly energetic beams of light having all waves in phase and of the same wavelength. Very high intensity lasers can be used for localized heating and melting, but certain types of lasers can also be used to produce very pure, modulated light signals and so are suitable for generating encoded beams used in optical fiber communications. To function, lasers must be "pumped", that is, an input of energy is required to produce the unstable energy situation necessary for laser action to occur. Pulse lasers are usually pumped by means of an extremely bright flash of light; these lasers are often made from specially doped glasses or single crystals such as Cr doped Al2O3 (ruby lasers) or Nd doped yttrium aluminum garnet (YAG lasers). Other laser types can provide continuous output and so must be continuously pumped, often with electrical energy. An especially interesting type of continuous laser can be made from a semiconductor or insulater crystal that has been selectively doped so as to produce a pn junction. When a dc electrical voltage is applied across this junction is a direction that tends to force electrons towards the p side and holes toward the n side, recombination of excess electrons and holes in the junction region will release light energy, causing the junction to glow. When the electrical input is small, the light waves generated are not in phase, and the glowing junction is called a light emitting diode (LED). LEDs are popular for constructing all sorts of electronic displays. When the electrical energy input is large, and certain other geometric requirements are met, the light emitted by the junction will be intense and in phase, the junction will behave as a laser. In lasing mode, the magnitude of the light emitted varies with the magnitude of the applied electrical signal, and the lasing behavior also "switches off" sharply when the pumping signal drops below a threshold level. Consequently, diode lasers are particularly well suited for converting electrical signals into modulated light beams, and thus are especially valuable as signal generators in optical fiber communications systems.
Transparent polycrystalline electrooptic materials, such as PLZT ceramics, can be used for a variety of devices in which transmission of a polarized light beam is throttled by changing the optical characteristics of the material with an applied electric field. The uses include rapidly darkening windows to shield pilots or other personnel from the intense flash of a nuclear explosion or a laser weapon, goggles for welders, shutters for optical devices, optical displays, and even image storage devices. whenever a polycrystalline ceramic is intended for use as a transparent material, very careful processing is necessary from starting powder through forming and firing in order to eliminate light scattering pores and inclusions. It is not unusual to hot press such ceramics in order to ensure the absence of porosity.
Selected from: "High Tech Ceramics", P.Vincenzini, Elsevier, 1987
Words and Expressions
photonic 光子的
opacity 不透明度
propagate 传播
index of refraction 折射率
diverge 散射
dispersion 色散
oscillate 振荡
polarization 极化
birefringence 双折射
ancillary 辅助的
attenuation 衰减
inclusion 包含
modulate 调制
ruby 红宝石
threshold 阈值
goggles 护目镜
