Interacting with and Generating Light

Abstract

In this chapter we examine four key properties of ceramic materials all of which we can classify as optical. (1) Ceramics can be transparent, translucent, or opaque for one particular composition. (2) The color of many ceramics can be changed by small additions: additives, dopants, or point defects. (3) Ceramics can emit light in response to an electric field or illumination by light of another wavelength. (4) Ceramics can reflect and/or refract light. We will discuss why these effects are special for ceramics and how we make use of them. The optical properties of ceramics result in some of their most important applications. In their pure form, most dielectric single crystals and glasses are transparent to visible light. This behavior is very different from that of metals and semiconductors, which, unless they are very thin (<1 µm), are opaque. Many ceramics and glasses also show good transparency to infrared (IR) radiation. This property has led to the use of glasses for optical fibers for high-speed communications. We can produce polycrystalline ceramics that are highly transparent. The ability to make translucent and transparent polycrystalline ceramics was developed in the 1960s when it was discovered that small additions of MgO to Al2O3 powder could produce a fully dense ceramic by sintering. This product is widely used in streetlights (the golden glow). If a transparent ceramic or glass is doped, for example, by the addition of transition metal ions, the material becomes colored. We will discuss the different types of colors that can be produced in ceramics. The doping of Al2O3 with Cr3+ produces ruby, which is used as an optical cavity in a solid-state laser. The ruby laser was the first solid-state laser. There are now many more examples of solid-state lasers using colored ceramics and glasses as the optical cavity. We will describe how solid-state lasers work and the different wavelengths of radiation that can be produced. Phosphors produce light as a result of excitation by, e.g., electron irradiation. Again, we are using doped ceramics for this application because, as we will see, doping changes the electronic band structure of the ceramic.