1. Color Centers
Color center are responsible for beautiful colors of some ancient artwork. Such color centers are observed in many other materials as well, table salt for example. The figure below shows an absorption coefficient, ?, of table salt, also known as NaCl. The absorption feature related to the color centers is strongly peaked in the visible and gives rise to the slightly yellow color of table salt. If you would take a salt crystal and irradiate it with energetic ions or X-rays, you would find that the crystal turns dark yellow. Color centers are formed by the creation of socalled Frenkel defects, an ion displaced from its usual site in the crystal. The vacancies that remain after the ions are displaced (by e.g. an energetic particle) can trap charges. Some configurations in which charge can be trapped are shown and named in Figure 2.
2. Metallic nanoparticles
Some of the beautiful colors of 17 th century church windows and ancient glass objects derive their color from the presence of nanometer size noble metal particles. We would like to study the optical properties of a piece of SiO 2 glass with a refractive index of n SiO2 = 1.5 that has a low concentration of Ag nanoparticles (radius R) embedded in it. We illuminate the glass on one side with a white light source and notice a beautiful yellow color in transmission.