EMS172L_Lab06_OpcticalProperties

EMS172L_Lab06_OpcticalProperties - UC Davis EMS 107L...

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UC Davis – EMS 107L Electronic, Magnetic, and Optical Properties Laboratory – Prof. Ricardo Castro 1 UNIVERSITY OF CALIFORNIA, DAVIS Department of Chemical Engineering and Materials Science EMS 172L: Electronic, Magnetic, and Optical Properties Laboratory Laboratory Class VI – Optical Properties Absorption of Light Optical properties are among the most useful in new and old technologies. These properties we usually learn in high-school, but without knowing exactly what is the whole of the material in that specific behavior. When light (or photons) find a material, it will interfere in the energetics of the system, and this interference will be responsible for the observed properties. When incident light strikes a solid, it is re-emitted in one way or another (reflection, transmission) or its energy is vanished as heat, for example. In any of the cases, some interaction between the light and the matter will occur. One of the most interesting phenomena is absorption, and it can be understood based on the same band theory described for the electrical properties. Adsorption occurs when the electrons in the crystal capture the energy of the photons, and electrons are excited to a higher energy state if this is available (see figure), as in an interband transition (see figure). The energy to excite an electron is the high the higher the gap between the valence and the conduction band. The energy of the light can be described by E = ν h Where h is the Planck’s constant and ν is the light frequence given by the number of vibrations per seconds or Hertz (Hz). In glasses, the energy gas is very high (6.2 eV for SiO 2 glass), and adsorption only occurs for wavelengths of 200 nm or less. For semiconductors, as Si, the gap is small (1.1 eV), and adsorption already occurs in the infrared region. Therefore, they are opaque in the visible
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EMS172L_Lab06_OpcticalProperties - UC Davis EMS 107L...

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