Silicon Photonics: Fundamentals and Devices

4.3 Basic Concepts

4.3.1 Absorption and Emission

There are three basic processes through which EM radiation can interact with matter. The processes are illustrated by Figure 4.1 using a hypothetical atom having only two energy levels, E₁ (lower level) and E₂ (upper level). Under normal conditions, all materials absorb light. The absorption process is illustrated by the left part of Figure 4.1, in which a photon of energy ω = E₂ – E₁ is absorbed, transferring an electron from the lower level to the upper level. The intensity of the incident light gets attenuated due to the absorption. The excited atoms eventually return to their normal ground state, and the excess energy ω is emitted in the form of EM radiation. The radiation is directed along a random direction and has no phase relationship among them. The third process is known as stimulated emission, which differs from spontaneous emission in that the process is initiated by an incident photon. This process is illustrated by Figure 4.1c. In stimulated emission the frequency, phase, and direction of propagation of the emitted and incident radiation are all identical. All lasers, including semiconductor lasers, emit light through the process of stimulated emission, while the light-emitting diodes (LEDs) emit light by the spontaneous ...