8 Fundamentals of Optical Devices In this structure, the thin layer has slightly smaller bandgap, which attracts the concentration of carriers when the junction is forward biased; therefore this layer is referred to as the active region of the device. The strong carrier confine- ment is a result of band gap discontinuity. The sandwich layer can be controlled to be on the order of $0.1 mm, which is several orders of magnitude thinner than the depletion region of a homojunction; therefore very high levels of carrier concentration can be realized at a certain injection current. In addition to providing carrier confinement, another advantage of using double heterostructure is that it also provides useful photon confinement. By using a material with slightly higher refractive index for the sandwich layer, a dielectric waveguide is formed. This dielectric optical waveguide provides a mechanism to confine photons within the active layer, and therefore very high photon density can be achieved. 1.1.4 Spontaneous Emission and Stimulated Emission As discussed, radiative recombination between electrons and holes creates photons, but this is a random process. The energy is conserved in this process, which determines the frequency of the emitted photon as v ¼ DE/h, where DE