In the previous chapter we have examined optical processes in semiconductors and discussed optical absorption, gain, recombination, etc. In this chapter we will discuss the origins of dielectric response and how one can modify the response. It is well known that the dielectric response of a solid is different from that of vacuum. This difference arises from the presence of charges in the solid so that the “local field” felt inside a solid is different from the externally applied field. The presence of an external electric field polarizes the solid by creating relative displacement of charges. The atomic electron charge cloud can be disturbed, the ionic charge (for solids with cations and anions) can be shifted, the ions themselves can move physically, the “free” charge in the conduction band can be disturbed, etc.
In this chapter we will develop several models for polarization and dielectric response. Particularly interesting are cases where the response can be controlled by external stimulus and thus be exploited for device applications. The dielectric response can change as a function of applied field, applied strain, temperature, optical intensity, etc. As a result we can use the dielectric response change for strain sensors, tunable capacitors, interference-based optical devices, etc.
POLARIZATION IN MATERIALS: DIELECTRIC RESPONSE
We have seen that solids can be characterized as insulators, semiconductors, and metals. Insulators, where current flow is negligible in the presence of an electric field (due to the absence of mobile charges) show very interesting dielectric response. When an external electric field is applied the charges in the solid are disturbed, creating a dipole moment and polarization (dipole moment per unit volume).