In Chapter 5 we examined how light detection and emission occurs in devices. In addition to lasers, LEDs, and detectors discussed in Chapter 5, we need devices that can: (i) modulate light, i.e. alter the strength of the light signal; (ii) switch light from one path to another for, say, the purpose of sending a beam to a particular route; and (iii) selectively filter a particular optical wavelength. Such devices are needed for optical communication networks as well as for display technologies and optical sensors.
In Chapter 6 we have seen how the dielectric response of a material is influenced by internal charges. The distribution of these charges and the polarization can be altered by electrical, mechanical, and thermal perturbations. Mechanical stress (strain) leads to devices that can be used as piezoelectric sensors and transducers. Similarly temperature changes can be exploited for infrared sensors. In this chapter we will examine how electrical signals can be used to alter the optical response of a device.
Optical signals are electromagnetic waves, which propagate through free space and solids in accordance with Maxwell's equations. As noted in Chapter 5, in light absorption and emission we need to use quantum mechanics (i.e., treat electromagnetic waves as particles) to understand the physical properties (absorption coefficient, gain, etc.). Once these properties are established the propagation is described by the classical wave equation of Maxwell. The properties of light waves are described by physical parameters, such as polarization, intensity, wavelength, speed of the wave, etc. Most optical modulators and switching devices are based on phenomena that allow altering the polarization or intensity of light.