Читать книгу Introduction To Modern Planar Transmission Lines - Anand K. Verma - Страница 175

The uniform plane wave in the unbounded medium is the TEM‐type wave. The monochromatic EM‐wave is characterized by amplitude, phase, and polarization states. The microwave to optical wave devices can appropriately manipulate these characteristics to steer the EM‐waves in the desired direction with shaped wavefront. The modern metasurfaces, discussed in sections (22.5) and (22.6) of chapter 22, can achieve such controls on the reflected and transmitted waves.

In general, both and fields have two orthogonal field components in a plane normal to the direction of propagation, the x‐direction, as shown in Fig. (4.9a and b). The field components are in the (y‐z)‐plane. For the TEM mode, it is possible to get either (E_y, H_z) or (E_z, H_y) pair of fields. Both pairs of field components can also exist. The orientation of the electric field component and the movement of the tip of the resultant E‐field determine the polarization of the EM‐wave. Thus, the (E_y, H_z) pair of the EM‐wave is called a y‐polarized wave, as only the E_y component of wave exists. The (E_z, H_y) pair of the EM‐wave is called the z‐polarized wave. The (y‐z)‐plane is known as the plane of polarization. It is normal to the direction of propagation, i.e. the x‐axis. Both these polarizations are linear polarization.

Figure (4.9a and c) show that for the EM‐wave propagating in the x‐direction, the tip of the E_y field component moves along the y‐axis from +E₀ to 0 to −E₀. The movement and rotation of the tip of the ‐vector could be seen using the instantaneous ‐vector of the wave propagating in the x‐direction. In Fig. (4.9c), the path of the tip of the E_y‐field vector, in the plane of polarization, traces a line with respect to time. The linear trace demonstrates the vertical linear polarization. However, if both E_y and E_z field components are present, any of the three kinds of wave polarizations can be obtained: (i) linear polarization, (ii) circular polarization, and (iii) elliptical polarization. These polarizations are shown in Fig. (4.10a–c). The type of wave polarization depends upon the magnitude and the relative phase of the orthogonal E_y and E_z field components. The polarization states are briefly discussed below. Finally, the Jones vector and Jones matrix descriptions are summarized to describe elegantly the polarization states and their control by the polarizing devices.

Figure 4.10 Type of polarizations.