Directive excitation of guided electromagnetic waves through polarization control

Experimental evidence is reported on the control of the directionality of guided electromagnetic microwaves by the polarization of the exciting wave. Experiments are conducted using a two-dimensional waveguide made of two parallel aluminum plates. The upper plate, which has a linear array of holes, is externally illuminated by a polarized wave whose incident wavevector is contained within the mirror-symmetry plane defined by the linear array. Surprisingly, the measurements show that the propagation inside the waveguide is highly asymmetrical, and it is controlled by the polarization of the incoming wave. This extraordinary phenomenon is explained in terms of a simple model involving a set of dipoles that are excited at the hole positions. Our finding provides a powerful method to sort different polarizations of a free-space beam to different propagation directions of guided electromagnetic waves.

Figure 1

(a) Schematic of a circular hole array with linear incident wave polarization. (b) Manufactured circular array. (c) Schematic of a rectangular slit array with circular incident wave polarization. (d) Manufactured rectangular array.

Figure 2

(a) Simulation of the vertical electric field at the bottom plate of the PPW for a linearly polarized plane wave incident at an angle into the array of small circular holes shown in Fig. 1. (b) Analytical 2D model of an array of dipoles placed at opposite ends of each hole driven by the incident plane wave. (c) Radiation diagram of TEM waves inside the PPW for each pair of dipoles with opposite signs representing a single hole. (d) Array factor of the periodic repetition of holes are excited by a phase difference of Δ$\varphi$.

Figure 3

Numerical simulations of the power directionality coefficient $P_{\mathrm{right}}/(P_{\mathrm{right}}+P_{\mathrm{left}})$ for the rectangular slits under (a) linear polarization and (b) circular polarization. The selected geometry is marked as a black dot in panels (a) and (b), but it has only been experimentally assessed in the case of circular polarization of panel (b).

Figure 4

(a) Measured (raw) electric field distribution at 9.3 GHz within the PPW following Fig. 1, with the circular hole array located along axis $y$ = 0. (b) $E$ field processed by means of a FFT. Some backward waves (values with $k_x$ < 0) are observed in (b), which are filtered from the $k$ space in (c). (d) Inverse FFT of (c) showing only the forward spectral components of the raw measurement in (a).

Figure 5

(a) Measured (raw) electric field distribution at 5.5 GHz within the PPW following Fig. 1, with the rectangular slit array located along axis $y$ = 0. (b) $E$ field processed by means of a FFT. Some backward waves are observed in (b), which are eliminated from the $k$ space in (c). (d) Inverse FFT of (c) showing only the forward spectral components of the raw measurement in (a).

Figure 6

Measured (symbols) and simulated (lines) unidirectionality coefficients as a function of the normalized frequency $f_{\mathrm{norm}}=f{p}_{\mathrm{array}}/c$ for the hole (blue solid) and slit (red dashed) coupling structures under linear and circular polarizations, respectively.

Figure 7

Spatial Fourier space for the guided modes inside the PPW.